34 



AN ECONOMIC ENTOMOLOGY. 



occurs on each body segment ; but as a matter of fact some of 

 the segments, especially in the adult, lose them, the insect de- 

 pending upon a more limited number as the main trunks become 

 enlarged. The insect blood requires oxygen, as in the higher 

 animals, and, as there are no lungs to which the blood can be 

 carried, it is necessary that the air itself should be taken to all 

 parts of the body, and this is accomplished by means of tubes 

 called trachecs. Trachea occur in insects, in spiders, and in 

 myriapods, all of which are, therefore, called ' ' Tracheates. ' ' 

 Spiders have eight legs instead of six as in the true insects, and 

 have the head and thorax combined in one piece or cephalo- 

 thorax. They have also a decided tendency to the formation of 

 lung-sacs. The ''myriapods" or "centipedes" are more worm- 

 like in structure, and are furnished with many legs, — one or two 

 pairs on each body segment, — sometimes exceeding one hundred 

 in number. 



One tracheal trunk or tube starts from each spiracle, and be- 

 gins at once to divide and subdivide, sending its ramifications 

 to all parts of the body, even to the tips of all appendages, so 

 that air permeates every part of the insect, and tracheal tissue is 

 present everywhere. The structure of these tubes is interesting : 

 they consist of a membranous cylinder, stiffened by chitinous 

 rings or spirals, in appearance very much like the closely-coiled 

 springs in automatic shade-rollers. This gives rigidity to the 

 tubes, keeping them always open, while at the same time it 

 allows the utmost freedom of motion. A series of longitudinal 

 tubes on each side of the body unite the main trunks from each 

 spiracle, and in some cases there are transverse tubes joining 

 the longitudinal tracheae. The entire system is thus connected 

 in one or two ways, and at a pinch an insect is able to get its air 

 supply from one spiracle only and from almost any part of its 

 body. 



Where so many openings into the insect body exist there 

 must, necessarily, be some arrangement for protecting them and 

 preventing the entrance of foreign material. There is, therefore, 

 a great variety of sifting structures, their character depending 

 largely upon the habits of the insects. There may be a series 

 of teeth or spurs on each side of a narrow, irregular slit ; a series 

 of hairs or bristles crossing in front of the opening ; a covering 



