92 NATURAL HISTORY OF ARTHROPODS. 



in the insects there is no true gastrula, the primitive stomach always being formed in 

 the yolk. The anterior and posterior divisions are lined with a hardened chitinous 

 layer, Avhile the median portion has no such lining. The anterior portion, the function 

 of which is the preparation of the food and its introduction into the stomach, is usually 

 a simple tube or oesophagus, but frequently it is divided into several portions, with 

 other functions. In some forms, as the moths and butterflies, there is a " sucking 

 stomach " in the head which acts as a pump, drawing the fluids into the mouth and 

 forcing them back to the true stomach. Others, as the crickets, have an enlargement 

 farther back (the crop) lined with chitinous teeth, the object of which is to still further 

 comminute the solid food upon which these forms live. The proventriculus or gizzard 

 is another enlargement of the anterior portion found in most insects. 



The stomach proper is either a simple tube or it may have pocket-like prolongations 

 which greatly increase its digestive surface. These pockets are especially noticeable 

 in some of the flies, the grasshoppers, and especially in the spiders. Digestion is 

 accomplished by the passage of the nutritive portions through the Avails of the stomach, 

 when they enter into the general circulation without the intervention of a system of 

 lacteals like those found in the human body. It is a peculiar feature that in the young 

 of some of the bees and wasps and flies the stomach ends blindly, there being no 

 connection between that organ and the intestine, though in later life the connection 

 is made. 



The hinder portion of the alimentary canal, the intestine, is usually short. Into it 

 open numerous tubes, the so-called Malpighian vessels, the function of which is the same 

 as the kidneys of the higher forms. These urinary tubules are found only in insects. 

 A still further feature or accessory of the digestive canal are the salivary glands, which 

 pour their secretion into the mouth. These organs are present in almost every member 

 of the group, and, on the contrary, are entirely absent in the Crustacea. Usually there 

 is but a single pair of salivary glands, but this number may be increased, there being 

 not unfrequently two, and even three pairs of these organs. 



The heart of insects may be said to resemble in a general way that of most 

 Crustacea. As in that group, it is a long, many-chambered organ lying above the 

 intestine. It forces the blood forward through an aorta of varying length, which runs 

 from the anterior end of the heart to the vicinity of the brain. In returning to the 

 heart the blood collects in two venous trunks, by which it is brought to the posterior 

 portion of the central organ. Finer subdivisions of the arterial and venous systems 

 are absent, and during a portion of its course the blood flows in open spaces between 

 the muscles and viscera. 



In the phenomena of respiration, and the organs concerned therein, insects present 

 one of the greatest differences from the Crustacea. In the latter group, as we have 

 seen, organs of respiration, when present, take the form of gills, borne on some of the 

 feet, and the blood in passing through them is brought in contact with the oxygen 

 dissolved in the water. Insects, on the other hand, are fitted for breathing air by 

 means of tubes or trachea which penetrate to all parts of the body, and which give the 

 name, synonymous with Insecta as here used, of Tracheata to the group. In the 

 thoracic and abdominal regions of the body there occur small openings on the sides, 

 never more than one on a side in each segment, which are known as spiracles or 

 stigmata. It is through these, and not through the mouth, that an insect breathes. 

 In some larvae there are eleven pairs of these spiracles, while in the adults the number 

 is frequently much less. In most hexapods there are but nine pairs. 



