i8 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 8^ 



Owing to the uniformity in the relation of the genital apertures to 

 the eighth and ninth abdominal segments, these segments in the 

 majority of insects become specifically the genital segments. Their 

 appendages form the principal organs of egg-laying and copulation, 

 and may therefore be designated gonopods. In some of the simpler 

 insects the gonopods are lacking and the genital segments have no 

 distinctive external features; but usually the segments show some 

 conspicuous structural adaptation to the functions of copulation or 

 oviposition. 



The eighth scguient. — Modifications of the eighth segment (fig. 6, 

 VIII) occur principally in the female, since it is on the ventral part 



VII 



Vin IX X XI Cer XII(Tel) 



-a 



2Gon 



\ 

 iGon 



FiG. 6. — Diagram illustrating the concept of the structure of the abdomen 

 adopted in this paper. 



a-a, dorso-pleural line separating tergal region from pleuro-sternal region; 

 Ccr, cercus ; Eppt, epiproct ; iGon, sGon, first and second gonapophyses ; la, 

 lamina subanalis ; LB. limb basis; Papt, paraproct (lobe of eleventh sternum) ; 

 sa, lamina supra-analis ; Sp, spiracle; Stn. primary sternum; Sty, stylus; T, 

 terguni; Tel, telson (twelfth segment, greatly reduced or obliterated in insects). 



of this segment that the first gonapophyses (iGon), or genital proc- 

 esses of the eighth gonopods, are developed, and become the ventral 

 blades, or first valvulae, of the ovipositor in all species provided with 

 an ovipositor. The female genital opening is normally situated be- 

 tween the bases of the first gonapophyses in the membrane behind 

 the primitive eighth sternal plate, but the latter is frequently prolonged 

 beneath the base of the ovipositor, forming the subgenital plate of the 

 female. The bases of the gonopods of the eighth segment are never 

 united with the eighth sternum in female insects having an ovipositor. 

 In the Thysanura they are large, stylus-bearing plates or lobes which 

 retain the normal position of limb bases, but in pterygote insects they 



