NO. 6 INSECT ABDOMEN — SNODGRASS 95 



provided with four muscles. Three of these muscles, distinguished 

 as an abductor, a depressor, and an elevator muscle, take their origins 

 on the tenth al)doininal tergum ; the fourth arises on the supra-anal 

 plate or epiproct. The muscle from the epiproct, Ford says, is absent 

 in GrvUotalpa, but it is present in Gryllus, Neocojwcephaliis, Ceutlio- 

 philits. and iMclaiioplits. In Gryllus. because of the union of the tenth 

 tergum and the epiproct, this muscle, how^ever, has shifted forward to 

 the area of the tenth tergum. The writer has found only three mus- 

 cles in Dissostcira inserted directly on the base of the cercus, two 

 arising on the tenth tergum, and one on the epiproct. 



The origin of the anterior muscles of the cerci on the tenth segment 

 might be construed as evidence in favor of the view that the cereal 

 appendages belong to the tenth segment ; or, on the other hand, it 

 might be taken as favoring Heymons' claim that the true eleventh 

 segment has been obliterated. However, it is not necessary to assume 

 that the muscles associated with the cerci are primarily muscles of 

 these appendages. The great bundles of fibers that operate the 

 pincer-like cerci of J a pyx almost fill the large tenth abdominal seg- 

 ment (fig. 40 C, nicl) , but they appear to be the longitudinal dorsal 

 muscles normal to this segment, which secondarily function as cereal 

 muscles by reason of their posterior attachments at the bases of the 

 cerci. Ford (1923), observing that most of the cereal muscles in 

 Orthoptera arise from the tenth tergum, asserts that these muscles are 

 " intersegmental muscles between the tenth and eleventh segments," 

 while the muscles from the epiproct, she says, represent " the inter- 

 segmental muscles between the eleventh and twelfth terga." (Her 

 reference of the posterior muscles to the twelfth tergum is m accord 

 with her acceptance of Heymons' claim that the eleventh segment 

 has disappeared in the adult.) 



Whatever may be the nature of the dorsal muscles of the cerci, 

 the fact is significant that the organs have no ventral musculature — 

 in this respect cerci differ from styli and gonapophyses. The absence 

 of muscles from the paraprocts to the cerci, moreover, weakens the 

 comparison between the paraprocts and the stylus-bearing plates of 

 the preceding abdominal segments, since the stylus muscles always 

 take their origin in these plates. The termination of the ventral mus- 

 culature of the abdomen in the paraprocts, on the other hand, makes 

 it almost certain that the paraprocts are terminal lobes of the eleventh 

 sternum. 



Cerci are usually absent in holometabolous insects, but cercus- 

 like appendages occur on the eleventh abdominal segment in females 

 of Panorpa (fig. 8 H, Cert), and on the terminal segment of adult 



