ARTHROPODS. CLASS INSECTS 117 



parts of the body. In the insects the blood serves almost 

 entirely to carry the food, and the oxygen is conveyed 

 through the animal by a remarkable contrivance found 

 only in the insects, the spiders, and a few related forms. 



114. Respiratory system. If we examine an insect, the 

 grasshopper for example, we find a number of small brown 

 spots on each side of the abdomen, each of which under a 

 magnifying-glass is seen to be perforated by a narrow slit. 

 Carefully opening the body, we find that each slit is in 

 communication with a white, glistening tube that rapidly 

 branches and penetrates to all parts of the animal. When 

 the body is expanded the air rushes into the outer openings, 

 on through the open tubes, and is distributed with great 

 rapidity to all the tissues of the body. In many insects 

 some of these tubes connect with air-sacs which probably 

 serve to buoy up the insect during its flights through 

 the air. 



115. Wingless insects (Thysanura). The simplest of all 

 insects are the fishmoths and springtails, relatively small 

 organisms covered with shining scales or hairs. The first 

 of these is occasionally seen running about in houses feed- 

 ing upon cloth and other substances, while the latter live 

 in damp places under stones and logs. They are without 

 wings, but are able to run rapidly and to leap considerable 

 distances. In addition to the ordinary appendages, the 

 abdomen bears what are perhaps rudimentary legs, a fact 

 which, together with their relatively simple structure, 

 strengthens the belief that the insects have descended 

 from centiped-like ancestors. 



116. Grasshoppers, crickets, katydids, etc. (Orthoptera). 

 Rising higher in the scale of insect life, we arrive at the group 

 of the cockroaches, crickets, grasshoppers, locusts, and 

 other related insects. Four wings are present, the first pair 

 thickened and overlapping the^ second thinner pair. The 

 latter are folded lengthwise like a fan, which is said to have 

 given the name Orthoptera (meaning straight-winged) to 



