554 ScHAKFF — On the Slugs of Ireland. 



The Colours of Slugs. 



A good deal has been written in various Zoological works on tlie colours of 

 animals in general, and Poulton has recently published a most interesting work, 

 chiefly on the colours of Insects. He finds that a variety of causes influence the 

 production of colours, but that by far their most widespread use is to assist an 

 animal in escaping from enemies or in capturing its prey. 



The view that colour is of direct physiological value to slugs has been ably 

 argued by Simroth in the cases of Arion ater and Limax maxinms, and Eimer (6) 

 seems to support Leydig's view, that the darker colour of A. ater on the sea-coast 

 may be caused by the greater moisture of the atmosphere. However, I hope 

 I shall be able to show that neither of these views are altogether borne out by 

 facts. Cockerell (4), judging from some specimens sent to him from a mountain 

 in county Waterford, draws the conclusion that altitude influences the colour of 

 slugs, but this also, I believe, is not supported by sufficient evidence. 



I think that the colours of slugs in Ireland are at all ages, as a rule, protective. 

 Simroth (38) agrees with this as far as the smaller species are concerned, but he 

 excepts Limax maximus and Arion empiricorum {— ater\ because they are often 

 distinguished by strikingly vivid colours. 



He made numerous experiments with the latter species, kept it in a hot atmo- 

 sphere, and offered it to various birds as food, and finally came to the conclusion 

 that the colour in the brick-red variety is a warning colour. The object of a so- 

 called warning colour, I may say, is to render the animal as conspicuous as pos- 

 sible, in order to enable its enemies to easily learn and remember the animals 

 which are to be avoided on account of any noxious properties they may possess. 



Simroth (38), moreover, points out that all the very variable species of slugs, 

 such as Arion ater and Limax maximus, are darkly coloured both at theii* southern and 

 northern limits of range, the shores of the Mediterranean, and Scandinavia. He 

 supposes this to be due to a natural protection against heat and cold, i.e. he believes 

 that colour is of direct physiological value. 



We know, however, that dark colours absorb radiant heat easily, while light 

 colours do so with difficulty ; and it seems therefore surprising that Simroth (38) 

 did not take into account the fact that the white variety of A. ater in Scandinavia 

 is almost as common as the black [Esmark (7)]. Both on the Continent and in 

 Ireland the young of A. ater are brilliantly coloured during winter, and most 

 specimens darken at the approach of summer. 



If Simroth' s theory were correct, one would expect the slugs which are destined 

 to resist the severe cold of a Continental winter to be coloured dark. All young 

 specimens, however, whether they produce black or brown adults, are light-coloured 



