Snails do not eat lichens when they are dry and hard, but on damp or dewy nights, and on rainy days, all kinds, both large and small, come out of their shells and devour the lichen thalli softened by moisture. Large slugs (Limax) have been seen devouring with great satisfaction Pertusaria faginea, a bitter crustaceous lichen. The same Limax species eats many different lichens, some of them containing very bitter substances. Zopf[1233] observed that Helix cingulata ate ten different lichens, containing as many different kinds of acid.
Other creatures such as mites, wood-lice, and the caterpillars of many butterflies live on lichens, though, with the exception of the caterpillars, they eat them only when moist. Very frequently the apothecial discs and the soredia are taken first as being evidently the choicest portions. All lichens are, however, not equally palatable. Bitter[1234] observed that the insect Psocus (Orthoptera) had a distinct preference for certain species, and restricted its attention to them probably because of their chemical constitution. He noted that in a large spreading thallus of Graphis elegans on holly, irregular bare spots appeared, due to the ravages of insects—probably Psocus. In other places, the thallus alone had been consumed, leaving the rather hard black fruits (lirellae) untouched. In time the thallus of Thelotrema lepadinum, also a crustaceous lichen, invaded the naked areas, and surrounded the Graphis lirellae. The new comer was not to the taste of the insects and was left untouched.
Petch[1235] says that lichens form the staple food of Termes monoceros, the black termite of Ceylon. These ants really prefer algae, but as the supply is limited they fall back on lichens, though they only consume those of a particular type, or at a particular stage of development. Those with a tough smooth cortex are avoided, preference being given to thalli with a loose powdery surface. At the feeding ground the ants congregate on the suitable lichens. With their mandibles they scrape off small fragments of the thallus which they form into balls, varying in size from 1·5 mm. to 2·5 mm. in diameter. The workers then convey these to the nests in their mandibles. It would seem that they carry about these balls of food, and allow the ants busy in the nest to nibble off portions. Lichen balls are not used by termites as fungi are, for “gardens.”
Other observations have been made by Paulson and Thompson[1236] in their study of Epping Forest lichens: “Mites of the family Oribatidae must be reckoned among the chief foes of these plants upon which they feed, seeming to have a special predilection for the ripe fruits. We have had excellent specimens of Physcia parietina spoiled by hidden mites of this family, which have eaten out the contents of the mature apothecia after the lichens have been gathered. One can sometimes see small flocks of the mites browsing upon the thallus of tree-dwelling lichens, like cattle in a meadow.” The Oribatidae, sometimes called beetle-mites, a family of Acarinae, are minute creatures familiar to microscopists. They live chiefly on or about mosses, but Michael[1237] is of opinion that a large number frequent these plants for the fungi and lichens which grow in and about the mosses. In Michael’s Monograph of British Oribatidae, four species are mentioned as true lichen-lovers, Leiosoma palmicinetum found on Peltigera canina and allied species; Cepteus ocellatus and Oribata parmeliae which live on Physciae, the latter exclusively on Physcia (Xanthoria) parietina; and Scutovertes maculatus which confines itself to lichens by the sea-shore. Another species, Notaspis lucorum, frequents maritime lichens, but it is also found on other substrata; while Tegeocranus labyrinthicus, though usually a lichen-eating species, lives either on mosses or on lichens on walls. Zopf[1238] reckoned twenty-nine species of lichens, mostly the larger foliose and fruticose kinds, that were eaten by mites. Lesdain[1239] in his observations on mite action notes that frequently the thallus round the base of the perithecia of Verrucaria sp. was eaten clean away, leaving the perithecia solitary and extremely difficult to determine.
Fig. 126. 1, Tetranychus lapidus, enlarged; 2, Verrucaria calciseda with eggs in situ, slightly enlarged; 3 and 4, eggs attached to lichen fruits, much magnified (after Wheldon).
J. A. Wheldon[1240] found the eggs of a species of mite, Tetranychus lapidus, attached to the fruits of Verrucaria calciseda, Lecidea immersa and L. Metzleri, calcicolous lichens of which the thallus not only burrows deep down into the limestone, but the fruits form in shallow excavated pits ([Fig. 126]). The eggs of this stone mite are found fairly frequently on exposed limestone rocks, bare of vegetation, except for a few crustaceous lichens. “There is usually a single egg, rarely two, in each pit apparently attached to the old lichen apothecium. The eggs are very attractive objects under a lens; they measure ·5 mm. in diameter, and are disc-like with a central circular depression from which numerous ridges radiate to the circumference, like the spokes of a wheel. When fresh, they have a white pearly lustre, becoming chalk-white when dry and old.” Wheldon’s observations were made in the Carnforth and Silverdale district of West Lancashire.
A minute organism, Hymenobolina parasitica[1241], first described by Zukal and doubtfully grouped among the mycetozoa, feeds, in the plasmodium stage, on living lichens. The parasitic habit is unlike that of true mycetozoa. It has recently been recorded from Aberdeenshire.
b. Insect mimicry of Lichens. Paulson and Thompson[1242] give instances of moth caterpillars, which not only feed on lichens, but which take on the coloration of the lichens they affect, either in the larval or in the perfect moth stage. “One of the most remarkable examples of this protective resemblance to lichens is that of the larva of the geometrid moth, Cleora lichenaria, which feeds upon foliose lichens growing upon tree-trunks and palings, and being of a green-grey hue, and possessed of two little humps on many of their body-segments, they so exactly resemble the lichens in colour and appearance as to be extremely difficult of detection.” Several instances are recorded of moths that resemble the lichens on which they settle: perfect examples of such similarity are exhibited at the Natural History Museum, South Kensington, where Teras literana, Moma orion, and other moths are shown at rest on lichen-covered bark from which they can hardly be distinguished.