22 INSECTS. 



cells, to which, in the vertebrata, the blood is brought 

 for aeration by the heart ; in insects the air is carried in 

 tubes, or air passages, to every part of the body. Down 

 each side of the insect runs a large air tube, commu- 

 nicating by short tubes running out of it with the 

 breathing holes, or spiracles, which lie along each side of 

 the abdomen. From these two main passages shoot 

 little clusters of smaller tubes, which ramify again and 

 again, until their minute branches are to be found in 

 every part of the body. Thus, while the vertebrate 

 animal inhales only through the double passage ter- 

 minating in the mouth (or, as in fish and reptiles, in the 

 gills), the insect breathes through a series of openings 

 in its abdomen ; and the air, instead of being carried to 

 the chambers in which the blood visits it, is carried to 

 the blood in every part of the body. 



It now remains to define the especial character of the 

 true insect, and to show in what it differs from other 

 animals not separated from it by barriers of so decided a 

 nature as those just mentioned. 



The name insect is now much more restricted in its 

 use than it formerly was. Spiders, centipedes, scorpions, 

 woodlice, shrimps, and even lobsters, have been included 

 under the term, but are now considered as belonging to 

 other orders. The true insect, as at present received, is 

 an animal arriving at maturity through a series of moults, 

 or metamorphoses. It is without internal skeleton, 

 having the body enclosed in a jointed covering, and is 

 composed of three principal parts head, thorax, and 

 abdomen ; the head bearing antennee, the thorax bearing 

 six jointed legs, and (with certain exceptions) four wings. 



The reader will bear in mind that this definition applies 

 to the perfect insect only, and that caterpillars, footless 



