66 ENTOMOLOGY 



plate, or tergum, and a ventral plate, or sternum, the two being 

 connected by a pair of plcural membranes, which facilitate the 

 respiratory movements of the tergum and sternum. Most of 

 the abdominal segments have spiracles, one on each side, situ- 

 ated in or near the pleural membranes of the first seven or 

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

 follows : 



In most embryo insects there are eleven pairs of spiracles 

 (three thoracic 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 general- 

 ized insects as Thysanura and Ephemerida eleven occur in va- 

 rious adult Orthoptera, with traces of a twelfth, while Hey- 

 mons has detected twelve abdominal segments 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 Cy- 

 nipidae the tergum, of. segment two or three occupies most of the 



