4 INTRODUCTION. 



dilate in proper arteries and veins ; but is driven from the 

 fore part of the heart into the head, and thence escapes into 

 the body, where it is mingled with the nutritive juices that 

 filter through the sides of the intestines, and the mingled 

 fluid penetrates the crevices among the flesh and other in- 

 ternal parts, flowing along the sides of the air-])ipes, whereby 

 it receives from the air that influence which renders it fitted 

 to nourish the frame and maintain life. 



Insects are never spontaneously generated from putrid ani- 

 mal or veo-etable matter, but are produced from ego-s. A 

 few, such as some plant-lice, do not lay their eggs, but re- 

 tain them within their bodies till the young are ready to 

 escape. Others invariably lay their eggs where their yoimg, 

 as soon as they are hatched, will find a plentifid supply of 

 food immediately within their reach. 



Most insects, in the course of their lives, are subject to 

 very great changes of form, attended by equally remarkable 

 changes in their habits and propensities. These changes, 

 transformations, or metamorphoses^ as they are called, might 

 cause the same insect, at difl'erent ages, to be mistaken for 

 as many difl'erent animals. For example, a caterpillar, after 

 feeding upon leaves till it is fully grown, retires into some 

 place of concealment, casts ofl' its caterpillar-skin, and pre- 

 sents itself in an entirely difterent form, one wherein it has 

 neither the power of moving about, nor of taking food ; in 

 fact, in this its second or chrysalis state, the insect seems to 

 be a lifeless oblong oval or conical body, without a distinct 

 head, or movable limbs ; after resting awhile, an inward 

 struggle begins, the chrysalis-skin bursts open, and from the 

 rent issues a butterfly or a moth, whose small and flabby 

 wings soon extend and haixlen, and become fitted to bear 

 away the insect in search of the honeyed juice of flowers 

 and other liquids that suffice for its nourishment. 



The little fish-like animals that swim about in vessels of 

 stagnant Avater, and devour the living atoms that swarm in 

 the same situations, soon come to maturity, cast their skins, 



