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 trachez 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 larvee 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 Fig. 5. 
groups or sub-classes,* founded on important 
differences in their internal anatomy and the 
arrangement of their external parts. 
I.—Hexapop 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. 
Fieurn 5.—‘Furniture-bug,” or Shiner, (ZLepisma), natural size. A smooth, 
shining, neuropterous insect, covered with minute silvery scales, but destitute of 
wings. It lives in houses among books pipers clothing, or in furniture, ete., 
eating them in various ways PiPi# rere fondro cili® paste of books, and will often 
loosen wall-paper. 
