INSECTA. 807 



some strong muscles approaching the dorsal or ventral portions, which 

 appear intended to give to the breast a movement of compression or dilatation. 



The 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 mov^e these parts extend along the anterior margia 

 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- 

 cal, 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 Ugniperda) Lyonnel 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 nmscles 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 antenna;, and to the mouth. From its posterior extremi- 

 ty arise two chords or one pair of nerves, which, forming a collar, embrace 

 the oesophagus, and, uniting below in a ganglion, give olT 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 

 exttemity. 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 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 cilijB, 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 having travx-rsed 

 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 fluid, and adheres strongly. The cornea 

 IS composed of a hard elastic membrane, with the surface reticulated or 



