NO. 6 INSECT ABDOMEN — SNODGRASS 23 



z'iridissiiua, consisting of two pairs of appendicular [)rocesscs on the 

 tenth abdominal segment closely resembling the two pairs of valvulae 

 on the ninth segment, the outer pair corresponding with the valvulae 

 formed of the basal plates, the inner pair with those formed of the 

 gonapophyses. It is scarcely to be supposed, however, that an abnor- 

 mality of this kind is a " reversion " to an ancestral normality. The 

 embryonic limb rudiments of the tenth abdominal segment in all the 

 more generalized insects are normally suppressed before hatching. 

 In the Holometabola, on the other hand, appendicular structures fre- 

 quently occur on the tenth segment in postembryonic stages, and there 

 is little doubt that such organs on the larva, typically represented 

 by the postpedes of caterpillars, are true limb structures; in adults 

 they include the socii of Lepidoptera and Trichoptera, and the cercus- 

 like processes of Tenthredinidae, which appear to be derived from 

 the larval postpedes. The appendages of the tenth, or pygidial, seg- 

 ment of the abdomen may be generally designated the pygopods. 



The eleventh segment (nro-segnient). — The eleventh abdominal 

 segment represents the last true somite of the body, and is present in 

 the embryos of many insects as a well-developed ring bearing the 

 rudiments of the terminal pair of appendages (fig. 5 A, Cer). The 

 segment is present in adult Protura as a fully formed annulus with 

 tergal and sternal plates (B, XI), and in some of the lower Insecta 

 having 11 distinct segments in the abdomen the eleventh segment 

 is retained likewise as a complete annulus. This condition is well 

 shown in Nesomachilis (fig. 7 B) where the eleventh segment, though 

 mostly concealed within the tenth (A), consists of continuous tergal 

 and sternal regions (C, D), and bears laterally the long, filamentous 

 cerci {Cer). The tergal region is produced into the median caudal 

 filament (c/), and the sternal bridge supports a pair of broad subanal 

 lobes, the paraprocts (D, Papt), separated by a median cleft. In 

 Thermobia the eleventh segment has a distinct tergal plate, or epiproct 

 (E, F, Eppt), but the sternal bridge is lost, and the sternal region 

 of the segment is represented only by the paraprocts (F, Papt), upon 

 which are borne the cerci {Cer). The median ventral region of the 

 eleventh segment is generally obliterated in pterygote insects that 

 have a well-developed tenth segment, but in some of the Homoptera. 

 as in the cicada (fig. 8 C), the venter of the eleventh segment is not 

 only present but it contains a distinct sclerotic sternal remnant {XI S). 



The adult abdomen of most of the lower Pterygota ends with a 

 supra-anal plate (fig. 6, Eppt) which is in every way suggestive that it 

 corresponds with the epiproct, or tergum of the eleventh segment, in 

 the Thysanura. Some entomologists, however, leasing their opinion on 



