INSECTS AND OTHER ORGANISMS 383 



viridana). These moths are flying at midsummer, and 

 after pairing lay on the twigs their eggs which are not hatched 

 until the succeeding spring. In May and early June the 

 small slaty caterpillars feed in thousands on the leaves, and 

 in many seasons the trees are completely stripped of foliage. 

 The caterpillars make for themselves cylindrical shelters, 

 each rolling round the tip of a leaf and fastening it by silken 

 threads ; in this little case the caterpillar has a house, the 

 walls of which are good to eat. A shock to the leaf may result 

 in the caterpillrr crawling out of its shelter and lowering itself 

 to the ground, playing a thread of silk out of its mouth ; by 

 means of thiis thread the larva can climb up again when 

 conditions once more become normal. The leaf-shelters 

 serve also to protect the pupae. An oak-wood where these 

 caterpillars have been present in multitudes — as they often 

 are in hot seasons — looks, with the brown and withered 

 remnants of its leafage in early summer, as if scorched by 

 fire. It has been pointed out by A. T. Gillanders (191 2) 

 and others that it is the pedunculate form of our native oak 

 which sufl"ers especially from the ravages of these insects, 

 the sessile variety being comparatively immune, perhaps 

 because coming earlier into leaf it can better afford to pay 

 toll to the caterpillars. Such conditions vary in different 

 areas, and it appears generally that trees suffer most 

 severely when their sprouting season coincides with that 

 of the hatching of the insect larvae which devour them. 

 The larvae of Tortrix viridana^ though usually restricted 

 to oak, can feed on the leaves of other trees ; ash, beech, 

 lime, and hazel, for example. They have been observed 

 to forsake oaks whose leaves they had completely devoured 

 before they had attained full growth, and to migrate 

 to some other trees. The foliage of oaks is also eaten 

 by caterpillars which can live and flourish on a variety 

 of deciduous trees ; the Buff-tip {Pygaera hucephala) and 

 the Vapourer {Orgyia antiqua), for example. There is 

 obvious advantage for an insect that is adaptable in its 

 feeding habits and not absolutely dependent on one plant 

 only for its sustenance. The *' Vapourer " has some 



