341 



pupal wing may at times be quite devoid of tracheoles, while 

 at other times they may lodge two tracheoles, if not more, 

 seems to show that the nervures have no relation whatever to 

 these respiratory tubes, and that the latter have grown into 

 them merely because they represent a path of diminished 

 resistance to growth. It seems to follow, also, that attempts 

 to arrive at any conclusions as to the phylogeny of families 

 and genera (in the chalcid wasps, at any rate) on the basis 

 of pupal wing structure are fallacious, unless special distinc- 

 tions are drawn between the wing nervures and the tracheoles 

 which they may contain. 



The Abdomen and its Appendages. 



The abdomen of the wasp is built up from the last ten 

 segments of the larva, and in its general features the develop- 

 ment is identical with that of the thorax, i.e., the narrow 

 imaginal discs of each segment grow outwards beneath the 

 cuticle of the larva and assume the general shape of the 

 abdomen of the wasp. The imaginal discs, moreover, are 

 similar in appearance to those of the thorax (fig. 2), being 

 in the form of narrow strips of tissue, running vertically 

 down each segment close behind the spiracle (in those seg- 

 ments which possess one). The imaginal disc of the last seg- 

 ment occupies the whole of its lower lateral regions. 



The general shape assumed by the abdomen is ovoidal; 

 but in this the first two segments do not co-operate ; on the 

 other hand, a remarkable migration takes place here, and the 

 whole of the first abdominal segment, and the upper half of 

 the second, become merged in with the thoracic segments, to 

 form the middle region of the insect, the hymenopteran 

 ''alitrunk," while the lower part of the second abdominal 

 segment forms the petiole, which in the adult wasp connects 

 the ''abdomen" with the alitrunk, and articulates with the 

 upper part of the second segment, with which, therefore, it 

 always remains in fairly close intimacy. 



It is in the larva about twelve hours after defaecation 

 that this process of migration is first indicated. At this stage 

 a horizontal splitting is seen in the second abdominal seg- 

 ment, and shortly after, the first segment, and the upper 

 half of the second, begin to move forwards, while the lower 

 portion of the second retains its position, and eventually form,s 

 the petiole. At the time when the larva moults, these two 

 segments have distinctly left the remainder of the abdomen, 

 and have produced the ''alitrunk" (fig. 7). At this time, 

 also, the abdominal discs have completely encircled the body, 

 and the lower portion of the second abdominal disc has so 

 constricted as to give it the form of the petiole; already in 



