94 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



tween the lateral angles of the last tergal plate and the upper 

 angles of the paraprocts (fig. 40 A, B), but the terminal plate 

 of the dorsum is here clearly a composite sclerite formed of the united 

 tenth tergum (A, X) and the epiproct (XI). Frequently the cerci 

 are more closely associated with the paraprocts than with the tergal 

 plates. In any case, however, the intermediate position of the cerci 

 in adult Pterygota gives no positive evidence of the segmental rela- 

 tions of these appendages in this group of insects. 



On the other hand, the ontogenetic evidence of the nature of the 

 pterygote cerci seems to be quite definite, for it is stated by Ayeis 

 (1884), Cholodkowsky (1891), Wheeler (1893), and Heymons 

 (1896a) that the cerci in the embryos of Orthoptera are formed 

 directly from the appendages of the eleventh abdominal segment (figs. 

 5 A, 9 A, B, Cf/'). Heymons claims that the eleventh segment itself 

 disappears from the adult abdomen, and that the cerci thus come 

 to have an apparent intersegmental position between the tenth and 

 the twelfth segments. As already shown, however, it appears more 

 probable that the eleventh segment is usually represented in the 

 adult by the epiproct and the paraprocts, and that it is the twelfth 

 segment which is lacking, or reduced to a circumanal fold (fig. 12 A, 

 Prpt). 



The association of the cerci with the upper basal angles of the 

 paraprocts, or their actual connection with these plates in some cases, 

 as in Thermobia (fig. 7 F), has given rise to the idea that the cerci 

 and the paraprocts have a genetic relation to each other. Thus, 

 Crampton (1920, 1921) contends that the paraproct is the base of a 

 segmental appendage of which the cercus is the distal part. Accord- 

 ing to Heymons (1896), on the other hand, the embryonic cercus 

 represents the entire appendage of the eleventh segment, including the 

 basis, which in the pregenital segments unites with the primary seg- 

 mental sternum to become a lateral part of the definitive sternal plate. 

 In the adult insect, Heymons says, the cereal base usually disappears 

 as an evident lobe, though a rudiment of it is retained in young 

 nymphs of Gryllus and Decticus as a small basal ring supporting the 

 free part of the organ (fig. 8 B). 



The musculature of the cerci, so far as it is known, is always dorsal, 

 there being no muscles from the sternal region of the abdomen or from 

 the paraprocts in any way associated with the cerci. The origins of the 

 muscles present, however, give no clew to the segmental relations of 

 the cerci, since the muscles arise either on both the tenth and eleventh 

 terga, or on the tenth tergum alone. In her study of the abdominal 

 muscles of Orthoptera, Ford (1923) finds that each cercus is typically 



