NO. 6 



INSECT ABDOMEN — SNODGRASS 



103 



Continuity of muscle fibers is of frequent occurrence wherever the 

 intersegmental connections are lost, or where segmental boundaries 

 are obliterated. 



Clialastogastrous Ilyincnopfcra. — The larvae of the sawflies and 

 horntails are provided with terminal appendicular organs of several 

 varieties, all borne by the last abdominal segment, some arising from 

 the dorsum, others from the venter. 



On the dorsum there is in some forms, as in Pteronidea ribesii 

 (fig. 43 A), a pair of small, immovable lateral processes (a) arising 

 from the end of the last segment above the anal opening (An). These 

 processes have been regarded as rudimentary cerci (Crampton, 1919), 

 but they are more evidently mere cuticular outgrowths, as claimed 

 by Middleton (1921), analogous with the urogomphi of coleopterous 

 larvae (fig. 44 C, ng), since they have none of the characteristics of 



Fig. 43. — Terminal appendicular structures of larvae of chalastogastrous 

 Hymenoptera. 



A, Pteronidea ribesii (Tenthrediniclae). B, Ci»ibex amcricana (Cimbicidae). 

 C, Cephaleis sp. (Pamphiliidae). 



a, paired processes of dorsum of terminal segment; Ati, anus; b, median proc- 

 ess of dorsum of terminal segment ; Pp. postpedes, or pygopods ; rz'S, retractor 

 muscle of plantar lobe ; Vs, planta. 



true cerci, and, according to Middleton, take no part in the formation 

 of the lateral, cercus-like appendages of adult Tenthredinidae. In 

 certain other chalastogastrous larvae a median process, or postcornu 

 (fig. 43 C, b), is borne on the end of the tenth abdominal segment. 

 This process varies characteristically in shape and size in dififerent 

 families, as tabulated by Middleton (1921). It occurs in larvae that 

 bore into stems or that live in silk-spun tents or within the hollow of 

 curled leaves. 



The ventral appendages of the pygidial segment of chalastogastrous 

 larvae likewise vary in form in dififerent groups. Those of species 

 that live in the open closely resemble the anal legs of lepidopterous 

 larvae. In Pteronidea ribesii, for example (fig. 43 A), a ventral pro- 

 jection of the tenth segment beneath the anus, bearing two small 

 terminal lobes (Vs), is clearly the homologue of a pair of appendages 



