39 6 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 kortensis, 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 (Forfeit/a 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 disagree- 

 able 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 (Ltniax) 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 man}' 

 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. 



