224 INSECTS. 



nervous threads of the head seem to be derived from it. Many 

 consider these ganglia or knots as so many little centres of 

 nervous energy, and thus explain why, when an insect is cut 

 into small portions, it displays for some time marks of sensation. 



In Insects two kinds of eyes occur, the first compound, or 

 composed of facets, the others simple and smooth. These 

 eyes are immoveable, and destitute of cilise or eyelids. The 

 optic nerve, at some distance from its origin, is extended to form 

 the retina, and divided into a number of hexagonal threads, which, 

 passing through the network of a circular trachea, go to a mem- 

 brane, generally cellular, called the choroid coat, and after hav- 

 ing traversed the posterior part of the cornea, are applied against 

 the facets or multiplied faces of the external eye, take their 

 figure, and become individual retinae. Those species which shun 

 the light are destitute of the choroid coat. The pigment which 

 covers the upper surface, and that which covers the opposite 

 side of the cornea, is opaque, slightly liquid, and adheres strong- 

 ly. The cornea is composed of a hard elastic membrane, with 

 the surface reticulated or divided by small hollow lines, often 

 furnished with hairs and a number of hexagonal facets. Lewen- 

 hoeck has counted 3181 in the cornea of a beetle, and 8000 

 in that of a moth. The eye of the butterfly has 1 7,235. Each 

 facet may be considered as a crystalline lens, concave within and 

 convex 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 female, as in the grasshopper and cricket. The 

 sense of smell is more evidently manifested 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 pro- 

 per for the deposition of their ova, and where they were not 

 previously to be seen. The seat of this faculty some naturalists 

 are inclined to believe is in the antennae, while others, as M. 

 Dumeril, suppose it to be at the entrance of the tracheae or stig- 

 mata. On the other handj 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 developed as the palpi are minute 



