570 PHYSIOLOGY. 



in comparison with their collective numbers. The reason of this is, 

 that many young larvae, if they were disclosed from the egg early in 

 the spring, would not find their necessary food ; and other eggs are depo- 

 sited in substances which are found only in the summer, as leaves of 

 plants, the larvae of other insects, &c. But we may maintain as a 

 general fact that those insects especially hybernate as eggs, which deve- 

 lope in the course of one year two or three generations, as we have 

 already mentioned of the plant lice ; most of these do not hybernate, 

 except as eggs. Those also hybernate as eggs whose development 

 as perfect insects takes place very late in the year, as most of the 

 Orthoptera, for instance, Acheta, Gryllotalpa, Locusta, Gryllus. 

 They can therefore pair only very late in the year, so that the 

 young, should any be disclosed, would no longer find food during the 

 same year. The females consequently excavate holes in the earth 

 and bury their eggs therein, when they die, as the males have before 

 done. The same is the case with the second autumnal generation ; in 

 this instance also the larvae would no longer find food, the eggs are 

 therefore disclosed in the spring. Amongst the Lepidoptera, Gaslro- 

 tropacha, Neuslria, and Liparis dispar hybernate as eggs. In each 

 case the mother deposits her eggs on the twigs, and never on the 

 leaves, of such trees the leaves of which the caterpillars feed on. 

 Geometra grossulariata, on the contrary, is disclosed the same year, 

 and hybernates as a larva. It is remarkable that only those Lepidoptera 

 whose caterpillars live upon perennial plants hybernate as eggs or as 

 caterpillars, and the rest chiefly as pupae, a phenomenon which even 

 Roesel observed, and which finds its natural cause in that the leaves of 

 all annual plants appear later than those of perennial ones. The pupae 

 consequently are developed later than the eggs, because the imago finds 

 food only in the flowers, whereas the caterpillars find it in the leaves. 

 The degree of cold that exposed eggs can endure is not trifling. 

 Spallanzani placed the eggs of the silkworm in a temperature of 

 38 Fahr., and yet larvae were disclosed from them, as well as from 

 others that had been exposed to a temperature of 56 Fahr. 



Many insects are found as larvae also during the winter, namely, all 

 such which pass more than one year in this state, as, for instance, 

 Melolontha vulgaris, Orycles nasicornis, Lucanus cermis, the large 

 capricorns, many Elalcrs, Bnpresies, and a multitude of Lepidoptera, 

 namely, Euprepia malronula, Cossus lignipcrda, &c. Many of them 

 are said to form a sort of dwelling, for example, Cossus, in which they 



