HEXAPODA 



37 



The blood consists of two elements, a fluid plasma and cells similar to the 

 white corpuscles of the blood of vertebrates, the leucocytes. It differs greatly 

 in appearance from the blood of vertebrates, on account of the absence of red 

 blood corpuscles. In most insects the blood plasma is colorless ; but in many- 

 species it has a yellowish, greenish, or reddish color. 

 The blood receives the products of digestion of 

 food, which pass in a liquid form, by osmosis, through 

 the walls of the alimentary canal. On the other 

 hand it gives up to the tissues which it bathes the 

 materials needed for their growth. In insects oxy- 

 gen is supplied to the tissues and gaseous wastes 

 are removed chiefly by the respiratory system and 

 not by means of the blood as in vertebrates. 



The nervous system. — The central part of the 

 nervous system consists of a ganglion in the head 

 above the oesophagus, and of a series of double 

 ganglia, typically one for each segment of the body, 

 lying on the floor of the body cavity, and connected 

 by two longitudinal cords. In the head, one of these 

 cords passes on each side of the oesophagus, from 

 the brain to another ganglion in the head below the 

 oesophagus, thus forming a nervous collar about the 

 alimentary canal. From each ganglion nerves arise, 

 which supply the adjacent parts; and from the 

 thoracic ganglia nerves extend to the legs and wings. 

 This series of ganglia is really a double one; but 

 the members of each pair of ganglia are more or less 

 closely united on the middle line of the body, and 

 often appear as a single ganglion. Figure 70 gives a general view of the 

 central nervous system of Corydalus cornutus. 



In addition to the central nervous system there are two sympathetic 

 nervous systems and the peripheral sensory nervous system, the latter 



composed of a network of fine 

 nerves all around the body just 

 beneath the hypodermis of the 

 body walls. 



How insects breathe — The 

 respiratory system. — A com- 

 mon mistake made by begin- 

 ners in the study of Entomology is to suppose that insects breathe through 

 the mouth as do the higher animals. Many a beginner has carefully 

 poured chloroform on the head of an insect in the expectation of killing it 

 in that way, and has been surprised at his poor success. 



The truth is, insects breathe through their sides. If an insect be 

 carefully examined, there can be found along the sides of the body a 

 series of openings (Fig. 71). These are the openings through which the 

 air passes into the respiratory system and are termed spiracles. 



The number of spiracles varies greatly in different insects. There is, 

 however, never more than one pair on a single segment of the body. 

 They do not occur on the head, but are borne by each of the last two tho- 

 racic segments., and by the first eight abdominal segments. Thus ten seg- 

 ments may bear spiracles, but usually one or more segments lack them. 



Fie. 70. — 



of Corydalus 



Nervous system 

 (After Leidy.) 



Fig. 71. — Side-view of locust with wings removed. 



