eINSECTA. 807 
some strong muscles approaching the dorsal or ventral portions, which 
appear intended to give to the breast amovement of compression or dilatation. 
Tne abdomen in insects is composed of many imbricated rings, of which 
the one nearest the breast passes over the second, the second over‘the third, 
&c. The muscles which move these parts extend along the anterior margin 
of one segment, to the posterior margin of that which precedes it, and give 
more or less motion upwards or downwards, according to the structure and 
junction of the separate parts. The feet are provided with flexor and exten- 
sor muscles at every articulation. When the thigh is slender and cylindri- 
eal, the motion is confined to walking; when thick and tumid, to give room 
for the requisite muscles, the motions indicated are leaping and swimming. 
The muscles of insects in general are extremely numerous, very irritable, 
and many of them extremely minute. In the caterpillar of one species, 
(Cossus ligniperda) Lyonnet reckoned upwards of four thousand different mus< 
cles, while those of the human body do not exceed five hundred and twenty- 
nine; and the strength of these muscles is such, that some caterpillars are 
able to suspend themselves horizontally in the air for hours, supported by 
their posterior feet on a vertical! surface. 
The nervous system of insects is the same as in the annelides, the crusta- 
cea and arachnides. From the brain, or what is considered equivalent to 
the brain, the nervous matter in the head, originate threads, which extend 
to the eyes, to the antenne, and to the mouth. From its posterior extremi- 
ty arise two chords or one pair of nerves, which, forming a collar, embrace 
the cesophagus, and, uniting below in a ganglion, give off filaments to the 
‘surrounding parts. Two chords more or less approximated, often united, 
are prolonged from thence along the under part of the body, forming at in- 
tervals other ganglions, varying in number, till the filaments reach the anal 
extremity. What is called the brain differs but little from the other ganglia, 
and is distinguished by this appellation, only because the nervous threads of 
the head seem to be derived fromit. 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 ciliz, or eyelids. Tle optic nerve, at some distance from its origin, is 
extended to form the retina, and divided into anumber 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 having traversed 
the posterior part of the cornea, are applied against the facets or multiplied 
fares of the external eye, take their figure and become individual retine. 
Tnose 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 fluid, and adheres strongly. The cornea 
1s composed of a iard elastic membrane, with the surface reticulated or 
