4 BOARD OF AGRICULTURE. 



the wings originate from the upper part of the same rings that 

 bear legs below (fig. 4). Most insects breathe air by means of 

 a complicated system of finely branched air tubes, having a 

 sort of spiral spring to keep them open (fig. 3), which are con- 

 nected with valvular openings, called spiracles, along each side 

 of the body. Some species of spiders have respiratory cavities 

 that somewhat resemble lungs, and contain numerous thin 

 membranes, arranged like the leaves of a book. These are, 

 however, connected with openings in the lower side of the 

 body, and may be regarded as a peculiar modification of the 

 air tubes or tracheae found in other insects. In many flying 

 insects the air tubes expand in certain parts into large hollow 

 vesicles, which give greater lightness to their bodies. In all 

 insects we can distinguish three regions of the body: the 

 head, composed of several rings closely united together, and 

 bearing the organs of the mouth and senses as many pairs 

 as there are rings ; the thorax, composed of either three or 

 four rings, which bear as many pairs of legs, and sometimes 

 one or two pairs of wings above ; the abdomen, composed of 

 numerous rings, which are not consolidated, and generally bear 

 only the external reproductive organs ; but in the spiders they 

 bear the spinnerets, in many Iarva3 several pairs of fleshy legs, 

 in centipeds, etc., numerous legs, (figures 1 and 6), and in 

 some insects long, slender, feeler-like organs (figure 5). 



Insects are naturally divided into three great 

 groups or sub-classes,* founded on important 

 differences in their internal anatomy and the 

 arrangement of their external parts. 



I. HEXAPOD INSECTS. 



The highest sub-class contains the Hexapod 

 or six-legged insects, including all the flying 

 insects, and many that are destitute of wings. 

 In these the head, thorax, and abdomen, are 

 distinctly separated as three regions of the body. 



* According to some writers these divisions are called orders. 

 FIGURE 5. "Furniture-bug," or Shiner, (Lepisma), natural size. A smooth, 

 shining, neuropterous insect, covered with minute silvery scales, but destitute of 

 wings. It lives in houses among books, papers, clothing, or in furniture, etc., 

 eating them in various ways ; it is very fond of the paste of books, and will often 

 loosen wall-paper. 



