OUTLINES OF ENTOMOLOGY. 15 



times their own lieiglit, or to sustain, without injury, weights several 

 hundred times that of Iheir own. The lofty and long-continued flights 

 of some species, the capacities of others for running, burrowing, bor- 

 ing into hard substances, and for carrying or dragging heavy burdens, 

 all attest the wonderful strength and elasticity of their muscles. 



The Nervous system consists of two delicate cords which extend 

 longitudinally, the one above the other, along the ventral side of the 

 body. Insects have no brain, properly so called, but the lower or 

 external cord has a series of swellings or nerve knots called ganglia, 

 varying in number from two to ten, from which nerve fibers are dis- 

 tributed in various directions. The ganglion in the head is, in many 

 species, larger than the others, but does not differ from them in any 

 other particular. 'Next to that the ganglia of the thorax are most 

 developed, especially in perfect insects, since from them the supply of 

 nerve-force for the \^ings and legs must be derived. The upper or 

 internal cord is a simple thread without nerve knots or branches. It 

 lies very close to but scarcely in contact with the ganglionic cord. We 

 find in this nearly equal distribution of nerve force the reason why 

 many insects can live for a considerable time after a part of the body 

 has been crushed or severed, and why the separated parts seem to be 

 alike endowed with vitality. 



The Circulatory system of insects is as yet but imperfectly under- 

 stood. The blood is cold and colorless, or with a slight yellowish tint. 

 It does not flow through tubes corresponding to our arteries and veins, 

 but seems to bathe the other tissues without being confined to special 

 channels. There is, however, a long, narrow membranous sac, situated 

 near the upper surface of the body, which forms a sort of heart, the 

 pulsations of which can be distinctly seen in many thin-skinned insects, 

 especially in larvae. This tube is called the dorsal vessel, and is divided 

 into several chambers by valves which permit the blood to pass only in 

 a forward direction. The blood enters the dorsal vessel through open- 

 ings in its sides, and, flowing toward the head, is expelled through a 

 large artery called the aorta, from whence its course can no longer be 

 traced. In its progress it is aerified by contact with the air vessels 

 and mixed with chyle from the stomach, and is thus prepared to nour- 

 ish the organism. It is scant in quantity compared with the blood of 

 vertebrates, and the circulation seems to be slow. 



Respiration or breathing in insects is performed, not through a 

 single trachea or air-pipe communicating with a pair of lungs, as in 

 most of the higher animals, but through a series of delicate tubes which 

 divide and subdivide so as xo permeate all parts of the body. The 

 openings to these tubes are in most insects on the sides of the seg- 

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