392 



INSECTA. 



It should he mentioned that the boundary between the thoracic and abdominal 

 regions is, in many Insect larvae, less sharply marked. This is connected with 

 the fact that, in larvae, the thorax is frequently of less significance for the 

 locomotion of the whole body than in the imagines, either because locomotory 

 organs develop on the abdomen also (e.g., in caterpillars), or that such organs 

 are altogether wanting on the thorax as well (maggot-shaped larvae). More 

 careful examination, especially of the inner organs, will, however, reveal in 



these cases also important differences between the 

 thoracic and the abdominal segments. As we find 

 that the separation of the thorax from the abdomen 

 is very marked in the Thysanura, we may regard it 

 as a feature inherited long ago by the Insect phylum, 

 and may consider the apparent obliteration of these 

 boundaries in certain larval forms as merely a second- 

 ary phenomenon. 



The loss of extremities in the abdominal 

 region is an important feature which dis- 

 tinguishes the Insecta from the Myriopoda. 

 With regard to the derivation of the Insecta 

 from the latter group, or from forms re- 

 sembling the Myriopoda, the fact that the 

 rudiments of abdominal extremities appear 

 in the insect embryo and disappear later is 

 of importance (pp. 296-300). The ventral 

 stylets found on the abdomen in the Thysanura 

 have repeatedly been regarded as vestiges of 

 extremities, and this seems all the more 

 probable as, in Machilis, these stylets actually 

 function as locomotory organs. Recently, 

 however, following Haase (No. 153), and 

 supported by the occurrence of similar 

 stylets on the coxae of the thoracic limbs 

 of Macliilis, and on most of the limbs in 

 Scolopendrella, zoologists have been inclined 

 to regard these appendages merely as coxal spurs (p. 299). On the 

 first abdominal segment of Campodea, on the contrary, there is a 

 true limb-rudiment. 



While, in the Myriopoda, the number of the body-segments varies 

 greatly in the different genera and species, the number seems to be 

 fixed and universally prevalent in the Insecta. The thorax is always 

 composed of three segments, each of which carries a pair of legs 

 (a fact which gave rise to the name of Hexapoda). In the same 

 way it seems to be clearly established by ontogeny that the abdominal 

 region is universally composed of ten trunk-segments and one sub- 



Fic. 193. — Campodea staphy- 

 linus (after Lubbock, from 

 Lang's Text-book). 



