NO. 8 INSECT ABDOMEN SNODGRASS 37 



and the eggs discharged from the gonopore located on the floor of 

 the chamber are passed into the channel of the ovipositor between the 

 bases of the first valvulae. This condition is well illustrated in Acridi- 

 dae, Tettigoniidae, and Gryllidae. Since the first valvifers, however. 

 are always distinct from the eighth sternum, they may be displaced 

 posteriorly to such an extent that the ovipositor base lies entirely pos- 

 terior to the genital chamber. This condition is found especially in 

 insects in which the seventh sternum is prolonged, for in such cases 

 the eighth sternum is generally reduced (fig. 8 C, VlllStn) and the 

 genital chamber {GC) becomes a mere pocket of the anterior wall of 

 the vestibulum {Vsi). An unusual condition, to be described in 

 detail later, occurs in some of the Cicadidae, in which the highly de- 

 veloped genital chamber has a second posterior opening on the ninth 

 venter (fig. 8D). 



THE OVIPOSITOR 



An ovipositor formed from appendicular processes of the abdomen 

 is in general present only in insects having the female genital opening 

 on the eighth abdominal venter, or between the eighth and ninth 

 sterna. With the transposition of the genital opening to the ninth 

 segment, the ovipositor has been suppressed in all cases (except in 

 Cicadidae having two genital openings, in which the tgg passage leads 

 directly into the channel of the ovipositor ; and it should be noted 

 also that the female exit apparatus of the Homoptera in general is not 

 well understood). Apparent rudiments of the gonopods of the ninth 

 segment, however, may be retained, as in Panorpidae and Coleoptera. 

 A comparative study of the plates and appendicular structures as- 

 sociated with the female genital opening in Coleoptera has been made 

 by Tanner (1927), who claims that the terminal hooks or spurs are 

 styli ; but the general structure and musculature of the organs does not 

 correspond closely with that of the parts of a typical ovipositor be- 

 longing to the ninth segment. The principal groups of pterygote in- 

 sects in which an ovipositor is well developed are the Orthoptera, 

 Hemiptera, Thysanoptera, and Hymenoptera. In various other orders, 

 however, an ovipositor may be present in some members, either as 

 a fully developed organ or in a rudimentary form. The fundamental 

 structure of the ovipositor is always the same, and the wide occurrence 

 of the organ suggests that an ovipositor formed of the appendages of 

 the eighth and ninth abdominal segments was a character of the 

 common ancestors of the Thysanura and Pterygota. 



The pterygote ovipositor, in its typical form, consists of a shaft 

 and a basal mechanism, and usually includes a pair of accessory lobes. 



