730 TIIK GENERATIVE ORGANS. 



In other words, there are certainly three somites in the 

 pupa, representing \vhat is apparently one somite in the larva. 



The manner in which the eight anterior segments of the 

 abdomen of the pronymph of the Blow-fly originate has been 

 already described, and it is evident that they correspond with 

 the eight obvious abdominal somites of the larva. In the 

 female imago, the external orifice of the genitalia is situated 

 behind the eighth of these somites, which are all represented 

 by complete chitinous rings ; but there is another segment 

 between the genital orifice and the anus which has no repre- 

 sentative in the larva. This somite bears a pair of appendages, 

 and is apparently developed from the hypodermis of the last 

 larval somite at a later period. 



In the male insect, instead of one post-genital segment there 

 are probably three all developed, with the eighth or progenital 

 somite, from the last obvious somite of the larva. 



Kunckel d'Herculais [25] figures and describes three pairs 

 of appendicular discs in the last larval segment of Yolucellu. 

 It is evident, therefore, I think, that the number of somites in 

 the embryo and larva cannot be regarded as equal to the 

 number in the fully-developed imago, and it has long been 

 known that in the Myriapoda the number of somites is in- 

 creased by each ecdysis. The same phenomenon is seen to 

 an even more marked extent in the Crustacea ; so that little 

 surprise need be felt if it also occurs in the Insecta. 



b. Development of the External Generative Organs in Insects 



Generally. 



As the study of the development of the external sexual 

 organs in the Blow-fly is exceedingly difficult, I have had little 

 success in the investigation of the earlier stages in this insect, 

 partly from the fact that these stages occur at a very early 

 period in the pupa, when it is exceedingly difficult to obtain 

 satisfactory sections near the extremity of the abdomen, owing 

 to the density of the cuticular exuviae of the larva ; and partly 



