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SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



segment of the insect abdomen, and its appendages, the uropods 

 (2oApd), evidently correspond with the cerci. The telson (Tel) 

 being suppressed in insects, the tergum of the eleventh abdominal 

 segment (twentieth somite) becomes the supra-anal plate, or epiproct. 

 The cerci, therefore, may be regarded as the uropods of insects. 



The twelfth segment. — Among adult Hexapoda a twelfth segment 

 of the abdomen is developed as a complete annulus with tergal and 

 sternal plates only in Protura (fig. 5 B, XII). In the arthropods gen- 

 erally the terminal segment is the periproct, or end piece of the body 

 containing the anus, anterior to which the true appendage-bearing 

 somites are formed. In the malacostracan Crustacea the periproct 



Fig. 10.- — Terminal abdominal structures of a crustacean, Aiiaspidcs tasmaniac. 



A, posterior part of abdomen, showing the uropods (2oApd) as terminal 

 appendages of penultimate segment. B, ventral view of telson, showing lamina 

 supra-analis (sa) and laminae sub-anales (la) surrounding anus (An). 



forms the telson (fig. 10 A, Tel), typically a broad terminal lobe of the 

 abdomen, having the anus situated in the basal part of its ventral 

 surface (B, An) between two lateral valve-like flaps (la). A distinct, 

 anus-bearing, terminal lobe of the body is said to be present in the 

 embryos of some insects (fig. 5 A, Prpt) , but in adult insects there 

 is never more than a vestige of the periproct, or rudiment of a seg- 

 ment beyond the cercus-bearing eleventh somite (fig. 6, XII). 



The best example of the retention of a twelfth segment in insects 

 is furnished by the larvae of some Odonata, in which the anus is con- 

 tained in a small circular fold (fig. 12 A, Prpt) ordinarily concealed 

 between the bases of the epiproct (Eppt) and the paraprocts (Papt). 

 In the walls of this circumanal fold, as Heymons (1904) has shown, 



