IV INTRODUCTORY. 



The abdomen contains the ovipositor ; sometimes, as in the 

 bees and wasps, forming a sting. In the spiders (Fig. 1), 



however, there are no an 

 tennae, and the second 

 maxillae, or labium, is 

 wanting. Moreover, there 

 are four pairs of legs. 

 The centipedes (Fig. 2, a 

 Myriopocl) also differ from 

 the rest of the insects 

 in having an indefinite 

 number of abdominal 

 rings, each bearing a pair 

 of legs. 



On examining the ar- 

 1. Spider (Tegenaria). rangement of the parts 



Within, we find the nervous cord, consisting of two chains of 

 swellings, or nerve-knots, resting upon the 

 floor or under side of the body ; and the heart, 

 or dorsal vessel, situated just under the skin 

 of the back; and in looking at living cater 

 pillars, such as the cut- worm, and many thin- 

 skinned aquatic larva?, we can see this long 

 tubular heart pulsating about as often as our 

 own heart, and when the insect is held against 

 its will, or is agitated, the rapidity of the pulsa 

 tions increases just as with us. 



Insects do not breathe as in the higher ani 

 mals by taking tjie afr into the mouth and filling 

 the lungs, but there are a series, of iiOies or 

 pores along the side of the body, as seen in the 

 grub. of the humble bee, through which the air 2. Centipede, 

 enters and is conveyed to every part of the body by an immense 

 number of air tubes, (pig. ^ tufees, or tracheae, in the cau- 



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