WHAT IS AN INSECT? V 



dal appendage of the larva of a dragon fly). These air tubes 

 are everywhere bathed by the blood, by which the latter becomes 

 oxygenated. 



Indeed the structure of an insect is entirely different from 

 that of man or the 

 quadrupeds, or any 

 other vertebrate ani- 

 mal, and what we 



call head, thorax, 3 - Caudal appendage of larva of Agrion. 

 abdomen, gills, stomach, skin, or lungs, or jaws, are called so 

 simply for convenience, and not that they are made in the same 

 way as those parts in the higher animals. 



An insect differs from a horse, for example, as much as a 

 modern printing press differs from the press Franklin used. 

 Both machines are made of iron, steel, wood, etc., and both 

 print; but the plan of their structure differs throughout, and 

 some parts are wanting in the simpler press which are present 

 and absolutely essential in the other. So with the two sorts 

 of animals ; they are built up originally out of protoplasm, or 

 the original jelly-like germinal matter, which fills the cells com- 

 posing their tissues, and nearly the same chemical elements 

 occur in both, but the mode in which these are combined, the 

 arrangement of their products : the muscular, nervous and skin 

 tissues, differ in the two animals. The plan of structure, namely, 

 the form and arrangement of the body walls, the situation of 

 the appendages to the body, and of the anatomical systems within, 

 i. e., the nervous, digestive, circulatory, and respiratory systems, 

 differ in their position in relation to the walls of the body. 

 Thus while the two sorts of animals reproduce their kind, eat, 

 drink and sleep, see, hear and smell, they perform these acts by 

 different kinds of organs, situated sometimes on the most oppo- 

 site parts of the body, so that there is no comparison save in 

 the results which they accomplish; they only agree in being 

 animals, and in having a common animal nature. 



