LIMAX ARBORUM. 93 



Although naturally of nocturnal or crepuscular habit, yet during or after 

 showery weather the animals emerge from their retreats, indifferent as to 

 the time of day, and ascend to the tops of the highest trees, afterwards 

 resting immobile during the day in the cavities beneath the branches, the 

 armpits as it were, or within the dense tufts of Orthotrichum phyllanthemum 

 and other mosses with which the tree trunks are sometimes clothed. 



Some individuals, however, may for a length of time remain contentedly 

 within the sheltering cavities they frequent, and rest for a time partially or 

 even wholly submerged in the accumulated water. 



This species, though usually regarded as the mucus thread-spinner par 

 excellence, does not really spin so well or so readily as A. agrestis. 



According to Bouchard-Chantereaux, R. Standen, and others who have 

 studied the subject, it can, however, especially when young, and when not 

 gorged with food or overladen with moisture, spin well and easily, descend- 

 ing considerable distances without difficulty, and if necessary can reascend 

 by the same thread. 



As a consequence of its adaptation to rocks and trees, the staple food of 

 the species has become changed from fungi to the cognate lichens, and, 

 according to Simroth, although the contents of the stomach turn alcohol 

 green, this is not due to leaves and vegetables, but to the colouring matter 

 contained in the hchens. Malm, however, has seen it devouring the small 

 fungoid growths growing upon diseased places on oaks, birch, and other 

 trees, and many observers have stated that it also feeds upon the soft young 

 growth of bark and decaying wood. In confinement they will on occasion 

 prey upon each other, and Gain states that out of 1 85 different kinds of 

 food, they eat fairly freely only of lettuce-stalk, turnip, and cooked onions, 

 and that such mosses, lichens or fungi as he offered were left untouched. 



Parasites and Enemies. — In addition to the enemies of slugs in 

 general, L. arborum is, according to Fischer, liable to be attacked and 

 destroyed by a species of Carahus, which tears open the skin with its man- 

 dibles and feeds upon the viscera. 



It is also liable to be infested by an entozoan worm, which Van den 

 Broeck discovered living within the vitelline sac of the embryo. 



Fossil. — This species is reported from the Pleistocene beds of Moravia, 

 by Spiridion Brusina, and by Mrs. McKenny Hughes from the same depo- 

 sits at Barnwell Abbey in Cambridgeshire. Sandberger records it for 

 Britain from the Lower Pleistocene freshwater bed at West Runton, Nor- 

 folk, and also from our Upper Pleistocene brick earths. 



Variation. — Umax arhorum is subjected to some amount of variation 

 in the fundamental colour of the body, and also in the character and dis- 

 tinctness of the markings thereon. 



According to Simroth the body markings are constituted by the presence 

 of the inner, main, and outer bands, as in the true Limaces, but the outer 

 band is usually missing, and the main band when present is generally repre- 

 sented by a more or less faintly indicated line. 



The keel is very variable in its length and prominence, and is said to be 

 most strongly developed in elevated or mountainous regions, where it may 

 extend to almost four-fifths of the total length of the body ; this peculiarity 

 is invariably accompanied by the greater diffusion of the darker secondary 

 colouring, which may extend to such a degree that the whole body becomes 

 uniformly dark or even black, the slightly paler areas at the fore-part of the 

 ■body being due in this as in other species to the lime plentifully deposited 

 within the tissues of the body walls. 



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