NO. 6 INSECT ABDOMEN — SNODGRASS lOI 



bases of the claws. The claws themselves are in every way suggestive 

 of the crochets of the caterpillar's foot. 



Notwithstanding the general structural resemblance between the 

 terminal appendages of Chauliodes or Corydalus and the abdominal 

 legs of caterpillars, it can scarcely be supposed that the foot struc- 

 ture in either case has been derived immediately from that of the 

 other, since the two-clawed condition would be a highly specialized 

 one in the Lepidoptera. All that may be claimed is that the funda- 

 mental structure of the larval abdominal limbs is the same in both 

 the Neuroptera and the Lepidoptera. The neuropterous appendage 

 is the more primitive in that it retains the stylus, which has the form of 

 a segmented appendage in Sialis (fig. 35 A, C). We may assume that 

 the gill-bearing tubercles of the Corydalus larva have been formed 

 secondarily from the foot lobes, or retractile vesicles, as a better adap- 

 tation to aquatic life in this genus, and that the vesicles have been 

 lost from all the appendages in Sialis, and from all but the terminal 

 appendages in Chauliodes. 



Lepidoptera. — The so-called anal legs, or postpedes, of caterpillars 

 are so similar to the legs of the preceding abdominal segments as 

 scarcely to need a separate description. The musculature of the two 

 sets of appendages, however, differs in some respects. The plantar 

 lobe of each anal leg is retracted by a large dorsal muscle (fig. 42 A, 

 B, C, rvsd) and a small ventral muscle {rvsv), both arising from the 

 intersegmental fold (/) before the ninth abdominal segment. The 

 lateral muscles of the leg are reduced to a few fibers (D, h) lying 

 external to the large dorsal retractor of the planta. Between the bases 

 of the legs there is a sheet of transverse ventral muscles (B, tv). 

 which appear to belong to the wall of the last body segment rather than 

 to the appendages. 



The great development of the dorsal retractor muscles of the anal 

 legs, the reduction of the lateral muscles, and the presence of the 

 ventral retractors of the plantae are all features correlated with the 

 function of the postpedes in the caterpillar, which usually have a 

 stronger independent forward movement than do the legs of the pre- 

 ceding segments. 



The large terminal segment of lepidopterous larvae appears to be 

 a compound segment composed of the tenth abdominal somite, with 

 its appendages, the pygopods, and the reduced eleventh segment, bear- 

 ing the anus, but lacking cerci. Figures of the embryo of Picris rapae 

 given by Eastham (1930) show clearly a well-developed tenth abdom- 

 inal segment bearing the last pair of appendages, and beyond it a 

 large terminal lobe, containing the anus, which is evidently the 



