730 THE GENERATIVE ORGANS. 
In other words, there are certainly three somites in the 
pupa, representing what 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. 
Kiinckel d’Herculais [25] figures and describes three pairs 
of appendicular discs in the last larval segment of Volucella. 
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 exuviz of the larva ; and partly 
