342 



the larva fourteen hours after defaecation the petiole is clearly 

 seen. As on the rest of the body, the attainment of adult 

 proportions is rapidly followed by the secretion of a delicate 

 cuticle, after which the larva moults. 



The changes which occur during pupal life in the 

 abdomen are quite parallel to those occurring on the 

 remainder of the body. A certain amount of shrinking 

 takes place some twelve hours after pupation ; bristles form 

 on various parts of the body ; the small sculpturings of the 

 wasp's body are moulded on the soft epithelium of the pupa, 

 and then the whole abdomen undergoes chitinisation. The 

 blackening of the abdomen takes place on the fourth day, 

 soon after that of the head and thorax, i.e., the wasp blackens 

 from before backwaids. 



The two abdominal segments which have become merged 

 into' the alitrunk undergo but slight changes during pupal 

 life. Just as the first thoracic segment grows downwards 

 to form the front wall of the thorax, so the two migrated 

 abdominal segments also grow downwards to form the rear 

 wall of the ''alitrunk." The two processes take place at the 

 same time, and are to be explained, I believe, as the result 

 of a pull, exerted upon them by the contraction of the great 

 longitudinal thoracic muscles, which pass horizontally from 

 the one to the other (see belov/, p. 426). This pulling down- 

 wards results in the more distinct separation of the alitrunk 

 from the rest of the abdomen. The chitin in this region 

 becomes extraordinarily thick, and the whole surface under- 

 goes remarkable sculpturing. In the two-day pupa, the upper 

 part of the second abdominal segment is still clearly visible 

 as a small square segment embedded in the one preceding it ; 

 it is about equal in size to the petiole, with which it articu- 

 lates. As chitinisation advances, however, it becomes more 

 and more difficult to detect. 



That the alitrunk contains the first abdominal segment is, 

 of course, well known. But that the upper portion of the 

 second segment is also incorporated in the alitrunk does not 

 seem to have been recognized hitherto. Thus Sharp (1895) 

 writes: — "The structure of the posterior part of the alitrunk 

 has given rise to an anatomical discussion that has extended 

 over three-quarters of a century, with the result that it is 

 now clear that the posterior part of M^iat appears to be thorax 

 in Hymenoptera is composed of the abdominal segment. Tliis 

 part has been called 'Latreille's segment,' the 'median seg- 

 ment,' and the 'propodeum.' "... "Although the true 

 first segment of the abdomen is detached from its normal 

 position and added to the thorax, yet the term abdomen is 

 conveniently restricted to the part that commences with the 

 true second segment" (part 1, p. 492). 



