lo INSECTS 



Matters now become a little more difficult; but if 

 we continue our examinations by comparing a lobster, 

 a spider and a beetle, we find that the latter has a 

 distinct head, separate from the rest of the body, while 

 in the others the head and middle section of the body 

 form a single mass known as a " cephalothorax. " Our 

 beetle also will be found to be breathing from the sides 

 of the body through a series of ringed tubes known as 

 trachea: and this makes it a Tracheate — an honor that it 

 shares with centipedes or myriapods. Finally, if we 

 persist in our policy of exclusion, we leave as true 

 insects only those forms that have no more than three 

 pairs of jointed legs, attached to the thorax or middle 

 region of the body, the body itself made up of no more 

 than thirteen obvious rings or segments, grouped into 

 three regions, the head, thorax, and abdomen, contain- 

 ing respectively one, three and nine segments. 



We are now ready to define an insect as an articulate, 

 arthropod, tracheate hexapod; but it will be equally 

 correct and much easier to say that it is a ringed ani- 

 mal, with six jointed legs, breathing by means of air 

 tubes or trachea;; this definition applying more particu- 

 larly to the adult stage, and only to the adult stage of 

 many of those having a complete metamorphosis. 



This method of breathing, by the bye, carries with 

 it a modification of the circulating system. The air 

 being carried in tubes to all parts of the body, there is 

 no need for lungs nor for any system of veins or arteries. 

 The blood simply flows about in the interstices of the 

 body cavity, kept in motion by a tube-like heart, divided 

 into chambers, which lies just under the back or dorsal 

 surface, and oxygen is taken up from the tracheaj any- 

 where in its course, while the products of digestion are 

 taken up from the specialized cells about the digestive 

 system. And, after all, the process of maintaining life 



