60 ENTOMOLOGY 



The development of aviation was due largely to thorough studies 

 of the flight of birds and insects. 



Abdomen. The chief functions of the abdomen are respiration and 

 reproduction, to which should be added digestion. The abdomen as a 

 whole has undergone less differentiation than the thorax and presents a 

 simpler and more primitive segmentation. 



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

 tergum (notum) and a ventral plate, or sternum, the two being connected 

 'by a pair of pleural membranes, which facilitate the respiratory move- 

 ments of the tergum and sternum. Abdominal tergites and sternites 

 are often termed urotergites and urosternites, respectively. 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 2 i 3 



Japyx 1 4 7 I! 



Machilis 2 7 9 



Lepisma 2 8 10 



Nicoletia 2 8 10 



Orthoptera 2 8 10 



Odonata 2 8 10 



Heteroptera. 2 6(7) 8(9) 



Lepidoptera 2 7 9 



Diptera 2 7 9 



1 Japyx actually has four thoracic and seven abdominal spiracles, as described and 

 illustrated by Grassi (1888), Willem (1900) and Verhoeff (1904); a study of their figures 

 indicates, however, that the spiracles may have migrated forward, and that the fourth 

 thoracic pair (there being two pairs in the metathorax) belongs morphologically to the 

 first abdominal segment. 



Number of Abdominal Segments. Though only ten abdominal 

 segments are evident in many adult insects and many larvae as well, 

 the typical number is eleven, and the maximum twelve. In embryos of 

 Thysanura, Orthoptera, Ephemerida, Odonata, Coleoptera and Hy- 

 menoptera, eleven abdominal neuromeres (primitive ganglia) have been 

 found by Heymons and others; each neuromere representing a segment; 

 and the twelfth segment is present as a telson, a terminal segment con- 

 taining the anus, but without a neuromere and never bearing a pair of 

 appendages. This telson is present also in the adults of some generalized 

 insects, as Orthoptera. 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 



