LESSONS IN ZOOLOGY. 277 



The digestive organs after the mouth are the (esophagus, into 

 which open the salivary glands; then a suctorial stomach in some 

 cases, and in others a crop or a straight or coiled intestine; but 

 whatever the name or form, one portion is adapted to digestion 

 of food and another concerned in the ejection of feces. There is 

 no liver, but certain glands in the walls of the stomach seem to 

 perform the office of such an organ, and there are also indica- 

 tions of pancreatic glands and urinary glands. 



The circulatory apparatus of insects is a simple dorsal vessel 

 divided into eight chambers; as this vessel or heart expands 

 blood flows into it, and as it contracts the blood is driven into all 

 parts of the body without being conveyed by a distinct system of 

 tubes. The blood is usually colorless, and always contains 

 amoeboid blood cells. 



Respiration by tracheae is one of the chief characteristics of an 

 insect. The tracheae are delicate tubes penetrating all portions 

 of the insect body, which receive their supply of air through 

 openings called stigmata situated in the membrane connecting 

 the dorsal and ventral plates of the abdomen. The number of 

 stigmata vary, but there are seldom more than nine or less than 

 two pairs present. The tracheae are kept open by a tough spiral 

 fiber of chitin, so that their structure is quite like that of the 

 tracheae of higher animals. Many insects seem to be able to de- 

 velop animal heat by increasing the vigor of respiratory move- 

 ments. The nervous system of insects is highly developed, all va- 

 riations between a common ganglionic mass in the thorax and a 

 ventral chain of ganglia occur. In some cases the brain attains 

 considerable size, which, according to one writer, " gives origin to 

 the sense nerves, and seems to be the seat of the will and of the 

 psychical activity." The ventral chain seems to correspond to the 

 spinal cord, and there seem to be instances of a true sympathetic 

 system. The muscular system lies just beneath the integument, 

 and seems to be continuous with it. The muscles are numerous, 

 especially about the head, some larva? having as many as 4,000 

 distinct muscles. They are generally of the cross-striped variety, 

 and give the insect great power and activity of movement. Some 



