NO. 6 INSECT ABDOMEN — SNODGRASS 15 



periproct (fig. 5 A, Prpt), which carries the anus, but does not have 

 appendages. It is the terminal piece of the body anterior to which 

 the true somites are formed, segmentation of arthropods being intra- 

 somatic and not a process of budding. Some investigators claim that 

 a pair of coelomic sacs is formed in the periproct. The presence of not 

 more than six segments in the abdomen of CoUembola is usually 

 taken to be the result of reductive specialization, or " degeneration," 

 in these insects, considering that the existence of even six segments 

 is obscure in some forms by a loss of intersegmental lines. Tillyard 

 (1930), however, sees in the CoUembola a primitive " protomorphic " 

 condition in which only nine postcephalic somites have ever been de- 

 veloped. He» points out that segmentation in the collembolan embryo 

 produces six abdominal segments and no more, and that we have, 



r ■;:: n"^ 7,:^ iXApd 



tr . ^ VXApd 



J J "Prpt 



XIIT 



A '~ ' '^''' An^ ' ' XIIS B 



An-' . - -' (XII 



Fig. 5. — Examples of the presence of twelve segments in the he.xapod abdomen. 



A, posterior end of abdomen of young embryo of Gryllotalpa (from Heymons, 

 1895), with large periproct {Prpt), or twelfth segment, behind the last true 

 somite (XI) bearing the cerci (Ccr). B, end of abdomen of adult proturon, 

 Accrcntulus confinis (from Berlese, 1910), showing twelve distinct segments. 



therefore, no evidence of a greater number of somites ever having 

 been present in this group of arthropods, which he would not ally 

 closely with the other insects. A reduction in the number of abdominal 

 segments is the rule in both immature and adult insects generally. 

 As just stated, evident remnants of the periproct are rare except in 

 the Protura. While ii segments are distinct in many of the more 

 generalized insects, in the higher orders, especially in the Holome- 

 tabola, not more than 10 segments are usually present, and sometimes 

 only nine are evident. In the more specialized insects there is a 

 tendency toward elimination of the first abdominal segment, but 

 generally reduction takes place at the posterior end of the body. 

 Since the periproct is commonly lacking, or reduced to a circumanal 

 membrane, the eleventh somite, which carries the last pair of seg- 

 mental appendages, becomes tiie definitive anal segment. The tenth 



