160 PROTECTION AGAINST INSECTS 



course of five generations, and in one summer ten such 

 generations may occur. The average number of eggs of the 

 commoner injurious insects varies between 100 and 200 

 (Katzeburg). Warm, dry weather and plenty of food, and 

 breeding-places, such as diseased wood, or branches broken 

 by snow, are very favourable to prolific multiplication. Under 

 such circumstances, insects that are generally of limited 

 numbers may appear locally in destructive swarms. Besides 

 the case of bark-beetles, such an abnormal increase is frequently 

 met with in the case of the grey-tussock moth (Dasycliira 

 pudilmnda, L.), allied to that species which ravages the Indian 

 sal forests. 



One favourable season is not usually sufficient to produce 

 an insect-calamity, but two or more successive favourable 

 springs and summers. 



Most insects are solitary, but many, such as bees, ants, 

 certain kinds of wasps, and termites, are social and have a 

 wonderful organisation, framed on the principle of subdivision 

 of labour. The larvae of some moths are also gregarious. 



SECTION V. USEFUL FOREST INSECTS. 



Carnivorous insects attack other species in various ways, and 

 have been subdivided by Eatzeburg as follows : 



Predatory insects follow and kill other insects in every stage. 

 Ground- and tiger-beetles belong to this group. 



Predaceous parasitic insects, like the former class, seize other 

 insects, but carry them to their nests, where their own larvae 

 feed on them. Such are the fossorial wasps (Sphegidae or 

 Crabronidae) ; they first stin^ their prey, but without killing 

 them, and thus render them inert. 



Finally, parasitic insects wound the larvae, pupae, or even the 

 eggs of other insects with their fine ovipositors to lay eggs in 

 them. The larvae hatching from these eggs feed on the juices 

 of their hosts. Ichneumon-wasps and some flies (Tachinae) 

 are examples of this group. 



A classification of these insects according to their utility is 

 scarcely possible ; of predatory insects, the largest are generally 

 the most useful, especially species of Carabus and Calosoma. 



