THE EXTERNAL GENERATIVE ORGANS. 729 
organs form either the ovipositor or the sting (aculeus). The 
ovipositor in the Orthoptera has a precisely similar construction 
and consists of four gonapophyses, those of the pro- and meso- 
genital somites ; in the female Locusts the meso- and meta- 
genital somites exist as distinct annuli, and there is usually a 
pair of jointed cerci articulated between the tergal and ventral 
plates of the meta-genital annulus; sometimes a second pair 
spring from the anal segment. 
From the examination of the skeletal structures of a great 
number of Insects, I have arrived at the conclusion that 
theoretically there are three genital somites, one in front of, and 
two behind, the external genital orifice, either or all of which 
may exist as distinct annuli in the perfect form; that each of 
these somites may have a pair of ventral appendages, the gona- 
pophyses and cerci; that in many cases when the annuli are 
undeveloped their gonapophyses remain, and that there is 
always a more or less developed anal somite behind the 
three genital somites, which frequently bears a pair of distinct 
appendages. 
Development of the Abdominal Somites—It is a very well- 
established fact that the number of abdominal somites present 
in an embryo Insect is from eight to eleven, a number which 
is too small to account for the formation of the four genital 
somites which the theory I have advanced requires; but it 
is very generally admitted that the posterior abdominal somite 
of the embryo is a complex of two or more somites. 
Poulton [853] says, speaking of the number of abdominal 
somites in the larva of the Lepidoptera, which is generally held 
to be ten: ‘There is no difficulty about the seven anterior 
abdominal segments, each of which bears a spiracle. Behind 
the seventh, however, there is a somewhat confused mass of 
segments, bearing a single spiracle on its anterior part; this 
spiracle is usually larger than those on the other abdominal 
segments. This confused mass is sometimes described as a 
single segment, and sometimes as two; a careful comparison 
with the pupa proves that it is certainly made up of three 
segments.’ 
