GENERAL CONSIDERATIONS. 3 



of full development. But the law of increase seems to rest upon the supply of food : where 

 this is sure and constant, the increase is much greater than where it is precarious or un- 

 certain ; thus those insects which subsist on animal food are comparatively limited, while 

 the vegetable feeders are more numerous : this is in keeping with the harmony of nature, 

 and the original arrangements of the Creator. 



The eggs of insects are made up, essentially, of the same parts as the egg of vertebrated 

 animals. It consists of a yolk, with its germinal disc and germinal spot, which seem really 

 to be nothing else than a cell with its nucleolus. The yolk is enveloped in a proper mem- 

 brane ; and in order to defend and protect it, it is supplied with a hard outside envelope, 

 analogous to the eggshell of the common fowl ; but the shell, the white, and its tough 

 membrane, are not essential parts in the constitution of the egg. 



The eggs of insects, like all other eggs, obey the law of temperature. The young are 

 hatched at an earlier day if the temperature is increased, and the hatching is retarded in a 

 medium of low temperatiu-e. It therefore happens that man is often a gainer in consequence 

 of the warmth of spring, which brings forward insect life at an early day, only to perish 

 by the frosts whiclr soon succeed. The eggs of insects are endued with the power to resist, 

 or rather withstand, wide variations of temperature. It is evident that they sustain all the 

 ordinary changes of the climate, and that it often happens that they are exposed to a 

 temperature equal to 20*' below zero. The ability to withstand either extreme of tempera- 

 ture depends upon the conditions of the egg : if it has progressed considerably towards 

 the development of an embryo, its power to withstand those extreme changes is diminished. 

 The pupa resists but feebly those changes when it first assumes this state ; but when it 

 has nearly reached the period of completing its metamorphosis, it is surprising that it can 

 resist a high degree of heat. I exposed the pupaj of numerous silkworms, enclosed in a 

 bottle, to 212°, without injju-ing them. I was siuprised to find, not many days afterwards, 

 that hundreds of millers of both sexes had escaped from their cocoons. The warmth of the 

 sun, together with its light, is sufficient to destroy the vitality of the pupa when it first 

 assumes that state ; but the eggs of insects require air, or oxygen, as much as the perfect 

 animal : when enclosed in a vacuum, they lose their vitality. Oxygen is essential to the 

 development of the embryo, and hence the outer covering must admit its passage. 



The eggs of some insects seem to grow : they increase in size, probably by the absorption 

 of moisture from the atmosphere, or from the surface upon which they are laid. 



The covering of eggs varies exceedingly : in some it is beautifully sculptured ; in others 

 it is smooth and shining. These different characters, if they could be fully delineated, 

 would constitute important marks for the discrimination of species, for it is probable that 

 they differ in the species to which they belong. 



The most remarkable I'act in the history of insects, is their metamorphosis. The egg, as 

 has been observed already, is similar to the eggs produced by other classes of animals ; but 

 it never gives birth to a jjerfect insect, the immediate product of the egg being really as 



