CHAPTER X
ECONOMIC AND TECHNICAL
A. Lichens as Food.
a. Food for Insects, etc. Some of the earlier botanists made careful observations on the important place occupied by lichens in nature as affording food to many small animals. In 1791 Jacques Brez[1226] wrote his Flore des Insectophyles, and in the list of food-plants he includes seven species of lichens. The “insects” that frequented these lichens were species of the genera Acarus (mites) and Phalena (moths). A few years later Persoon[1227] noted that lichens formed the main food supply of many insects, slugs, etc. Zukal[1228], quoting from Otto Wilde (Die Pflanzen und Raupen Deutschlands, Berlin, 1860), gives a list of caterpillars that are known to feed on and destroy lichens.
A very considerable number of small creatures feed eagerly on lichens, and traces of their depredations are constantly to be seen in the empty fruit discs, and in the cortices eaten away in patches so as to expose the white medulla. It has been argued by Zukal[1229] that the great formation of acid substances in lichens is for shielding them against the attacks of animals; Zopf[1230] on the contrary insists that these substances afford the plants no real protection. He made a series of experiments with snails, feeding them with slices of potato smeared with pure lichen acids. Many snails ate the slices with great readiness even when covered with bitter acids such as cetraric, or with those which are poisonous for other animals such as rhizocarpic and pinastrinic. The only acid they refused was vulpinic, which is said to be poisonous for vertebrates. The crystals of the acids passed unchanged through the alimentary canal of the snails, and were found in masses in the excreta. They were undissolved, but, enclosed in slime, their sharp edges did no damage to the digestive tract.
Stahl[1231] however upholds Zukal’s theory of the protective function of lichen acids against the attacks of small animals. Some few snails, caterpillars, etc., that are omnivorous feeders consume most lichens with impunity, and the bitter taste seems to attract rather than repel them; but many others he contends are certainly prevented from eating lichens by the presence of the acids. He proved this by soaking portions of the thalli of certain bitter species for about twenty-four hours in a one per cent. soda solution, which was sufficiently strong to extract the acids. He found that these treated specimens were in most cases preferred to fresh portions that had been simply moistened with water.
Even the omnivorous snail, Helix hortensis, was several times observed to touch the fresh thallus and then creep away, while it ate continuously the soda-washed portion as soon as it came into contact with it. Calcium Oxalate, on the other hand, formed no protection; omnivorous feeders ate indifferently calcicolous lichens such as Aspicilia calcarea and Lecanora saxicola, whether treated with soda or not, but would only accept lichens with acid contents, such as Parmelia caperata, Evernia prunastri, etc., after they had been duly soaked.
Experiments were also made with wood-lice (Oniscus murarius), and with earwigs (Forficula auricularia), and the result was the same: they would only eat bitter lichens after the acids had been extracted by the soda method. Stahl therefore concludes that acids must be regarded as eminently adapted to protect lichens which otherwise, owing to their slowness of growth, would scarcely escape extinction.
The gelatinous Collemaceae, as also Nostoc, the alga with which these are associated, are unharmed by snails, etc., on account of their slippery consistency when moist, which prevents the creatures from getting a foothold on the thallus. These lichens however do not contain acids, and if, when dry, they are reduced to powder and then moistened, they are eagerly eaten both by snails and by wood-lice. Peltigera canina, on account of a disagreeable odour it acquires on being chewed, is avoided to a certain extent, but even so it is frequently found with much of the thallus eaten away.
Hue[1232] in his study of Antarctic lichens, comments on the abundance and perfect development of the lichens, especially the crustaceous species, which cover every inch of rock surface. He ascribes this to the absence of snails and insects which in other regions so seriously interfere with the normal and continuous growth of these plants.