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 
