DAMAGE CAUSED BY ANIMALS. Ill 



beetle, butterfly, wasp, fly, &c., according as it corresponds to one 

 or other of the seven orders into which insects are classifiable. As 

 soon as the imago has appeared from the chrysalis, the time of 

 swarming commences, during which copulation and . reproduction 

 .take place. In the case of most insects, the male dies soon after 

 copulation, and the female after the ovi-deposition, so that, as a 

 rule, the life-period of the perfect insects is comparatively very 

 short. But at the same time, many insects, especially beetles and 

 ; weevils, hibernate as imagines, arid have, of course, a relatively 

 1 long life-period during the complete stage of their existence. 



The time intervening between the deposition of the ovum and 

 i the swarming and reproduction of the ova by imagines is termed 

 the generation or generative period of the insect. The generation 

 may be: 



Multiple or manifold, when several generations are produced 

 within one year, as in the case of plant-lice (Aphidse) 

 and Ichneumonidfe. 



Two-fold or double, when two generations are produced in every 

 year, as in the case of bark- beetles (Scolytidte) and saw- 

 flies (Tenthredinidw). 



Single, simple, or annual, when one generation is produced each 

 year, as in the case of most butterflies and moths 

 (Lepidoptera). 



Biennial, biannual, or two-yearly when the insect requires two 

 complete years for its development, as in the case of wood- 

 wasps (Uroceridse), the Pine Kesin-gall Tortrix (Tortrix 

 resinella), and many long-horned beetles (Cerambycidee). 

 Multannual or polyannual, when three or sometimes four years 

 are necessary, as in the case of the cockchafer (Melolontha 

 vulgaris). 



Forest insects either do injury only as larvae, as in the case of 

 jail butterflies and moths, and as regards a portion of the other 

 insects, or only as imagines, as in the case of some beetles, or else 

 jduring both stages of development as Iarva3 and imagines, although 

 Ithis is usually confined to the case of beetles, e.g., cockchafer, Pine- 

 I beetle (Hylesinus piniperda), &c. 



The distribution of injurious forest insects is very extensive, 

 (both horizontally and vertically, although the number of the 

 species, and of the individuals in them, decreases in going north, 

 or with elevation above the sea-level. Conifers are in a much 



