NO. 6 INSECT ABDOMEN SNODGRASS 5 



immediately below it (P) corresponds with the region of the subcoxal 

 pleural plates of the thorax (Sex). The pleuro-ventral line of the 

 abdomen is also frequently indicated by the presence of a groove or 

 other structural feature, as in certain Thysanura and in some holo- 

 metabolous larvae. Where abdominal limb rudiments are present, their 

 bases always occupy the areas between the folds or grooves marking 

 the dorso-pleural and pleuro-ventral lines ; but in most adult insects 

 in which the abdominal appendages have been entirely suppressed, 

 the limb base areas are continuously sclerotized with the primitive 

 sterna in the definitive sternal plates, and, unless styli or other repre- 

 sentatives of the free parts of the appendages are present, there is 

 usually no indication in the definitive structure as to the primitive 

 position of the pleuro-ventral line. 



The dorsum of an abdominal segment may be occupied by a single 

 tergal plate, which in some insects encloses the spiracles laterally, 

 though the spiracles often lie in membranous paratergal areas ; or 

 the sclerotization of the dorsum may consist of a principal dorsal 

 sclerite, and of one or more lateral paratergal sclerites. The abdominal 

 pleural areas contain the appendage bases, where appendages are 

 present, but the limb bases are commonly reduced to plates or lobes 

 forming a part of the body wall, or they are entirely suppressed as 

 individual structures, and their areas then become merely a part of 

 the general pleural integument. In the last case the limb base area may 

 be entirely membranous, or it may contain one or more pleural scler- 

 ites ; when fully sclerotized, it is usually continuous with the primitive 

 sternal sclerotization in a pleurosternal definitive sternum. The venter 

 is the lower surface between the bases of the appendages, and con- 

 tains the primitive sternal plates. The latter may retain their individu- 

 ality, but, as just noted, the primitive sternum of each abdominal 

 segment is more commonly united with the pleural sclerotizations. The 

 definitive sternal plate thus formed, finally, may be still further en- 

 larged by a fusion with paratergal plates on each side of the dorsum. 

 It becomes evident, therefore, that the sclerotization of the abdomen 

 is not necessarily an index to the primary regional divisions of the 

 abdominal surface. 



The abdominal appendages. — The appendages of the insect abdomen 

 fall into three principal groups, namely, those of the pregenital, or 

 visceral, segments, those of the genital segments (or segment), and 

 those of the postgenital segments. The pregenital appendages take on 

 such a variety of shapes and functions that it is impossible to give them 

 collectively a distinctive name. The genital appendages are the gono- 

 pods. The postgenital appendages are the pygopods on the tenth 



