808 INSECTA. 



divided by small hollow lines, often furnished with hairs, and a number of 

 hexagonal facets. Lewenhoeck 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 

 antennse, while others, as M. Dumeril, suppose it to be at the entrance of the 

 tracheae or stigmata. On the other hand, Brunnich, Olivier, and Marcel 

 de 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 antennm 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 emptyingitself by absorption and exuda- 

 tion. M. Carus, however, has discovered in the caudal laminae of some 

 larvae, and in the rudimentary wings, an excurrent and incurrent motion of 

 fluid in distinct tubes, which he conceives to be 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 

 to 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. ' 



Respiration in insects is effected by means of two tubes, one on each side of 



