198 BIOLOGY FOR BEGINNERS 



Internal Structure. The internal structure is very complex, some 

 insects having over twice as many separate muscles as we have in 

 our whole body. The digestive system is well developed, there 

 being salivary glands, a crop, stomach, digestive glands, intestine, 

 ancf rectum. 



Excretion is provided for by a large number of thread-like tubes 

 at the junction of stomach and intestine. Circulation, while not 

 entirely inclosed in blood vessels, is controlled by a six-chambered 

 heart on the dorsal (upper) side, from which the light-colored blood 

 is forced toward the head and around throughout the tissues, in 

 contact with the air tubes. 



Respiration. The respiratory system is highly developed. It 

 consists of an extensive network of air tubes called tracheae, there 

 being six main tubes running lengthwise, from which branch air 

 sacs and smaller tracheae reach every tissue in the body. 



These tracheae open by means of the spiracles, which are tiny 

 holes, protected from dust by hairs, found on the abdomen (8 pairs) 

 and on the thorax (2 pairs). By alternate expansion and contrac- 

 tion of the segments at the rate of sixty-five per minute air is 

 pumped in and out of these spiracles, and circulates through the 

 tracheae, where, by osmosis, the oxygen from the air and carbon 

 dioxide from the blood exchange places. A peculiar feature of the 

 insect respiration is the fact that the air goes to the blood by means 

 of the tracheae instead of the blood going to the air in capillaries 

 as in our lungs. Another curious fact is that the veins of the wings 

 are probably tracheae, adapted for the function of support rather 

 than respiration. 



Nervous System. The nervous system of insects reaches a 

 higher degree of development than that of any invertebrate group 

 and a comparison of the types studied can well be made at this time. 



The protozoan cell received its impressions directly, it responded 

 throughout, to heat, light, contact, and possibly other stimuli, 

 but vaguely and without the aid of any nervous tissue. 



In animals like the hydra, certain groups of cells seem more 

 sensitive than others to external influences and also appear to 

 control the activities of the animal. These are the simplest ex- 



