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The classification of lepidopterous larvae

1915; Linguagem: Inglês

10.5962/bhl.title.16772

0073-4748

S. B. Fracker,

Hymenoptera taxonomy and phylogeny

14homotypic with iv of the abdomen, it is shown to be really iii a, a seta which Dyar had either overlooked or considered of no importance.W. T. M. Forbes' work (1910) did not cover the subject of the homotypy of the setae.The few figures he labels include errors for which he was not responsible as he had not given the subject consideration.(See discussion of theta, page 34.) Siltala (1907) made a study of the chitinous armature, especially the setae, of caddice-worms.He finds considerable variation in the order (Trichoptera) but carefully compares the different arrangements.The setae of the first larval stage are very sparse while those of later instars are obviously special and secondary developments.In neither case can satisfactory comparisons be made with the Lepidoptera. THE CHAETOTAXY OF A TYPICAL SEGMENT"While previous workers have drawn so-called type segments, these have never included all the setae found in larvae of this order.Their significance has been limited to particular segments or to particular groups.For example, Dyar's ''typical segment" refers to the abdomen only, altho it has been extended to cover the last two thoracic segments of the larvae of Frenatae.In order to make the following pages clear, attention should be called at this point to the diagram (Fig. 1) which shows all the primary and subprimary setae normally present on the segments of any generalized lepidopterous larva.The evidence on which this diagram is based and also the reasons for the adoption of Greek letters instead of numerals in naming the setae are given on later pages.This hypothetical type is mentioned here as a point of reference in order that it may be possible to use the setae as illustrations of the general principles on which this study of homology is based.On the same and the following plates are shown some of the modifications of the plan.It has been necessary to introduce certain new terms in this paper and in a few cases to use the old terms in a special sense.A glossary will be found at the close of Part Two, giving the meaning of all words used in a special sense in this discussion.The explanation of plates (p.147) gives the names and grouping of all the setae.II. Nature op the EvidenceIt is necessary first to outline the principles underlying attempts at any determination of homotypes.The evidence on which decisions in doubtful cases were based is discussed here, for some of the principles are not axiomatic.Their correctness will not be disputed, I am sure, by those who consider the broad basis of fact on which they rest.15] LEPIDOPTEROUS LARVAE -FRACKER 15 DEFINITIONSAccording to the Standard Dictionary, homology refers either (a) to ' ' the correspondence of a part of one animal with another, determined by agreement in derivation and development from a like primitive origin," or (h) to homotypy, which is 'Hhe correspondence of a part or organ of one region with that of another region in the same animal."Smith, in the "Glossary of Entomology", adds the usual provision that "the organs must be identical in general structure and origin, tho they may have developed in different ways for different purposes.' 'There is an important difference between the homology of crustacean appendages with each other and the homology of the setae in insects.In the former case, work is based on similarity in the fundamental structure and development of the homologous organs ; in the latter, only pos- ition can be considered, as the setae are all similar in structure.For that reason it is necessary to secure a little more accurate definition as a basis of work.With this in view, I suggest the following: Two organs on different segments of the same animal are homotypic, regardless of their positions at the present time, when they have developed from homotypic organs of a generalized ancestor.In a generalized type, two similar organs on different segments are homotypes when they 'bear the same relations to the other orga7is of their respective segments.To show that a prothoracic seta, for example, is homologous with one on the mesothorax, it is necessary to show that at one time the anlagen from which these setae were developed were in similiar positions on their respective segments.On the other hand it is equally true that, when two setae of a specialized form are in similar positions on their respective segments, a demonstration that they diverge farther and farther as we ILLINOIS BIOLOGICAL MONOGRAPHS [16 §tages between radically different conditions.When the tubercles* have been modified to form scoli or verrucae, it often seems impossible to determine whether differences have arisen by coalescence or by the dis- appearance of some of the tubercles.In many cases this is a very real problem, on whose solution depends the correctness of the conception of large groups.Another obstacle is the lack of developmental series.In the case of wings, the tracheae which precede the veins in larval and pupal stages plainly show the relation of the adult venation to the generalized type.The single elementary instar which acts as a guide in the study of the chaetotaxy of caterpillars, however, is sometimes so unlike the mature larva in the arrangement of its setae, that it is of little assistance in inter- preting their homology.The change is usually sudden and occurs at molting.Were the recapitulation more often extended over several molts the problem would be easier.The third difficulty is one that, up to this time, has prevented prog- ress in this field.Apparently a lepidopterous larva has three or more entirely distinct types of arrangement of the setae (Figs.7,8).In only a few species is there any apparent relation between the plan of the pro- thorax and that of the mesothorax, or between the last abdominal segment and any of the others.While this obstacle is not a serious one in classification, it has prevented thus far the determination of a satisfactory no- menclature and therefore of a working basis.Since Wilhelm Miiller said in 1886 that he found "no support for a comparison of the prothoracic setae with those of the following segments ' ', workers seem to have left the prothorax severely alone.They seem not to have realized that there is evidence not found in the Nymphalidae on which Miiller was working.Fortunately there are partial hints and clews to assist in surmount- ing each of these obstacles.Study of varied forms often yields signs of intermediate stages in position.Sometimes the single first stage, our only evidence of the past, unmistakably points the way to an interpre- tation we would not otherwise think of making.Again, the presence of a single unusual seta on a single segment of a generalized form will unite the setal plans of otherwise seriously differing segments.In these ways the gaps are at least partially closed and the problem is taken out of the realm of guesswork and abstraction.*The word, "tubercle", is used in this paper as a general term to indicate the location of a seta, or of a definite group of setae, or a process of the body wall bearing such a group.17