170 



AN INTRODUCTION TO ENTOMOLOGY 



The laying of eggs in compact masses, however, is not correlated, 

 in most cases, with gregarious habits of the larva?. The water- 

 scavenger beetles, Hydrophilidas, make egg-sacks out of a hardened 

 silk-like secretion (Fig. 188); the locusts, Acridiidas, lay their eggs in 



oval masses and cover them with a 

 tough substance; the scale-insects 

 of the genus Pulvinaria excrete a 

 . \ large cottony egg-sac (Fig. 189); 





.> 



Fig. 187. Egg-mass of 

 squash-bug. 



the 



Fig. 188. Egg-sacof Hvdrophilus 

 (After Miall). 



the eggs of the praying mantis are laid in masses and overlaid with 

 a hard covering of silk (Fig. 190); and cockroaches produce pod-like 



egg-cases, termed 



ootheca, each 



containing many 



eggs (Fig. 191). 

 Among the 



more remarkable 



Fig. 189. Pulvinaria innnmerabilis, females on 

 grape with egg -sacs. 



of the methods of 



caring for eggs is 



that of the lace-winged flies, Chrysopa. These insects place 



each of their eggs on the summit of a stiff stalk of hard silk 



(Fig. 192). 



Duration of the egg-state. In the life-cycle of most insects, 

 a few days, and only a few, intervene between the laying of 

 an egg and the emergence of the nymph, naiad, or larva from 

 it. In some the duration of the egg-state is even shorter, the 

 hatching of the egg taking place very soon after it is laid, or 

 even, as sometimes in flesh-flies, before it is laid. On the 

 n " n ^ other hand, in certain species, the greater part of the life of an 

 tis. individual is passed within the egg-shell. The common 

 apple-tree tent-caterpillars, Clisiocampa americana, lays 

 its eggs in early summer; but these eggs do not hatch till the fol- 

 lowing spring; while the remainder of the life-cycle occupies only a 



190, 



Fig. 

 -Egg- 

 mass 

 of a 

 pray- 



