NO. 14 



INSECT ABDOMEN SNODGRASS 



71 



each may bear distally a stylus (Sty), and in some species, mesad of 

 the latter, an eversible vesicle (Vs). The coxal plates are present 

 also on the genital segments (F, G), but the sternal plate is absent 

 on the ninth segment of the male (G) and on both genital segments 

 of the female. Styli are characteristically present on the coxopodites 



Cer 



I'lG. 24. — Hexapoda-Diplura and Thysanura : .structure of the abdominal 

 sterna and external genitalia. 



A, Ncso)iiachilis inaaricits. ventral view of an abdominal segment, showing 

 sternal surface formed of a small primary sternal plate (Shi) and two large 

 coxopodite plates (Cxpd) bearing appendicular organs. B, same, dorsal view 

 of right half of sternal plates, showing muscles of vesicula and stylus. C, 

 Hctcrojapyx gallardi, ventral view of an abdominal segment, showing coxopodites 

 incorporated into the sternum. D, same, dorsal surface of right half of sternum, 

 showing muscles of stylus. E, Ncsomachilis maorkus, posterior part of male 

 abdomen, ventral view, showing penis arising between bases of coxopodites of 

 ninth segment. F, MachUis variabilis, ventral plate of eighth abdominal seg- 

 ment of male, with pair of small first gonapophyses (iGoii), dorsal view. G, 

 same, appendages and penis of ninth segment, with second gonapophyses (2Gon) 

 arising from bases of coxopodites at sides of penis. 



Apt, sternal apotome ; Cer, cercus ; cf, caudal filament; Cxpd, coxopodite; 

 iGon. first gonapophysis ; 2Gon, second gonapophysis ; Papt, paraproct ; rvs, re- 

 tractor muscles of vesicula; sa, supra-anal lobe; smcls, stylus muscles; Stn, pri- 

 mary sternal plate; Sty, stylus; T, tergum ; V, venter; Vs, vesicula; VII I -XI, 

 abdominal segments. 



of the genital segments of Thysanura, but they may be absent on the 

 other segments; they never occur on the tenth segment, which also 

 has no sclerotic plates in its venter (E, X.V). The abdominal styli 

 of Thysanura are movable by muscles arising on the coxopodite plates 

 (B, F, G, smcls), but the similar processes on the thoracic coxae of 

 MachUis lack muscles, as do also the abdominal styli of Orthoptera. 



