NO. 6 INSECT ABDOMEN — SNODGRASS 63 



rivatives of the segmental appendages ; it gives no positixe evidence as 

 to what part of a primitive Hmb may be preserved in the definitive 

 rudiment, since the latter, whatever it may be, develops directly from 

 the embryonic rudiment, instead of following what we should suppose 

 would be the course of the phylogenetic evolution of the organ. 

 Comparative anatomy is more likely to foster illusions than to lead 

 to definite results, for while certain categories of facts may seem 

 to align themselves satisfactorily in some limited scheme of suggested 

 homology, the plan invariably breaks down when wider generaliza- 

 tions are attempted. The writer, therefore, can offer nothing new on 

 the fundamental morphology of the abdominal appendages of insects 

 that is likely to be generally accepted. Even so, however, it will 

 be sufficiently worth while to bring together the principal facts at 

 present known concerning the anatomy of the various appendicular 

 structures. 



Though the appendages of the insect abdomen are rudimentary in 

 the sense that they do not in any case represent a fully-developed 

 limb, they are in all cases specialized by a structural adaptation to 

 some particular use. The abdominal appendages of most interest to 

 entomologists are those of the genital segments, and if we can dis- 

 cover a means of identifying these organs in the various insect orders, 

 this discovery alone will be of much practical value, and it then be- 

 comes a less consequential matter if we can not fully decide the exact 

 morphological nature of the organs themselves. 



It is not possible, however, to study with profit any modified or 

 specialized appendicular organ without having some concept of the 

 nature of the primitive limb structure from which it has been derived. 

 Since there are current several difterent ideas concerning the funda- 

 mental structure of a primitive arthropod limb, it is therefore neces- 

 sary for a writer to make clear at the outset of a discussion the par- 

 ticular theory from which he proceeds. The following sketch will 

 give briefly the \iew on this subject here taken, and a more extensive 

 discussion at the conclusion of this section will examine the possibil- 

 ities of interpreting the structures of the abdominal appendages of 

 insects according to the terms of the theory adopted, which is essen- 

 tially that of P)r)rner (1921), though with differences in special appli- 

 cations. 



A comparative study of arthropod appendages soon shows that the 

 number of segments in the limbs, the relative size of the segments, 

 and even the segmental musculature are so variable in different 

 arthropod groups that none of these features can be used as a guide 



