LIFE -HISTORY. 155 



much moisture, is very destructive to hibernating insects, 

 especially hairy larvae, which suffer from fungoid diseases. 



Insects' eggs and pupae are almost insensible to cold. 

 Hairless larvae are most sensitive, especially at moulting 

 periods. Cold damp weather and cutting winds will then kill 

 them off in myriads. Storms blow larvae by thousands from 

 the crowns of trees. Uniformly warm years without heavy 

 rains therefore tend to produce great swarms of insects. Very 

 dry summers greatly reduce the food of some insects and 

 consequently their numbers. 



SECTION III. LIFE-HISTORY. 

 1. Generative Periods. 



Insects are distinguished from more highly organised crea- 

 tures by having generally a definite limitation to their duration 

 of life. The time which elapses between the egg and the 

 fresh production of eggs is termed a generation. These 

 may be single or annual, multiple, biennial or plurennial. 



A single generation occurs when an insect goes through all 

 its stages within twelve months (not, however, within a single 

 calendar year), and is by far the commonest. Thus, nearly all 

 Ijepidoptera have one generation in the year. A few Lepidop- 

 tera have more than one brood in the year, and, still more 

 rarely, the life of an individual species may extend through two 

 years, as Tortrix resinella, L., or even longer. 



In the case of a multiple generation, several broods are pro- 

 duced during twelve months, so that the respective stages, 

 eggs or larvae, of the same species may be found in different 

 months. A double generation is here commonest, as, for 

 instance, in the case of many bark-beetles and sawflies. 

 Many plant-lice produce five or more generations in,a single 

 summer, and the Bengal multivoltine silkworm completes a 

 generation every month, except during the period from 

 November to February. Three broods are sometimes pro- 

 duced within two years, for instance, by. Tomicus bidentatus, 

 Hbst., but such cases are rare. A plurennial generation 

 denotes that the insect takes more than one year for its full 



