CIRCULATORY AND NERVOUS SYSTEMS. 19 



that its conducting vessels be maintained in a tubular farm : this is effected by means of 

 a spiral elastic thread wound closely upon itself. By this means the vessels are prevented 

 from collapsing, the air enters freely at the sides of the insect, and permeates perfectly the 

 whole system. The air, then, is constantly in contact with the thin membrane intervening 

 between the trachea and those vessels which transmit the blood to the different parts of 

 the insect's body and extremities. Recent observations prove, in regard to the relations of 

 blood and air, that the former moves through the space between the outer and inner 

 membranes, in which a spiral filament winds, so as to bathe the exterior of the air-tubes 

 themselves*. 



It would therefore seem that there is in insects a most complex system of spiral vessels 

 or air-tubes, in connection and close relation with the vascular system, both being dis- 

 tributed with the greatest minuteness throughout the whole body. There is still another 

 provision for keeping up a supply of air especially in those insects whose flight is rapid, 

 and requires to be long sustained : it is found in the air-sacs that are distributed in dif- 

 ferent parts of the body ; they serve not only to retain a due supply of air, but also to 

 buoy up the body and render it lighter. The pulmonary system, by a combination of elastic 

 tubes and sacs, receives an extraordinary development, so as to compensate in the most 

 perfect manner for the deficiencies resulting from a less perfect form of circulation ; for 

 powerful muscular motion requires a rich supply of oxygen, rather than food, in order to 

 give energy and activity to the muscular system. 



From these facts, it appears that insects occupy no mean place in the scale of being, 

 when they are ranked according to the development of their circulatory apparatus and 

 the accompanying tracheal system. This combination, so far as the power of muscular 

 motion is concerned in the estimate, serves to elevate the insect to a rank but little below 

 that of the vertebrate class. 



Nervous system. The nervous system consists of a pair of chords extending through 

 the insect, upon the interior abdominal face, and connected at each segment by a ganglion. 

 Thus disposed along the ventral face of the body, the nervous system forms a symmetrical 

 whole. The ganglionic masses of the head, however, are more largely developed than those 

 of the 'trunk : this is necessary, from the condition of the eyes and the perfection of the 

 masticating apparatus. The cephalic centres are placed also above the esophagus. So the 

 nervous centres which supply the wings and legs are larger than those of the abdomen. 

 We see here'again a provision for the locomotive apparatus, by which a due supply of 

 nervous power or force is secured. 



There are probably but few instances in the animal kingdom, that exhibit such minute 



• See Carpknteb, p. 382 of his General and Compar-iUre Physiology. 



