THE EXTERNAL GENERA TIVE 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 [353] 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.' 



