NO. 6 INSECT ABDOMEN — SNODGRASS 25 



Eppt), but would dissent from Cranipton's opinion that the cerci 

 and paraprocts pertain to the tenth segment, since in such forms 

 as Nesoiiiachilis (fig. 7 P> ) they clearly belong to the eleventh segment, 

 and embryologists agree that the cerci are appendages of the eleventh 

 segment ( fig. 5 A ) . That the paraprocts at least belong to the same 

 segment as the epiproct is evident from their usual anatomical rela- 

 tions with the latter. Ford (1923), in her review of the musculature 

 of orthopteroid insects, says, " from the musculature we find that the 

 sui)ra-anal plate and ]-)araprocts form a symmetrical group," and fur- 

 ther she observes that " the transverse muscles between the supra- 

 anal plate and jmraprocts show that all three belong to the same seg- 

 ment." The segment represented by the epiproct and paraprocts Ford 

 believes is the twelfth, but she bases her opinion largely on Heymons' 

 statement that the eleventh segment is suppressed in the adult. . 



Furthermore, the anatomical relations between the paraprocts and 

 the cerci do not sui)port Crampton's (1920, 1921) contention that 

 the paraprocts are the bases of the cereal appendages. The cerci may 

 be united with the ])araprocts (figs'. 7 F, 8 A), l)ut generally they 

 arise independently in membranous areas behind the tenth tergum 

 between the epi])roct and paraprocts. The cerci never have muscles 

 arising in the paraprocts, and the ventral musculature of the ])ara- 

 procts indicates that these plates are merely lol)es of the eleventh 

 sternum (fig. 6, Pa pi), as they are actually in NcsouiachUis (fig. 

 7 D ) . With the usual suppression of the eleventh sternal area, 

 however, the paraprocts may appear to arise from the posterior margin 

 of the tenth sternum, and thev are sometimes continuons with the 

 latter ( 'ivg. 8 F, Pa pi ) . 



The cerci, as shown In' their de\-elopment, are the ciilirc ai)i)endages 

 of the eleventh segment. Their primitive l)ases may 1)e represented 

 by a small, ring-like segment at the root of each organ (Meymons, 

 1896, Walker. 1922), and, as noted al)Ove, they are sometimes united 

 with the sternal parai)rocts, but the muscles of the cerci always have 

 a tergal origin. As already observed, most of the cereal muscles arise 

 on the tergum of the tenth segment. These anterior muscles of the 

 cerci, however, appear to l)e derived from the intersegmental, longi- 

 tudinal muscle fibers ])rimitively extending between the tenth and 

 eleventh terga, which have secondarily become motors of the cerci. 

 In some Ortho])tera, each cercus has a single muscle arising on the 

 epiproct. 



The terminal structure of the generalized insect abdomen has a 

 certain resemblance to that of a malacostracan crustacean ( lig. 10 A). 

 The twentieth body segment of the crustacean re])resents the eleventh 



