THE INTERNAL ANATOMY OF INSECTS 123 



The leucocytes are nucleated, colorless, amoeboid cells similar to 

 the white corpuscles of vertebrates, in appearance and function ; they 

 take up and destroy foreign bodies and feed upon disintegrating tissue. 

 It is believed that the products of digestion of disintegrating tissue by 

 the leucocytes pass into the blood and serve to nourish new tissue. 



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 oxygen 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. 



VII. THE ADIPOSE TISSUE 



On opening the body of an insect, especially of a larva, one of the 

 most conspicuous things to be seen is fatty tissue, in large masses. 

 These often completely siuround the alimentary canal, and are held 

 in place by niimerous branches of the trachese with which they are 

 supplied. Other and smaller masses of this tissue adhere to the inner 

 surface of the abdominal wall, in the vicinity of the nervous system, 

 and at the sides of the body. In adult insects it usually exists in 

 much less quantity than in larvae. 



The chief function of the adipose tissue is the storage of nutriment ; 

 but it is believed that it also has a urinary function, as concretions of 

 uric acid accumulate in it during the life of the insect. 



VIII. THE NERVOUS SYSTEM 



a. THE CENTRAL NERVOUS SYSTEM 



The more obvious parts of the central nerv^ous system are the 

 following: a ganglion in the head above the oesophagus, the brain; 

 a ganglion in the head below the oesophagus, the subcesophageal 

 ganglion; a series of ganglia, lying on the floor of the body cavity in 

 the thorax and in the abdomen, the thoracic and the abdominal 

 ganglia; two longitudinal cords, the connectives, uniting all of these 

 ganglia in a series ; and many nerves radiating from the ganglia to the 

 various parts of the body. 



The connectives between the brain and the subcesophageal 

 ganglion pass one on each side of the oesophagus ; these are termed the 

 crura cerebri, or the legs of the brain; in the remainder of their course, 

 the two connectives are quite closely parallel (Fig. 124), 



