Excavations in wood are by no means caused only by insects: several of the larger Hymenomycetes--_Stereum_, _Thelephora_, _Polyporus_, etc.--tunnel the timber in characteristic ways and often after a fashion very suggestive of insects. They usually obtain access through fractures. _Tunnels_ in leaves are invariably due to the activity of miners belonging to the smaller moths and beetles--_e.g._ _Tinea_, _Orchestes_, etc.--the larvae of which eat out the mesophyll but leave the covering epidermis or cuticle untouched, and since the insect bores forwards only, in an irregular track, and leaves its excrement in the winding passage, the effect is very characteristic. Whitish leaf tunnels in Peas are excavated by _Phytomyza_. Characteristic foxy-red tunnels are mined in the leaves of Apples by _Lyonetia_, _Coleophora_, etc. _Falling of fruit_, of Apples, Plums, Apricots, etc., before they are ripe, is frequently due to insects, of which the various species of _Grapholitha_ or _Carpocapsa_ are conspicuous: the fallen fruits show a small hole leading by a labyrinth of passages to the "core" or "stone," and in which the grub and its excrement are visible. The cutting off of the vascular bundles and disturbance of the water supply only partly explain the premature fall. _Pith-flecks_ are minute brown specks or patches found in the wood-layers of many trees, and consist of dead parenchymatous thick-walled cells, reminding one of the structure of pith. They are explained as due to the borings of minute insects, _Diptera_ or Beetles, the larvae of which pierce the cortex and phloem and bore their way into the cambium. The latter then occludes the tunnels by filling them up with cells, and continuing its wood-forming activity gradually buries them deeper and deeper in the wood. Such pith-flecks are common in Willow, Birch, Alder, _Sorbus_, etc. It is possible that they may be due to other causes also in other trees. _Erosions_ or _irregular wounds_ on leaves are caused by large numbers of grubs and caterpillars and other insects, such as earwigs, as well as slugs, snails, and other animals; but it must by no means be assumed that all marginal leaf wounds, for instance, are caused by animals, since many fungi which rot the tissues, as explained below (p. 208), also cause such erosions, the putrescent parts falling out--_e.g._ the Potato disease. _Skeleton leaves_ frequently result from the ravages of caterpillars, which leave the coarser ribs and veins untouched, but much finer skeletons with the minute veins almost intact may be found on plants infested with certain insects--_e.g._ _Selandria_ on Cherries. Skeletonised patches on Cherry leaves, often pink or brown-pink, are eaten out by this grub.