52 ENTOMOLOGY 



Segments. A typical abdominal segment bears a dorsal plate, or 

 tergum, and a ventral plate, or sternum, the two being connected by a pair 

 of pleural membranes, which facilitate the respiratory movements of the 

 tergum and sternum. Most of the abdominal segments have spiracles, 

 one on each side, situated in or near the pleural membranes of the first 

 seven or eight segments. The total number of pairs of spiracles is as 

 follows : 



Thoracic. Abdominal. Total. 



Campodea 3 o 3 



Japyx -4 7 Ir 



Machilis 2 7 9 



Lcpisma 2 8 10 



Blattidae, Acridiidae 10 



Odonata 2 10 



Heteroptera . . 3 ' 7 IO 



Lepidoptera 7 9 



Diptera .... 79 



In most embryo insects there are eleven pairs of spiracles (three thora- 

 cic and eight abdominal); in adults, however, two pairs are commonly 

 suppressed the prothoracic and the eighth abdominal. 



Number of Abdominal Segments. Though consisting typically 

 of ten segments the number evident in such generalized insects as Thy- 

 sanura and Ephemerida eleven occur in various adult Orthoptera, with 

 traces of a twelfth, while Heymons has detected twelve abdominal seg- 

 ments in embryos of Orthoptera and Odonata. In the more specialized 

 orders, ten may usually be distinguished, with more or less difficulty, 

 though the number is apparently, and in some cases actually, less owing 

 to modifications of the base of the abdomen in relation to the thorax, 

 but especially to modifications of the extremity of the abdomen, for 

 sexual purposes. 



Modifications. In aculeate Hymenoptera the first segment of the 

 abdomen has been transferred to the thorax, where it is known as the 

 propodeum, or median segment; in other words, what appears to be the 

 first abdominal segment is actually the second; this, as in bees and wasps, 

 often forms a petiole, which enables the sting to be applied in almost any 

 direction. In Cynipidas the tergum of segment two or three occupies 

 most of the abdominal mass, the remaining segments being reduced and 

 inconspicuous. The terminal segments of the abdomen often telescope 

 into one another, as in many Coleoptera and Hymenoptera (Chrysididas), 

 or undergo other modifications of form and position which obscure the 



