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396 ECONOMIC AND TECHNICAL
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
omnivorous feeders ate
oxalate, on the other hand, formed no protection ;
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 1 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.
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 2
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
1 Hue 1915.
2
Zopf 1907.