62 First Stage of Insects. 



There are four separate stages in the development of insects: 

 The egg, the larva, the pupa, and the imago. 



THE EGG. 



This is not unlike the same in higher animals. It has its 

 yolk and its surrounding white or albumin, like the eggs of 

 all mammals, and farther, the delicate shell, which is familiar 

 in the eggs of birds and reptiles. Eggs of insects are often 

 beautiful in form and color, and not infrequently ribbed and 

 fluted as by a master hand. The form of eggs is very various 

 — spherical, oval, cylindrical, oblong, straight, and curved 

 (Fig. 15, 6). All insects seem tcr be guided by a wonderful 

 knowledge, or instinct, or intelligence, in the placing of eggs 

 on or near the peculiar food of the larva, even though in many 

 cases such food is no part of the aliment of the imago. The 

 fly has the refined habits of the epicure, from whose cup it 

 daintily sips, yet its eggs are placed in the horse-droppings of 

 stable and pasture. 



Inside the egg wonderful changes soon commence, and their 

 consummation is a tiny larva. Somewhat similar changes can 

 be easily and most profitably studied by breaking and exam- 

 ining a hen'a egg each successive day of incubation. As with 

 the eggs of our own species and of all higher animals, the egg 

 of insects, or the yolk, the essential part — the white is only 

 food, so to speak— =-soon segments or divides into a great many 

 cells which soon unite into a membrane, the blastoderm, which 

 is the initial animal. This blastoderm soon forms a single 

 sack, and not a double sack, one above the other, as in our 

 own vertebrate branch. This sack, looking like a miniature 

 bag of grain, grows by absorption, becomes articulated, and 

 by budding out is soon provided with the various members. 

 As in higher animals, these changes are consequent upon heat, 

 and usually, not always, upon the incorporation within the 

 eggs of the sperm cells from the male, which enter tie eggs at 

 openings called micropyles. The time it takes the embryo 

 inside the egg to develop is gauged by heat, and will, therefore, 

 vary with the season and temperature, though in diiferent 

 species it varies from days to months. The number of eggs 

 which aa insect may produce, is subject to wide variation. 



