46 DEFINITION OF THE TERM INSECT. 
model; and in the rest, with the above exceptions, it may 
be distinctly traced. 
The head of insects is clearly analogous to that of 
vertebrate animals, except in one respect, that they do 
not breathe by it. It is the seat probably of the same 
senses as seeing, hearing, smelling, tasting—and more 
peculiarly perhaps of that of touch. The eyes of insects, 
though allowed on all hands to be organs of sight, are 
differently circumstanced in many particulars from those 
of the animals last mentioned ; they are fixed, have nei- 
ther iris nor pupil, are often compound, and are without 
eyelids to cover them during sleep or repose; there are 
usually two compound ones composed of hexagonal fa- 
cets, but in some instances there are four; and from one 
to three simple in particular orders. ‘The antenne of 
insects in some respects correspond with the ears of the 
animals we are comparing with them; but whether they 
convey the vibrations of sound has not been ascertained: 
that they receive pulses of some kind from the atmo- 
sphere I shall prove to you hereafter—so that if insects 
do not hear with them in one sense, they may, by com- 
municating information, and by aéroscopy, to use Leh- 
man’s term, not directly in his sense*, supply the place 
of ears, which would render them properly analogous 
to those organs. ‘That in numbers these remarkable 
organs are factors is generally agreed, but this is not 
their wnzversal use. That insects smell has been often 
proved; but the organ of this sense has not been ascer- 
tained. What has improperly been called the clypeus, 
or the part terminating the face above the upper lip 
(labrum), is in the situation of the nose of the Vertebrata, 
2 De Antennis Insect, li. 65. 
