384 THE BIOLOGY OF INSECTS 



interesting adaptations to the conditions of the woodland 

 trees on which it feeds. The moths, which have reduced 

 mouth-parts so that they cannot feed, are seen in autumn ; 

 the males brown-winged with white spots flit around the 

 trees on the twigs of w^hich rest the fat wingless females 

 clinging to the silken cocoon surrounding the now empty 

 pupal coat. After pairing the female lays on the cocoon 

 her mass of eggs which are the wintering stage for this 

 species. The " tussock " caterpillars, hairy, bristly, and 

 conspicuously coloured, are hatched in spring. They are 

 active and well protected so that they can set forth on 

 migratory journeys, passing on occasion from tree to tree 

 or crossing roads or clearings in their search for fresh 

 feeding-grounds, their activity compensating for their 

 mother's inability to extend the range of the species. 



The timber of oak and other forest trees is tunnelled 

 and eaten by the larvae of many long-horn beetles and bark- 

 borers, as w^ell as by the caterpillars of various moths. Of 

 the latter the large black-headed, pink and yellow cater- 

 pillar of the Goat Moth (Cossus) is the most formidable, 

 attaining a length of three inches or more, taking three or 

 four years to complete its grovi1;h, and feeding voraciously 

 in the trunks and branches. The strong jaws of these 

 caterpillars enable them to " tunnel right into the heart of 

 the hardest wood," and as two hundred of them may occur 

 in a single trunk their activity can end only in the death 

 of the tree which harbours them. This result raises the 

 question of the relation between the growth and multiplica- 

 tion of the insect species and their food-supply. In extensive 

 forests where the mass of foliage and timber furnished by 

 the trees seems inexhaustible there is no practical limit to 

 the increase in number of an insect which has freedom to 

 migrate in some stage of its life-history. Where migration, 

 however, is restricted or impossible, excessive multiplication 

 of an insect may lead to the complete or partial starvation 

 of some individuals. This condition is shown in the dwarfed 

 specimens of some of our common tree-feeding geometrid 

 moths, Hyhernia defoliaria for example, developed in certain 



