50 DEFINITION OF THE TERM INSECT. 
of the trunk and its members; at any rate they do not 
replace the two anterior pair of legs of the hexapod 
Aptera. When merely used as wings, they commonly 
consist of a fine transparent double membrane, strength- 
ened by various longitudinal and transverse nervures, or 
bones as some regard them, accompanied by air-vessels, 
of which more hereafter, as well as of their kind and cha- 
racters. I shall only observe, that insects are known 
from all other winged animals, by having four wings, or 
what represent them, and this even generally in those 
that are supposed to have only a pair. Another pecu- 
liarity distinguishes the trunk of insects that you will in 
vain look for in the vertebrate animals—these are one or 
two pair of lateral spiracles or breathing pores. Though 
_ the respiratory sacs, &c. of birds are almost as widely 
dispersed as the tracheze and bronchize of insects?, yet 
their respiration is perfectly pulmonary, and nothing like 
these pores is to be discovered in them. ~ 
The principal peculiarity of the third part of the body, 
the abdomen, is its situation behind the posterior pair of 
thoracic legs, and its rank as forming a distinct portion 
of what represents the skeleton. In most insects it is so 
closely affixed to the posterior part of the trunk as to 
appear like a continuation of it, but in the majority of 
the Hymenoptera and Diptera, and in the Araneidan 
Arachnida, or spiders, it is separated by a deep incisure ; 
and in the first-mentioned tribe is mostly suspended to 
the trunk by a footstalk, sometimes of wonderful length 
and tenuity. In the Mammalia the male genital organs 
are partly external; but in insects as well as in many of 
the vertebrate animals, except when employed, they are 
* N. Dict, d’ Hist, Nat, xxviii.; compare 104 and 110. 
