Wood-boring is also done by many of the bark-beetles as well as by Longicorns, e.g. Saperda in
Poplars and Willows, the young shoots of which often show characteristic swellings with lateral holes indicating the points of exit. From the external apertures comminuted wood, like saw-dust, is frequently ejected in quantity and betrays the presence of the insects. Certain wood-wasps (Sirex) and the larvae of moths (Cossus) also make large perforations in the wood of Willows and other trees, often destroying it completely. In the case of these larger borers, whose tunnels may be as broad as the little finger, the foul smell as well as abundant "saw-dust" betray the evil.
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 Lyonettia, 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 cuttting off of the vascular bundles and disturbance of the water supply only partly explain the premature fall.