DEVELOPMENT 



I2Q 



insect at hatching is the first instar, after the first moult the second instar, 

 and so on. 



Egg. The eggs of insects are exceedingly diverse in form. Com- 

 monly they are more or less spherical, oval, or elongate, but- there are 

 innumerable special forms, some 

 of which are quite fantastic. 

 Something of the variety of form 

 is shown in Fig. 208. As regards 

 size, most insect eggs can be dis- 

 tinguished by the naked eye; 

 many of them tax the vision, 

 however, for example, the ellip- 



FIG. 209. Three eggs of the cabbage butter- 

 fly, Pier is rapa. Greatly magnified, but all 

 drawn to same scale. 



tical eggs of Dasyneura legu- 



minicola, which are but .300 mm. 



in length and .075 mm. in width; 



the oval eggs of the cecropia moth, on the other hand, are as long as 3 



mm. 



The egg-shell, or chorion, secreted around the ovum by cells of the 

 ovarian follicle, may be smooth but is usually sculptured, frequently with 



ridges which, as in lepidopterous eggs, may serve 

 to strengthen the shell. The ornamentation of 

 the egg-shell is often exquisitely beautiful, though 

 the particular patterns displayed are probably of 

 no use, being incidentally produced as impressions 

 from the cells which secrete the chorion. Varia- 

 tions of form, size and pattern are frequent in eggs 

 of the same species, as appears in Fig. 209. 



Always the chorion is penetrated by one or 

 more openings, constituting the micropyle, for the 

 entrance of spermatozoa. 



As a rule, the eggs when laid are accompanied 

 by a fluid of some sort, which is secreted usually 

 by a cement gland or glands, opening into the 

 vagina. This fluid commonly serves to fasten 

 the eggs to appropriate objects, such as food 

 plants, the skin of other insects, the hairs of 

 mammals, etc. ; it may form a pedicel, or stalk, 

 for the egg, as in Chrysopa (Fig. 210); may surround the eggs as a gelat- 

 inous envelope, as in caddis flies, dragon flies, etc. ; or may form a cap- 

 sule enclosing the eggs, as in the cockroach. 



FIG. 210. Chrysopa, laying 

 eggs. Slightly enlarged. 



to 



