310 Studies in Forestry [CHAP. xiv. 



within the year ; two-fold or double in the case of bark-beetles 

 (Scolytidae] and Saw-flies (Tenthredinidae\ which produce two 

 generations in each year ; single, simple, or annual in the case 

 of most butterflies and moths (Lepidopterd), which yearly pro- 

 duce one generation ; biennial or two-yearly in wood-wasps 

 (Uroceridae\ the Pine Resin-gall Tortrix (Tortrix resinelld), 

 and many longicorn beetles (Cerambyddae] \ or finally mult- 

 annual or poly annual in the case of the cockchafer (Melolontha 

 vulgaris), which takes three, and sometimes four years to com- 

 plete its generative cycle. Occasionally two generations take 

 place in three years, as in the case of Tomicus bidens ; but this 

 is most probably only exceptional. 



Metabolic insects feed merely as larvae and mature insects, 

 and in some exceptional cases (e. g. Spanish Fly, Lytta vesica- 

 toria^ Pine weevil, Hylobius abietis, &c.) are only injurious to 

 woodland growth as imagines ; but with ametabolic insects the 

 nympha also requires food-supplies. In both groups the de- 

 structive power of the larvae is often excessive, as numerous 

 species daily consume many times their own weight of foliage. 



Classification of Injurious Forest Insects. 



The scientific or morphological classification of insects is of 

 comparatively little use to the sylviculturist, for whom a bio- 

 logical method of grouping them together is in many ways 

 preferable. Thus they may be classed with reference to the 

 species of trees on which they feed, as Spruce insects, Pine 

 insects, &c. ; this is, however, open to the objection that many 

 insects are not monophagous merely, but also polyphagous, or 

 even pantophagous, devouring everything (like Liparis monacha 

 when occurring in vast swarms). Or they may be classed with 

 reference to the nature of the damage they do, which may be 

 either physiological^ interfering with the imbibition, transpira- 

 tion, or assimilation of the food-supplies and nourishment, as 



