808 INSECTA. 
divided by small hollow lines, often furnished with hairs, and a number of 
hexagonal facets. LLewenhoeck has counted three thousand one hundred 
and eighty-one in the cornea of a beetle, and eight thousand in that of a 
moth. The butterfly has seventeen thousand two hundred and thirty-five. 
Each facet may be considered as a crystalline lens, concave within and con- 
vex without. 
The organ of hearing is not manifest in insects, although most of them 
possess this faculty to a certain extent; for in the coupling season, many 
males have the power of producing a noise to call the females, as in the 
grasshopper and cricket. The sense of smell is more evidently manifest- 
ed both in their larva and perfect state, from their instantly discovering and 
crowding to places where their food is to be found, or to substances proper 
for the deposition of their ova, and where they were not previously seen. 
The seat of this faculty some naturalists are inclined to believe is in the 
antennx, while others, as M. Dumeril, suppose it to be at the entrance of the 
trachee or stigmata. On the other hand, Brunnich, Olivier, and Marcel 
Je Serres are of opinion that the sense of smell resides in the palpi; and 
experiments on bees have rendered it probable that the chief sensations are 
communicated by the mouth. From the proboscis being more or less de- 
veloped, as the palpi are minute or wanting, M. Lamarck considers this 
supposition the correct one. The organs of touch have been generally con- 
sidered to be those named antenne or feelers; and insects destitute of them 
use their palpi and the tarsi of the anterior feet for the same purpose. The 
covering of the body being generally corneous, can communicate but feebly 
the sense of touch. 
In insects, distinct absorbing or circulating vessels have not hitherto been 
discovered. A dorsal vessel, or long transparent canal, reaches indeed from 
the head to the posterior extremity of the body; and this has been conceived 
to be equivalent to the heart and blood-vessels of the higher classes. But 
this vessel, though narrowed at intervals, corresponding to the segments of 
the body, and having an undulatory contraction and dilatation from the head 
to the posterior extremity, possesses none of the characters of a true heart or 
circulating system ; and it is considered to be the only chief reservoir of the 
principal fluid in insects, filling and emptying itself by absorption and exuda- 
tion. M. Carus, however, has discovered in the caudal lamin of some 
larvee, and in the rudimentary wings, an excurrent and incurrent motion of 
_ fluid in distinct tubes, which he conceives tobe a true circulation ; a circum- 
stance which has escaped the penetration of Lyonnet, who always found the 
undulatory motion of the dorsal vessel to proceed invariably from the head 
‘o the tail. As this circulation, or double motion, however, has only been 
observed in one stage of existence, it is rather to be regarded as a phenome- 
non connected with the passage of the animal into a different state, than as 
a circulation analogous to that of animals, with two distinct sets of vessels. 
Respvration in insects is effected by means of two tubes, one on each side of 
