DEFINITION OF THE TERM INSECT. 45 
in which the vertebral column becomes external or merges 
in the upper shell. The cyclostomous fishes also are not 
very wide of insects as to their integument. But on this 
subject I shall be more full hereafter. 
The forms of insects are so infinitely diversified that 
they almost distance our powers of conception: in this re- 
spect they seem to exceed the fishes and other inhabitants 
of the ocean, so that endless diversity may be regarded 
as one of their distinctions. But on all their variations 
of form the Creator has set his seal of symmetry; so that, 
if we meet with an animal in the lower orders in which 
the parts are not symmetrical, we may conclude in general 
that it is no insect. 
But it is by their parts and organs that insects may be 
most readily distinguished. In the vertebrate animals, 
the body is usually considered as divided into head, 
trunk, and limbs, the abdomen forming no part of the 
skeleton; but in the znsect tribes, besides the organs of 
sense and motion, the body consists of three principal 
parts—Head, Trunk, and Abdomen—the jirst, as was 
before observed, bearing the principal organs of sense 
and manducation ; the second most commonly those of 
motion ; and the third those of generation—the organs of 
respiration being usually common to both trunk and ab- 
domen. These three primary parts,—though in some in- 
sects the head is not separated from the trunk by any 
suture, as for instance in the Arachnida ; and in others, 
head, trunk, and abdomen form only one piece, as in some 
mites,—still exist in all, and in the great majority they 
are separated by incisures more or less deeply marked : 
this is particularly visible in the Hymenoptera and Di- 
ptera, which, in this respect, are formed upon a common 
