INSECTS. 223 



functions of vitality. There are generally two muscles concern- 

 ed in the motion of each part. The muscles which move the 

 head are situate within the thorax, and the principal ones serve 

 to raise or lower it. Within the thorax likewise are placed the 

 muscles which move the wings and feet ; and besides these are 

 some strong muscles approaching the dorsal or ventral portions, 

 which appear intended to give to the breast a movement of com- 

 pression or dilatation. 



The abdomen in insects is composed of many imbricated 

 rings, of which the one nearest the breast passes over the se- 

 cond, the second over the third, Sec. 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 extensor muscles at every articulation. When the 

 thigh is slender and cylindrical, the motion is confined to walk- 

 ing ; when thick and tumid, to give room for the requisite mus- 

 cles, the motions indicated are leaping and swimming. The 

 muscles of insects in general are extremely numerous, very ir- 

 ritable, and many of them excessively minute. In the cater- 

 pillar of one species (Cossus ligniperda) Lyonnet reckoned up- 

 wards of 4000 different muscles, while those of the human 

 body do not exceed 529 ; and the strength of these muscles is 

 such, that some caterpillars are able to suspend themselves ho- 

 rizontally 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 Anneli- 

 des, the Crustacea, 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 antennae, 

 and to the mouth. From its posterior extremity arise two 

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

 the oesophagus, and, uniting below in a ganglion, give off fila- 

 ments to the surrounding parts. Two chords more or less ap- 

 proximated, often united, are prolonged from thence along the 

 under part of the body, forming at intervals other ganglions, 

 varying in number, till the filaments reach the anal extremity. 

 What is called the brain differs but little from the other gan- 

 glia ; and is distinguished by this appellation only because the 



