66 EN TOMOLOGY 
plate, or ferguim, 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, 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 : 
Thoracic. Abdominal. Total. 
Campodea, 3 Oo 3 
Japyx, A 7 II 
Machilis, 2 ¥ 9 
Lepisma, 2 8 10 
Blattide, Acridiide, 2 8 10 
Odonata, 2 8 10 
Heteroptera, 3 7 10 
Lepidoptera, 2 7 9 
Diptera, 2 7 9 
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 knownas 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- 
nipidee the tergum of segment two or three occupies most of the 
