principles of systematics and nomenclature general system and phylogeny of insects systematics of Ephemeroptera


The division 1.1 from the book by N. Kluge 2000  


last update 1.III.2004




I.1 Reconstructing phylogeny
I.1.1 Lingering misconceptions
I.1.1.1 Is embryological method reliable?
I.1.1.2 Applicability of paleontological method
I.1.1.3 Structure complication in phylogenesis
I.1.1.4 Oligomerization of homologous organs
I.1.1.5 Lack of specialization in ancestral forms
I.1.1.6 Evolutionary scenario
I.1.2 Cladistic analysis
I.1.2.1 Terms "apomorphy" and "plesiomorphy"
I.1.2.2 Apomorphy-based phylogenetic analysis
I.1.2.3 Identifying character polarity
I.1.2.4 Interdependency of phylogenetic theory and character definition
I.1.2.5 Independently evolving homologous characters
I.1.2.6 Field of application of cladistic analysis
I.1.3 Why do numerical methods fail
I.1.3.1 Inavailability of parsimony principle
I.1.3.2 Fundamental scortcomming of numerical taxonomy
I.1.4 Using molecular characters


The word phylogeny or phylogenesis (from Greek phylum tribe, and genesis origin) was introduced by E. Haeckel in second half of the XIX century and recently is used in two somewhat different meanings. (1) Phylogeny in wide sense is a historical development of organisms, the same as "evolution" in modern meaning of the word [while originally the word "evolution" ("evolutio") had completely different meaning as unrolling of a roll or ontogenesis of organisms]. (2) Phylogeny in narrow sense includes not all aspects of historic development, but only succession of branching of a genealogical (i.e. a phylogenetic) tree. Here we will use the word "phylogeny" in its narrow sense.


A widely spread opinion exists, that reconstruction of phylogeny is possible if combine three mutually supplementing methods embryological, paleontological and comparative (or comparative-morphological) ones. Combination of these three methods suggested in the second half of XIX century by Haeckel is called "Haeckel's triad". However it will be shown below that among these three methods, only the comparative method (more exactly, a cladistic analysis, or an analysis of synapomorphies) has real meaning. Besides this, some authors, in order to ground phylogenetic reconstruction, try to use principles which they regard as universal regularities of evolution such as a principle of complication of organization, a principle of oligomerization, and others. However in reality there are no any general laws of evolution (or at least they are not discovered yet) and according to modern view, evolutionary transformations can be quite diverse.

I.1.1.1. Is embryological method reliable?

The embryological method is based on the "biogenetic law", according to which each organism during its ontogenesis repeats its phylogenesis in a shortened and modified form (here we use the word "phylogenesis" in its wide sense see above). By other words, according to the "biogenetic law", the most ancient characters have to appear in individual development earlier than characters which are evolutionary younger.

In order to ground the "biogenetic law", specially selected examples are used, in which animals from different taxa at earlier stages of ontogenesis have more close similarity one with another than at later stages of ontogenesis.

For instance, in many manuals a drawing is reproduced which shows selected ontogenetic stages of a fish, a bird and a mammalian, and at the earliest among the drawn stages these animals are very similar one to another (they have gill chinks, small limbs and large tail), and at the later stages they are sharply different. But numerous opposite examples are also well known, when the greatest differences are found not at the later stage, but vice versa, at an earlier stage of ontogenesis. For example, differentiation of embryonic layers (ectoderm, entoderm and mesoderm) is a general character of Eumetazoa, while ways of formation of these layers at earlier stages of embryogenesis are characters of subordinate taxa [see III.A- Eumetazoa (1)]. Even if we turn to the mentioned above example with ontogenesis of the fish, the bird and the mammalian, the same animals at their earliest stages of orthogenesis (cleavage, gastrula et al.) have more sharp external differences than at the stage of gill chinks.  

Many researches repeatedly tried to use the "biogenetic law" for clarifying polarity of characters (i.e. to determine in which direction evolution of this or that character took place) and for reconstruction of an ancestor. Often it leaded to evidently wrong hypotheses.

As the examples of unsuccessful attempts to use the biogenetic law in entomology we can name following: existent in XIX century theory about campodieform ancestor of insects, based on similarity of Campodea and some of insect larvae (see V-1.1.1: Status of Diplura); attempts to explain origin of insect wings, basing on development of wings in ontogenesis (see VI-1: Classifications of Pterygota VII); opinion about homology of ventral appendages of odonates with cerci [see VI-1.2.1: Odonata (13)] and others. Some authors believe that the earlier difference between animals appear in ontogenesis, the earlier divergention of these animals took place in phylogenesis; on base of this they particularly assume a polyphyletic origin of arthropods, basing it by the fact that in various groups of arthropods early stages of embryogenesis are different (Anderson, 1973). However, if logically continue this idea we would come to an absurdity conclusion that various groups of arthropods and even various representatives of such surely holophyletic taxon as Collembola (see V- arose from various protozoans, as for them various types of cleavage are described (while the cleavage represents a changing from an unicellular to a multicellular condition). However generally the "biogenetic law" appeared to be not popular among entomologists, because a deep larval specialization in majority of insects visually demonstrates incorrectness of this law.

The "biogenetic law" was formulated in the XIX century, when nothing was known about molecular bearer of heredity and about mechanism of their transformation in evolution. Recently we know that during ontogenesis genotype does not change, and the organism on all stages of its ontogenesis is built on the base of the same programs encoded in the same genotype; during phylogenesis the genotype is changed, but this takes place not by raising of new genes which would encode newly added stages of ontogenesis, but by changing (mutations) of formerly existed genes. Thus, modern knowledge about mechanisms of heredity in no manner clarify the question in which manner the repeating of phylogenesis in ontogenesis could be served. Characters inherited from an ancestor and indicating relations of a certain animal can appear at every stage of development with equal probability; sometimes such characters are found in early stage, and in this case somebody say about expression of the "biological law"; sometimes, vise versa, such characters are found in a late stage.

An assumption can be made that the embryological method as a special method of phylogeny reconstruction is not grounded neither by theoretical ideas, nor by empirical data.

At the same time embryological data are quite important for reconstruction of phylogeny, if they are used not for a special embryological method, but as a material for cladistic analysis.

For instance, adult crustaceans (Eucrustacea) have so diverse structure, that it is difficult to name any common characters of this taxon; at the same time in all Eucrustacea larvae (nauplii) or late embryos have evident common features [see III.B- Eucrustacea (1)], that allows to make a conclusion about integrity of this taxon. At the same time it is difficult to imagine that the nauplius repeats a structure of the common crustacean ancestor, as it has non-segmented body and only two pairs of large postoral appendages (Fig. 17В); judging by everything, crustaceans being multisegmented animals arose from a multisegmented arthropodous ancestor with large number of homonomous limbs, and that one arose from multisegmented annelids.

In many arthropods during ontogenesis body segments originate as homonomous ones (the nauplius of Eucrustacea in this sense represents one of exceptions). Often in certain stages segmentation is well expressed, while in adult animals it partly disappears or masks for example head of insects and many other arthropods appears to become practically non-segmented. Investigation of embryogenesis allows to clarify segmental composition of the head and some other divisions of the body, that allows to understand common characters of certain segments and basing on this to make conclusions about phylogenetic relations of various groups of arthropods.     

I.1.1.2. Applicability of paleontological method

The paleontological method means a supposition that we can directly see a result of evolutionary change if compare recent animals with their fossils ancestors. It is true that we can find in fossil condition the specimen which gave a posterity from which a recent species arose. In some cases preservation of fossils and exactness of estimating of their age are enough to make important trustworthy conclusions about structure and time of existence of these animals. But now we have no any direct methods to recognize if this or that fossil animal is a direct ancestor of any of recent animals. Conclusions about relation ancestor-descendant can be made by cladistic analysis only. In order to do this, it is necessary to reconstruct a common ancestor of several recent species on the basis of comparison of these species, and than to make sure that this reconstruction does not contradict to structure of the certain fossil animal. In this case we have no any guarantee that the fossil animal is really a direct ancestor of the recent ones, but not a member of a side branch which retained that features of the ancestor which we are able to reconstruct.

The paleontological method can be accepted in a form of such rule: fossils which have plesiomorphies (i.e. ancestral features of structure) are found mainly in earlier deposits than fossils of animals which have apomorphies (i.e. derived features of structure). But there are many exceptions from this rule, that is explained by interrupted and occasional nature of paleontological data. It is clear that if in the moment of geological history A in a certain phylogenetic line a morphological character X1 was transformed to X2, the character X2 can be found in some of animals living later than the moment A, but can not be present in any animals living before the moment A. However there are no any limits for distribution of the character X1, because when the character X2 appeared, there was not any reason for disappearance of the initial character X1, and it can be preserved for indeterminately long time in phylogenetic branches related to the branch where the character X2 had appeared. Because of this in the rule formulated above, there is a word "mainly", and if the fossils interesting for us are known as a few specimens of a few deposits, this rule is non-acceptable.

Paleontological chronicle is in many respects occasional in relation to phylogeny, but it has its own regularities (see Chapter II.2). For example, soft-bodied animals have much less chances to be preserved than animals with hard skeleton; because of this presence of animals with skeleton in ancient deposits and absence of soft-bodied animals there in no manner testifies that recent soft-bodied animals arose as a result of loss of the skeleton. As terrestrial animals and plants were preserved mainly in lake deposits, the largest number of their fossils belong to species living in lakes and banks; this does not mean that these species are more ancient than others. Reach collections of fossils are got from selected local deposits where conditions of sedimentation and all geological history helped to the good preservation of the fossils; but the fact that these or that species did not fall to these deposits does not mean that these species did nit exist at that time. Large number of exceptions make the rule formulated above to be of little use.

An example of unsuccessful interpretation of paleontological data is a story of investigation in Palaeodictyoptera and other Protorrhynchota (see VI-1.3). In the second half of XIX beginning of XX century it was generally accepted that this group of insects should be the most primitive one, because it is one of the most ancient known groups of winged insects (they appeared in Namurian the last century of Lower Carboniferous, and died out in Permian). Palaeodictyoptera were called a "synthetic group", which contain inside itself ancestors of recent orders mayflies, odonates, orthopterans, rhynchotans, hymenopterans et al. Particularly, in connection with this, the structure of Palaeodictyoptera s.str. was regarded to be an ancestral structure of recent odonates, and on this base dragonflies (Anisoptera) were regarded to be the most primitive among recent odonates, because they are more similar to the paleodictyopterans, than damselflies (Zygoptera) (see VI-1.2.1.B). However, when more paleontological material was accumulated, it was founded out that the paleodictyopterans and other protorrhynchotans form a holophyletic group which is characterized by a unique modification of mouth apparatus and is not ancestral to any of recent insects; correspondingly, that theories lost their meaning, which explained phylogeny of selected groups of winged insects basing on the idea about origination of these insects from the paleodictyopterans. In this case the mistake appeared probably because the large and actively flying paleodictyopterans were buried in lake deposits of Carboniferous, while really primitive insects had another kind of life and did not fall into that deposits.

The paleontological method as a special method of phylogenetic reconstruction does not give desired results. At the same time paleontological data have great importance for reconstruction of phylogeny. In many cases fossil animals have such characters or combinations of characters, which are not found in recent species. Because of this, a comparative analysis of fossils give results which would be impossible to get studying recent forms only. For instance, examination of Palaeozoic mayflies with homonomous wings (Permoplectoptera) allows to understand phylogeny of mayflies, and without examination of Jantardachus and other Mesozoic groups allied to thrips, it would be difficult to clarify phylogenetic relations of Thysanoptera. But in these cases not a special paleontological method is used, but a cladistic analysis applied to paleontological objects.

I.1.1.3. Structure complication in phylogenesis

An opinion occurs that evolutionary changes are directed toward complication of structure of the organisms. Such "law" of obligatory or preferable complication of organization in evolution could be explained if accept the position by J.B. Lamarck, which was actual in the end of XVIII beginning of XIX century. According to this antique theory, organism is influenced from outside by fluids, and it has some mechanisms because of which it reacts adequately in such a manner, that inside it are formed channels for guiding these fluids (i.e. veins, nerves and other integration systems); this complication of structure is inherited in some way; as a result of this, during adaptation to a new environment, besides structures which already existed, new ones are added; i.e. new adaptation leads to complication of structure, that leads to regular transformation of more simple organisms to more composite ones. The fluid theory was disproved even in the Lamarck's time, and his assumptions about adequate reaction of organisms to influence of environment and about inheritance of acquired characters have not got further confirmation, and recently are regarded to be wrong.

It is doubtless that earliest living organisms had simple structure; that more complicate eucaryotes arose from more simply organized procaryotes; that multicellular animals arose from unicellular ones and that ancestors of such complicatedly organized multicellular animals as arthropods or vertebrates were multicellular animals with more simple structure. While initially organisms with complicate structure were absent, they originated as a result of complication. But this in no way allows to conclude that there is a law of complication: the evolution equally can lead to complication and to simplification. When we discuss phylogenetic relations of certain animals, in some of which structure is more complicate, and in others more simple, we can with equal success assume either an origination of the more complicate from the more simple one, or vice versa, origination of the more simple from the more complicate one. Because of this comparison of organisms by their complexity gives nothing for phylogeny reconstruction.

The discussed below opinion about a "law" of oligomerization of homologous organs is an opposition to the "law" about complication of structure.

I.1.1.4. Oligomerization of homologous organs

Many modern phylogenetic theories, including that in entomology, are based on an assumption that evolution is supplied with the oligomerization (i.e. reduction in number) of these or that organs initially existed in greater number. For example, it is assumed that arthropods initially had large number of equal leg pairs and subsequently the number of leg pairs was reduced (see III.B-1.2: Classifications of Euarthropoda); winged insects initially had two pairs of equal wings, and subsequently only one pair was retained (see Charter VI); and so on. Most of these theories are reliable as are argued on the base of cladistic analysis (see below).

But it would be necessary to warn against absolutization of the idea about oligomerization of homologous organs. When somebody discuss phylogeny of a selected group of animals, he or she tries to chose the most simple explanation among all possible ones. If one animal has some organ, and another animal has no such organ, it is easier to assume that reduction of this organ took place in the second animal, than to suggest a hypothesis which would explain origin of this organ in the first animal. In such manner, going by the way of easiest explanation, some authors try to restrict all evolutionary changes to reductions.

For example in the papers by Kukalova-Peck (1983, 1985, 1987, 1991) a theory of phylogeny of winged insects (Pterygota) is suggested, according to which the hypothetical ancestral form is supposed as a conglomeration of useless details of structure, bristled up by various appendages: nearly all details of structure found in any really existent insects are moved in a polymerized condition to this reconstruction of the ancestor. Each its segment besides a pair of limbs, bears both eyes and wings; each limb has maximum number of segments and claws known for any limb of any insect; each segment bears a stylus, each stylus is segmented, and so on. Reasoning in such manner the author liberates herself from necessity to explain appearance of new organs in evolution: In this case somebody other have to work out the questions about emergence of details peculiar for Pterygota somebody who reconstructs earlier stages of arthropod evolution.

However, it is clear that if animals have these or that organs, origin of these organs should be explained.

In some cases a phylogenetic hypothesis based on opinion about oligomerization of homologous organs appears to be wrong.


In the past many authors regarded representatives of Myriapoda with large undetermined number of segments as primitive forms. Particularly, it was assumed that Epimorpha are the most primitive among centipedes (Chilopoda) because they have large varying number of segments (see IV-1.2: Classifications of Chilopoda I). However from position of cladistic analysis this version seems to be wrong and we have to accept a hypothesis according to which increasing of segment number in Chilopoda took place secondarily (see ibid., II). Similarly, according to recently existed theory about phylogeny of millipedes (Diplopoda), a large undetermined segment number which is peculiar for some Helminthomorpha is secondary, while initially millipedes had relatively small and determined segment number (see IV-

Thus, comparison of animals by their oligomerism or polymerism is not enough itself to make conclusion about direction of evolution.

I.1.1.5. Lack of specialization in ancestral forms

Often a species ancestral to a certain taxon is assumed as something non-specialized, combining in itself features of all members of the taxon together. With such point of view is connected a term "generalized" which is sometimes used as combinations "generalized character" or "generalized organism". Actually each animal species has its own peculiarities, its own specialization and evolves under natural selection in particular conditions of medium, but not under an aspiration to produce in future a large and diverse taxon. During its evolution not only specialization takes place, but also a despecialization and a changing of specialization.


As one of numerous examples of disappearance of a primary specialization, evolution of mayfly larvae (Ephemeroptera) can be given: It is convincingly shown that larvae of mayflies initially had a specialization for active swimming [see VI-1.1: Ephemeroptera (10)] which in various groups of mayflies disappeared or changed to specialization for another kind of life; sometimes such larvae, having lost their swimming specialization, look as "generalized" ones as resemble many insects together.

No rise to doubt is given by theories according to which the evolutionary way leading to bees with their social kind of life and feeding by pollen of plants passed through a specialization for parasitism on insects (which is peculiar for primitive Hymenoptera Apocrita); the way leading to tetrapods and human passed through a specialization to swimming (peculiar for fishes); and many others.


Most probable that the unknown for us ancestral species, such as the common ancestor of winged insects, the common ancestor of insects with complete metamorphosis and others, were quite specialized insects, and such fundamental characters of large taxa as wings or complete metamorphosis initially arose as particular adaptations for concrete conditions but not as an inexplicable aspiration to give birth to biological diversity.

I.1.1.6. Evolutionary scenario

Some authors believe that in order to reconstruct phylogeny, it is enough to build an evolutionary scenario, i.e. to describe an assumed way of evolution with description of possible reasons of each evolutionary change. In this case the evolutionary changes are discussed in connection with external conditions on which background these changes took place. For explanation why natural selection acts in certain direction, they use data about changes of climates and landscapes in various geological epochs. Actually each scenario of such kind is only one of many possible hypotheses.


For example some authors suggest scenarios describing a way from annelids to onychophorans to myriapods to insects. In this case they assume that multisetose annelids (Polychaeta) which initially inhabited in see, came out to the land; as a result of adaptation for terrestrial king of life their parapodia protruded by sides were moved to the ventral side of the body and transformed to non-segmented legs peculiar for Onychophora; further evolution of these animals represented a perfection of adaptation for terrestrial life, their legs got segmentation, anterior pairs of limbs were integrated to mouth apparatus, and other peculiarities characteristic for arthropods appeared, as a result of which Onychophora gave rise to Myriapoda and than to insects (Hexapoda). In this scenario no place is found for other arthropods Chelicerata, Eucrustacea and Trilobitomorpha, because of this the apologists of this scenario regard that these animals derived from annelids independently from Myriapoda and Hexapoda. In connection with this they suggest scenarios of independent origin of chelicerates and eucrustaceans from annelids. In this case they assume that eucrustacean limbs, being derived directly from parapodia of Polychaeta, were initially non-segmented (swimming lamelliform appendages fillopodia, peculiar for Branchiopoda), and were segmented during evolution of eucrustaceans. On the base of these evolutionary scenarios, a conclusion about polyphyly of arthropods was made (see III.B-1: Classifications of Gnathopoda II). However, a quite different theory about origin of arthropods exists, according to which arthropods form a holophyletic taxon (see III.B-1.2 Euarthropoda). For this theory an evolutionary another scenario corresponds, which is not less convincing than the previous one: Polychaeta gave rise to Trilobitomorpha, and their limbs became segmented not in connection with terrestrial life, but in connection with inhabitancy on bottom of the sea; segmented limbs, inherited from trilobitomorps, were retained in eucrustaceans (and Branchiopoda secondarily lost segmentation of certain pairs of limbs in connection with transformations of them to swimming appendages); anterior pairs of eucrustacean limbs transformed, becoming a part of mouth apparatus; than follows turning to terrestrial kind of life and appearance of peculiarities characteristic for Myriapoda and Hexapoda. These two hypotheses are mutually exclusive. It is clear that the same insects could not arise by two different ways, and only one of these hypotheses is correct. The both hypotheses seem to be verisimilar when we are limited by building of evolutionary scenarios only. Besides these two ones, many other mutually exclusive theories about origin of insects were suggested, in each of them each selected step looks as verisimilar.


Evolutionary scenario is not enough to prove a phylogenetic hypothesis. An argued opinion about phylogeny can be got only with help of cladistic analysis, i.e. a reconstruction of phylogeny on the base of apomorphies.


All discussed above (in the division I.1.1) opinions about directions and modes of evolution are based on already existed in literature conclusions about animal phylogeny, but do not serve as bases fore these conclusions. The conclusions about these or that phylogenetic connection and a general conclusion about origin of diverse animals from a common ancestor, are made on the base of the fact that diverse animals have common characters which occurrence can not be explained neither by occasional coincidence, nor by any universal laws of biology. Presence of common characters of such kind testifies that these characters were inherited by their owners from their common ancestor. The fact that various animals which have a common character, differ one from another by other characters, testifies that at least some of these animals differ from the named common ancestor as well; thus, we come to the conclusion about existence of evolutionary changes. In order to clarify which concretely evolutionary changes and in which succession took place, it is necessary to analyze distribution of common characters in these animals. Such analysis is named phylogenetic, or cladistic analysis (from Greek cladus branch), because it clarifies out concrete branches of phylogenetic tree. Here under the word combination "cladistic analysis" we understand a scientific cladistic analysis based on scientific hypotheses; besides it, a numerical cladistic analysis exists, in which scientific hypotheses are premeditatedly ignored (see I.1.3 below).

As a founder of the cladistic analysis, German entomologist Willi Hennig (19131976) is regarded, who in 1950 published a book "Grundzuge einer Theorie der phylogenetischen Systematik" (English translation of this book "Phylogenetic systematics", which became most known appeared in U.S.A. in 1966). However, the same methods which were described by Hennig, were used by biologists not less than one and half century before him. For example, even in the supplement to the first volume of "Philosophy of Zoology" by J.B. Lamarck, a phylogenetic scheme is given which could be built only with usage of the principles of cladistic analysis (while comments to this scheme are extremely short and do not explain which methods were used by the author). Especially many works on phylogeny appeared beginning from the second half of XIX century, when thanks to works by Ch. Darwin the evolutionary approach got a general recognition. In the majority of works phylogenetic reconstructions are in fact based on cladistic analysis, however they usually lack necessary comments about methods of phylogeny reconstruction. Many authors of phylogenetic reconstructions clearly understood the logic of cladistic analysis; other authors using this logic intuitively, by words explained principles of phylogeny reconstruction in some other manner. Because of absence of distinct formula of principles of phylogeny analysis, some people who read papers on phylogeny, got a wrong impression that the phylogenetic schemes are drawn arbitrarily. Thus the Hennig's desert is not a discovering of a new method, but is that he was the first who clearly and consistently described methods of phylogeny reconstruction and it were his publications which made subsequent authors to formulate clearly the principles which they use for their phylogenetic reconstructions.

One of the Hennig's deserts is also that he introduced several important terms.

I.1.2.1. Terms "apomorphy" and "plesiomorphy"

The term apomorphy (from Greek apo - from), or apomorphic character means a progressive (in the original meaning of this word, from Latin pro- forward and gressus movement), i.e. a derived, or secondary character. The term plesiomorphy (from Greek plhsioz near), or plesiomorphic character means a primitive (in the original meaning of this word, from Latin primus, primitivus first, primary) character (named also original, ancestral, or generalized character). Plesiomorphy and apomorphy are two alternative conditions of a character, correspondingly before and after an evolutionary change happened with this character. These two categories are relative ones: if evolution goes from condition A to B and than to C, the condition B is an apomorphy in relation to A and a plesiomorphy in relation to C.

The terms "progressive" and "primitive" are often connected with categories "more perfect" and "less perfect", or "more composite" and "more simple", that can lead to confusion. The terms "apomorphy" and "plesiomorphy" have only one meaning a direction of evolutionary change of a concrete character in a concrete case, that makes these terms more strict.

An apomorphy which differs a certain taxon from all others is called its autapomorphy, and an apomorphy common for several taxa is called their synapomorphy. A plesiomorphy common for several taxa is called their symplesiomorphy.

I.1.2.2. Apomorphy-based phylogenetic analysis

The cladistic analysis is based on an assumption that evolutionary changes are unique and irreversible (that is known as a Dollo's law). This assumption is based on consideration of probability: A probability that the same change in genotype can appear several times independently is so little, that it can be regarded to be equal to zero. Thus it is assumed that each apomorphy originated only once and was inherited by all descendants of the species in which it had appeared. Consequently, if different species have a same apomorphic character (i.e. a synapomorphy), this testifies that all these species have inherited this character from their common ancestor, and therefore these species form an integral phylogenetic branch. In order to prove that several taxa form a common phylogenetic branch, it is necessary to find out at least one their synapomorphy, which at the same time would be an autapomorphy for all this branch. Thus in the Fig. 1 an apomorphy 3 is a synapomorphy of taxa c and d, and an autapomorphy of a branch c-d. Presence of this apomorphy proves that the branch c-d really exists and was originated from a common hypothetical ancestor e, in which this apomorphy 3 had appeared. At the same time a presence of symplesiomorphies of several taxa does not testify that these taxa have a common ancestor different from ancestors of other taxa. For instance in the same Fig. 1, a plesiomorphy 3 does not allow to say anything about phylogenetic relations of the taxa a and b besides the fact that these taxa do not belong to the branch c-d. A symplesiomorphy 2 belongs to the phylogenetic branch b-c-d, but solely an existence of this symplesiomorphy would be not enough to prove real existence of this phylogenetic branch (the reality of this branch is proved by the apomorphy 1).

Fig. 1. 
Reconstructing of phylogenetic relationships of four taxa (a, b, c and d) basing on analysis of six characters (1-6)
e and f - hypothetical ancestral taxa; white square - plesiomorphy; black square - apomorphy; shading - synapomorphy.   

A phylogenetic tree built on the base of cladistic analysis is called cladogram; in difference to some other kinds of phylogenetic trees, the cladogram contains only information about succession of branching, but does not contain information about absolute time of branching, degree of difference between branches and number of species in each branch.

Here is important to note that for reconstruction of a tree which branches represent supra-species taxa, it is necessary not only to find out synapomorphies between the branches, but also find autapomorphies of each branch. If for a certain taxon composed of more than one species, no one autapomorphy is find, we have no reason to figure this taxon in a form of phylogenetic branch. In the example in Fig. 1, for reconstruction of phylogenetic relations of four taxa (a, b, c and d) not less than six apomorphies are necessary.

Now let us return to the example discussed in the division I.1.1.6. We saw, that for explanation of insect origin various mutually exclusive evolutionary scenarios can be suggested. To the scenario about evolution Polychaeta Onychophora Myriapoda Hexapoda, following cladogram corresponds:

And to the scenario about evolution Polychaeta Trilobitomorpha Eucrustacea Myriapoda Hexapoda, following cladogram corresponds:

 As a proof of the first of these theories and at the same time disproof of the second one, could be synapomorphies of the taxa Onychophora, Myriapoda and Hexapoda, which in this case would be autapomorphies of the taxon Ceratophora, as in the first theory the taxon Ceratophora is regarded as a separate phylogenetic branch. In the second theory, Onychophora are separated from the common ancestor of Myriapoda and Hexapoda earlier than it is separated from Eucrustacea, Trilobitomorpha and Chelicerata.

Hence, according to the second theory, Onychophora, Myriapoda and Hexapoda can not have such synapomorphies which would be absent in Eucrustacea, Trilobitomorpha and Chelicerata. Actually synapomorphies of Onychophora, Myriapoda and Hexapoda (i.e. autapomorphies of Ceratophora) are not found yet.

A proof of the second of these theories and at the same time a disproof of the first one, are firstly, autapomorphies of Gnathopoda; secondly, synapomorphies of Chelicerata, Trilobitomorpha, Eucrustacea, Myriapoda and Hexapoda i.e. autapomorphies of Euarthropoda; thirdly, synapomorphies of Eucrustacea, Myriapoda and Hexapoda i.e. autapomorphies of Mandibulata. These complexes of apomorphies really exist (see III.B-1, III.B-1.2 and III.B-1.2.3 below): autapomorphies of Gnathopoda are the reduction of coelom and opening of the blood system; autapomorphies of Euarthropoda are the presence of chitin-containing cuticle, segmented limbs and faceted eyes (which are present if not in all, but at least in most primitive representatives of Chelicerata, Trilobitomorpha, Eucrustacea, Myriapida and Hexapoda); autapomorphies of Mandibulata are peculiar structure of mouth apparatus and faceted eyes (both being retained not in all but only in selected representatives of Eucrustacea and Hexapoda). The theory about holophyly of arthropods (the second cladogram) explain all these cases of existence of common characters in diverse animals as a result of their inheritance from their common ancestor i.e. as synapomorphies. As for the theory about polyphyly of arthropods (the first cladogram), their the existence of phylogenetic branches corresponding to the taxa Gnathopoda, Euarthropoda, and Mandibulata is not recognized, and hence, it is assumed that their common characters are not synapomorphies. Because of this, apologists of the theory about polyphyly of arthropods tried to explain the presence of these characters in diverse animals as a convergention, which appeared as a result of adaptation to similar conditions of life. These explanations seem to be rather strained, as it is difficult to decide which similar conditions can be present in deep-sea and on land surface where arthropods inhabit, as well as it is difficult to decide which adaptive significance can have such features as the loss of coelom or the presence of an additional light-refracting lens in each ommatidium of faceted eye. Thus, when the two mutually exclusive theories are discussed, the cladistic analysis allows to make choice in favour of the theory about holophyly of arthropods: this theory is grounded by concrete apomorphies (while they are disputed by opponents), and the alternative theory about polyphyly of arthropods is not grounded at all.

I.1.2.3. Identifying character polarity

In order to clarify polarity of a character (i.e. in order to know out, which condition of the character is a plesiomorphy and which is an apomorphy), is used an out group criterion, which means the following. Let's assume that we have already proved existence of a certain phylogenetic branch (for example the branch (b-c-d in Fig. 1) and regard this branch as a systematic group. In order to clarify phylogenetic relations inside this group we shall use characters which differ in different representatives of this group (in the branch b-c-d these are characters 36). In this case that conditions of the characters would be plesiomorphic, which besides this group are found also somewhere outside of it (in our example in the branch a), and that conditions would be apomorphic, which are not found anywhere outside this group. This principle is based on the fact that an apomorphy which appeared in a certain phylogenetic branch inside the given group, is a unique one, being limited in its distribution by this branch only, and because of this can not be found outside this group.


For example, in limits of the taxon Hexapoda, in some representatives antennae have muscles in the first segment only, in other in all segments except for the last one. If we would ground only on the antenna structure itself, we would have endless and useless discussion what is primary and what is secondary here more simple antennae with limited mobility, or more complicate antennae able for various movements. None reasoning that evolution should be obligatory directed somewhere toward complication or, vice versa, toward simplification; toward increasing of adaptation to the medium or, vice versa, toward diminishing of adaptation; toward more perspective for further evolution or, vice versa, toward an evolutionary deadlock such reasoning in this case do not give correct answer, because actually evolutionary changes can be infinitely variable and can have any direction. The principle of out-group comparison gives a unique answer on a question about primary structure of antennae in Hexapoda: the plesiomorphy (i.e. the primary condition) is the presence of muscles in all antenna segments except for the last one, and the apomorphy the presence of muscles in the first segment only: out of the taxon Hexapoda, multisegmented antennae with muscles in the first segment only are not found, while antennae with muscles in many segments are found in various groups of arthropods. From here the conclusion comes, that the structure of antennae with muscles in the first segment only, is an apomorphy which appeared in limits of the taxon Hexapoda, hence this character is considered to be an autapomorphy of a subordinate taxon Amyocerata inside Hexapoda (see division V-1.2).

In fact, the principle of out-group comparison is the only argued way to determine a polarity of character, i.e. a direction of evolution (about attempts to use other methods see I.1.1 above). At the same time usage of this principle is possible only when a natural group is outlined and when it is proved that this group represents an integral phylogenetic branch. Particularly, the reasoning given above would be impossible if there would not be proved that Hexapoda represents an integral phylogenetic branch. This also could be done only with help of cladistic analysis, using the principle of out-group. Thus, the process of cladistic analysis has no beginning. Somebody believes that having objective facts about animal characters and correctly using rules of the cladistic analysis, we can reconstruct step by step a phylogenetic tree, not making mistakes; actually this is impossible. The reconstruction of phylogeny with help of cladistic analysis is an endless process of search of non-conflicting theory and correction of previously done mistakes. As it is shown below, the characters on which reconstruction of phylogeny is grounded, are also not initial data, but themselves depend upon results of the phylogenetic reconstruction.

I.1.2.4. Interdependency of phylogenetic theory and character definition

In superficial view, it looks that animal characters are objectively existing and investigators only have to ascertain them. However actually in nature characters themselves are absent, and only structure of organisms exist. The characters are formulated by us on the base of the objectively existent organism structure, which is refracted by our perception. And our perception of the characters depend on our opinion about phylogeny of these animals.

For example, on such easy question for young pupils, how much legs have a beetle and a spider, no a single correct answer can be given (Table 1).

Table 1. 
Leg number of some arthropods counted by different methods (colour indicates taxa: abandoned taxon  Crustacea auct. and recently accepted taxon Chelicerata




larval Diplopoda

larval Acari




Method 1: number of pairs







Method 2:
legs on postoral segments No:




























































































If we would simply count walking legs, we would get numbers shown in the line "method 1"; this is only on of many possible methods of counting: here, for example, claws of cancers (Decapoda) are counted as legs, while the similar claws of scorpions (i.e. pedipalps of Arachnida) are not counted. Making such counting we can find similarity of Hexapoda, larvae of Diplopoda and larvae of Acari (each with 3 pairs of legs) and similarity between Xiphosura and Decapoda (each with 5 pairs of legs); in many classifications of XIX century, both Xiphosura and Decapoda were placed to a taxon Crustacea.

Recently this character is formulated in another way ("method 2"). When it is formulated in such manner, it appears that Xiphosura and Decapoda have nothing common, as well as larvae of Acari and Hexapoda; but a synapomorphy between Arachnida and Xiphosura is found, on which base they are united to a taxon Chelicerata (see III.B- There is also a characters common for Hexapoda and Diplopoda, that can be interpreted as a synapomorphy. The interpretation of leg position which is shown here, is based on a number of assumptions accepted by us: (A) We regard that in arthropods legs of each determined pair are connected with a determined segment; with this statement some recent paleontologists do not agree; this statement is correct particularly for arthropods, but is not correct for other groups, for example for vertebrates. (B) We regard that the segment homologous in all arthropods is the segment situated directly behind mouth opening (i.e. the first postoral segment), and that for most certain this segment can be determined by its innervation [see III.A- Articulata (2)]; not all zoologists agree with this statement: according to opinion by some authors, the innervation is not a reliable indication of the first postoral segment; according to opinion by others the mouth opening can change its position during the evolution, thus the first postoral segment is not homologous in various arthropods (see ibid.). (C) We count segments beginning from the anterior one and believe that in this case segments with equal numbers have to be homologous; at the same time it is evident that if count segments from the posterior end of the body, it would be not so. However, in some cases the opposite situation is found: for example, in all Chilopoda two last segments (the genital and the pregenital ones) are surely homologous; at the same time number of trunk segments in different chilopods is variable, thus if count the genital and the pregenital segments from the front, their numbers will differ in different species and even specimens. (D) We regard that the appendages of the first postoral segment of Hexapoda and Diplopoda have disappeared secondarily [see III.B- Atelocerata (1)]; but according to opinion by some authors, this segment was never present, and in this case all scheme should be different. (E) We regard that the gnathochilarium of Diplopoda is formed by appendages of the 3rd postoral segment only, and the collum represents the 4th segment and lack appendages; this is also a subject of discussion [see IV-1.1.2: Collifera (1)-(2)]. (F) We regard that the three pairs of legs are homologous in all Diplopoda; this is not evident at all, and had demanded special proofs [see IV- Diplopoda (6)]. (G) We regard that in Diplopoda the segments marked here as V-VII are haplosegments; possibly this is wrong [see ibid.]. (H) We regard that the same segments of Hexapoda are haplosegments, but there are also doubts about this [see V-1: Hexapoda (1)]. (I) We regard that the superlinguae of Hexapoda are not limbs, as some authors believed [see III.B-1.2.3: Mandibulata (3)].

This list of assumptions an disagreements can be continued. Dependently which of the assumptions we accept and which do not, we find or do not find in these or that taxa common characters, and synapomorphies among them. Thus, the result of cladistic analysis depends on these assumptions. Particularly, upon this depends an opinion about the discussional taxon Labiata. At the same tame each of these assumptions is made on the base of a certain opinion about phylogeny: for example, the first assumption (A) is based on the ideas about phylogenetic integrity of Articulata and of integrity of each of the arthropod group which has a certain set of limbs. These ideas about phylogeny themselves are based on the cladistic analysis of some other characters, which interpretation is also not evident and depends on ideas about phylogeny, and so on, endlessly.


In this simple example we see that an idea about phylogeny depends on formulation of characters, and the formulation of characters depend on idea about phylogeny. In connection with this, a danger exists to fall into a vicious circle of argumentation, when a wrong idea on phylogeny and a wrong formulation of characters would prove one another. A vicious circle appears when a set of arguments is limited, thus in order to broke the vicious circle, it is necessary to introduce new arguments. In phylogenetic reconstruction new arguments can be introduces by two ways: (1) to expand a set of species examined; this can be done if found out new species previously unknown to science (for many groups of insects this is a quite realistic way of investigation, as annually thousands of new species are discovered); (2) to expand a set of characters analyzed; this can be reached if use mew methods and if examine such structures which previously were poorly examined or not examined.

I.1.2.5. Independently evolving homologous characters

The most difficulties in phylogenetic reconstruction are caused by existence of independently appeared homologous characters (that is known as a law of homologous rows by N.I. Vavilov).

Besides synapomorphies (i.e. characters inherited from the common ancestor), there are characters which are also homologous but independently appeared. This is connected with the fact that animals can inherit from their common ancestor not a phenotypic character, but only genes which encode it; in further evolutionary changes such hidden character can appear in phenotype, and this can take place in different phylogenetic branches. Such independently appeared homologous character can be wrongly taken for a synapomorphy, and in this case the phylogenetic analysis can lead to wrong results. The more close is relation of the taxa, the more probable is that they contain hidden homologous characters. If each of the analyzed taxa contains many various species, we can hope that the hidden characters common for each of these taxa would be exposed in phenotype of some of its species. In this case the independently appeared characters are founded out and we can exclude them from the cladistic analysis. Because of this, the higher is rank and the larger is volume (i.e. number of species) of each of the taxa included in the cladistic analysis, the more reliable is the result of this analysis. The less reliable is cladistic analysis for taxa of the species rank (the lowest rank for which the cladistic analysis is possible theoretically).

I.1.2.6. Field of application of the cladistic analysis

For the cladistic analysis, can be and should be used all heritable characters of organisms anatomical, histological, cytological, biochemical, physiological, ethological and others, belonging to any stages of ontogenesis.

The cladistic analysis in its classical form is intended for reconstruction of phylogeny based on divergentions, when the phylogenetic scheme has a form of hierarchically branching tree, but not of a net. However, in some cases to usual evolutionary mechanisms of divergention can be admixed such factors as hybridization and transduction, in result of which genes are transferred not only from the common ancestor to its divergenting descendants, but by other ways also, and the phylogenetic tree gets a net-like form.

In principle, the cladistic analysis can be used for taxa which have rank not less than species. Between infra-species taxa reproductive isolation is absent, thus their relations have a form of net but not of a phylogenetic tree, and can not be clarified by the cladistic analysis.

Many authors use the cladistic analysis for reconstruction of phylogenetic relations of selected species. Theoretically this is rightful, but practically probably have little use, because the analysis on species level does not allow to clarify and exclude from the analysis that characters which are able to appear in genotype in hidden condition and to be displayed in phenotype as independently appearing homologous characters (see above).


The distinct formulating of the principles of cladistic analysis provoked in many investigators a temptation to formalize the process of phylogenetic reconstruction and to use for this purpose a computer. Recently there are compiled computer programs for building phylogenetic trees on the base of previously founded apomorphies. However the reconstruction of phylogeny by the formalized methods runs against insuperable difficulties and is scarcely expedient. As a passion for these methods is widely distributed recently, here it is necessary to look into their shortcomings of principle.

I.1.3.1. Unavailability of parsimony principle

For computer building of phylogenetic tree, at first a matrix taxon/character is compiled, where for each elementary taxon (called operational unit) included to the analysis, a state of character is marked and it is indicated if this state is an apomorphy or a plesiomorphy. Than on the base of this matrix a phylogenetic tree is built; the program which builds a tree is based on the same principle as the scientific cladistic analysis (see I.1.2 above) i.e. on an assumption that the same apomorphic conditions of characters are synapomorphies, i.e. are inherited from the common ancestor. If everything would be limited by this, the usage of computer program would be evidently superfluous, because it is easier to make all these procedures by hands than by computer. But during building of tree on the base of matrix taxon/character, often non-solvable combinations of characters are found:

Let us assume that

x1 is a plesiomorphy, x2 is an apomorphy; 
y1 is a plesyomrrphy, y2 is an apomorphy
















For such matrix no one phylogenetic tree can be built, as in each case it will appear that some of characters (either x2, or y2) had appeared independently twice. Here we can assume that in some of cases polarity of characters is determined wrongly. If change polarity of any character (for example, x) to an opposite one, on the base of this matrix a non-conflicting tree can be built:

It is possible to use a program in which polarity is not given presumably, but it clarified during building of the phylogenetic tree: as the correct one, is regarded that polarity of characters, which allows to build a non-conflicting tree (for example, as in the previous example). However, the matrix can contain such combination of characters, which do not allow to build a non-conflicting tree independently of assumption about polarity of characters:


Let us assume that

one of the characters - x1 or x - is an apomorphy, and another is a plesiomorphy;

one of the characters - y1 or y2 - is an apomorphy, and another is a plesiomorphy



















Theoretically, such non-solvable combinations of characters can not exist, but practically they are found very often. Reasons of existence of such combinations can be various: (1) As a result of convergention, during adaptations to the same conditions, organisms can get similar characters which looks as identical ones but have different genetic bases; such cases cause difficulties not only for computer methods, but for any method of phylogeny analysis. (2) Identical characters which have the same genetical basis, can appear independently in related groups (see I.1.2.5 above). (3) The seeming synapomorphy can be actually an artificial character i.e. several completely different characters which are equally worded. While the first two reasons bring difficulties for any kind of cladistic analysis, the last of these reasons is characteristic specially for formalized computer methods: investigator can distinguish an evidently artificial character from a real one, but computer which deals not with characters but with their wordings only, is unable to do it. When a non-solvable combination of characters is found, it is necessary to reexamine these characters of the investigated animals, in order to find a mistake. But the aim of computer programs is to avoid reexamination of animals, and to dissolve a problem by purely computer methods.

In order to circumvent the appeared obstacle, an assumption is introduced that evolution can be reversible and (or) repeatable. If not to add here other supplementary conditions, the cladistic analysis loses any meaning and the computer builds many possible trees. In order to select among many trees a single one, a principle is used which is called a principle of economy, or parsimony. This principle is following: among all trees, such a single tree is selected, in which the violation of principle of irreversibility and (or) non-recapitulation takes place the less number of times. The principle of parsimony has no relation to any biological theories. A "most parsimonious" phylogeny selected with help of this principle, hardly can be regarded to be the most correct one, as we do not know a natural mechanism which would make animals to chose the most parsimonious (or optimal in any respect) way of evolution. Phylogeny of every group of animals is unique, the evolutionary processes which took place, happened only once, and because of this no one mechanism of optimization of evolutionary process could exist.

In order to justify the principle of parsimony, they say that this principle (known also as Okkam's Razor) is generally accepted in science. But actually the principle which is called parsimony in the numerical cladistics, basically differs from the generally accepted principle of parsimony in the science. In science, the principle of parsimony is connected with the principle that theories are disprovable. No one universal scientific theory (i.e., a theory explaining an infinitude of particular facts) can be proved, because in order to do this it would be necessary to examine an endless number of facts; but it can be disproved, if one would find a single fact which contradicts to this theory. Because of this, a valuable scientific hypothesis should be disprovable i.e., should be formulated in such a manner that it would be principally possible to assume existence of facts which would contradict to this hypothesis. Until these facts are actually not found, the hypothesis is regarded to be correct (more exactly, not disproved). In order to explain the same fact, endlessly much mutually exclusive theories can be suggested, and in order to disprove each of them, a time should be spent. In order to regulate this process, as a working hypothesis is chosen the most parsimonious (i.e. the most simple) hypothesis among all possible ones; if this hypothesis is disproved, the most parsimonious among the rest ones is chosen. Here the principle of parsimony is served not as a criterion of truth, but only to chose a succession in which the hypotheses will be verified by help of other criterions. In the numerical cladistics, vice versa, the principle of parsimony is suggested to be used as the only and the final criterion to chose a "true" phylogenetic hypothesis among many hypotheses which are already disproved (!). For detail discussion of this question, see Pesenko (1989).

I.1.3.2. Fundamental shortcoming of numerical taxonomy

Usage of numerical (particularly computer) methods of phylogeny reconstruction lack meaning by following reasons.

(A) In the methods discussed above, as well as in many others, all calculations are done on the base of arbitrarily compiled "convenient" mathematical formulas; such line of business can not be regarded a scientific one. When mathematics is used in science, a scientific theory is written in a form of mathematical formula, and it is this formula which is used for calculation in the cases foreseen by this theory. At the same time the theory which is reflected in the formula, is a subject of scientific discussions, because it is formulated in such a manner that allows possibility to its disprovement.

(B) In the majority of recently used mathematical methods for phylogeny reconstruction, is used such a quantity as number of characters a number of common or diverse characters of several taxa, a number of apomorphies or plesiomorphies, et al. Phylogenetic tree built by these methods depends upon number of synapomorphies in these or that taxa. Here we should pay attention to the fact that in contrast to the numerical, in the scientific cladistic analysis (see I.1.2) number of synapomorphies have no any influence to the result: there a phylogenetic branch is regarded to be existent one if its members have at least one synapomorphy; a larger number of synapomorphies gives more confidence in correctness of the tree reconstruction, but does not influence to the tree itself. The characters number used in numerical methods, is a category meaningless from biological point of view, because the same character can be formulated in various manners, being short or long.

For example, the character of the taxon Entognatha, described in the division V-1.1 under the number "(1)" can be written as a single word: "enthognathy". The same character can be written more long (see ibid.). A long text can be made as a single compound sentence, and in this case it will be regarded as a single character. We can make the same text as many sentences, and give separate number to each of them, in this case all of them will be regarded as different characters. In the same manner the character of the taxon Pleomerentoma, connected with abdominal limb structure (see V-1: Classifications of Hexapoda III) can be written as one or several sentences and can be regarded as one or several characters. If in the characteristic of Entognatha number of sentences (i.e. counted "characters") will be more than in the characteristics of Pleomerentoma, the following phylogenetic scheme will appear: 

          +--Ellipura  |
       +--|            | Entognatha
 ------|  +--Diplura   |

If vice versa, quite different conclusion about phylogeny will appear: 

 -----|  +--Diplura    |
      +--|             | Pleomerentoma
         +--Amyocerata |                      

Even if we will take absolutely simple objects, for example a square and a ring, we would not be able to answer the question how many differences are between them, i.e. how many peculiar characters each of them has.

It is possible to say that there is a single difference the first object is a square, and the second is a ring. It is possible to say that there are 4 differences in the first case fore angles, in the second no one; with the same success we can count both angles and sides, curvation of sides and so on. Actually each geometrical object includes endless number of points and because of this is described by an equation which has endless number of values.

Structure of an organism is determined by genes; number of genes in a multicellular organism is very large and recently can not be calculated, but it is not endless. We take into account not genes, but characters; in contrast to genes, number of character is not large, but endless.

In numerical methods, arithmetical actions are done with numbers of characters division, subtraction, et al. Actually numbers of characters in all cases are equal to infinity, and only a person who does not understand what does he do, can make arithmetical actions upon them.

Everything said here about calculation of characters, is related not only to numerical methods in phylogeny

reconstruction, but to any other numerical methods in systematics. In 60th years of XX century there was such current as phenetics, or numerical systematics, which disclaimed meaning of phylogeny in systematics and suggested to base classification on an "objective" calculations. The modern methods of numerical cladistics are in great part adopted from phenetics. (In detail see Mair, 1969; Pesenko, 19891991).

(C) Independently if inadequate mathematical methods are used (see A) or not, if an artificial character calculation is used (see B) or not, usage of computer for phylogenetic reconstruction can not bring a desired result.

All advantage of computer is that it can quickly make a long chain of logical operations; if it is necessary to make one after another only a few logical operations, it is easier to do this mentally. If it would be possible to divide the process of phylogeny reconstruction to two successive steps on the first step to make a description (i.e. formulating) of characters, and on the second step reconstruction of phylogeny on the base of these formalized characters, this second step could be done with help of computer. However formulating of characters is possible only if we have an idea about phylogeny of the investigated group, because each description of animal already contains the authors opinion about nature of this animal or of this character, and because of this can not be objective (see I.1.2.4 above). Thus, all work on phylogeny reconstruction should be done simultaneously, without division to steps. It means that after a short chain of logical operations (so short that it can be easily done mentally) it is necessary to return to reexamination of animals in order to specify or correct the formulation of characters.


For example, if we got two conflicting trees shown above, it is useless to continue logical operations with the existent formulation of characters of Entognatha and Pleomerentoma. Instead of this, it is necessary to dissect heads of various representatives of Entognatha, compare them with other insects, and taking into account the appeared doubt about common origin of Entognatha, anew describe structure of their mouth apparatus. The same should be done with abdominal appendages of various insects, in order to revalue critically the characters of Pleomerentoma.

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