CHAP, xiv.] Insect Enemies 311 



in the case of bark-beetles and moths, or technical, interfering 

 with the utilization of the timber for technical purposes, as in 

 the case of wood-wasps (Siricidae\ longicorns (Cerambycidae), 

 and certain Scolytidae ; but this would be met by the objection 

 that in such cases the damage depends on too many different 

 factors (species and age of tree, time of attacks, surrounding 

 local circumstances, &c.) to be convenient. Or classification 

 may be made with reference to the extent of the damage 

 usually done; but this would be a very unsafe guide, for 

 some of the most destructive species (e.g. Liparis monacha] 

 are often for many years seldom to be seen, until at last cir- 

 cumstances favour their development in such enormous masses 

 that they devastate woodlands over large tracts of country. 

 Or they may be grouped with reference to the age of the 

 crops generally attacked (seedling-growth, thickets, pole-forest, 

 tree-forest) ; yet this would also be unsatisfactory, as many 

 species are dangerous throughout all stages of the development 

 of the crop. Finally, and preferably, they may be classified with 

 reference to the portion of the tree on which they principally 

 effect injuries. Adopting this last-named biological method of 

 classification, and commencing with the roots, we may group 

 the most dangerous species as follows : 



Root-destroyers, including the grubs of the mole-cricket 

 (Gryllotatpa vulgaris) and the cockchafer (Melolontha vul- 

 garis), and the caterpillars of the Seed Owlet-moth (Agrotis . 



Wood-borers, comprising the larvae of wood-wasps (Siri- 

 cidae), cervicorn beetles (Cerambycidae), and certain Scolytidae 

 like Tomicus lineatus, T. monographus, and T. dispar, also the 

 caterpillars of certain moths like Cossus ligniperda and Zeuzera 

 aesculi. 



Bark-beetles, including most bark-beetles (Scolytidae) i.e. 

 most of the true bark-beetles (Bostrychini), and the cambial 

 beetles (Hylesinini\ and several weevils (Curculionidae\ which, 

 often in both active forms, either destroy the cambial layer of 

 the bark and the sapwood (e. g. most Bostrychini and several 



