292 NATURAL SCIENCE. April, 



dark, those of the former species approximating to insects of the 

 spring brood, which are normally dark. He has also succeeded in 

 producing very dark examples of Lasiocampa qnercns, by cooling the 

 pupae, and very light specimens of its northern dark variety, calluna, 

 by heating the pupas (lo). In these and other species, there seems 

 no reason to doubt that low temperature applied to the pupas does 

 induce darkening of the imagine. On the other hand, the upholders 

 of the moisture theory deny that there is such a general darkening of 

 lepidoptera in arctic latitudes as should exist if low temperature be 

 the main cause of melanism, and insist that the climate of the dis- 

 tricts where melanism prevails is often mild and equable. This is 

 true, but the summer temperature of these regions is lower than that 

 of southern England, and much lower than that of Continental 

 Europe. It is very possible that moisture may be a true cause of 

 darkening; careful experiment might go far to settle the matter. 

 It seems clear, however, that moisture or cold, or both, tend to the 

 production of melanic varieties, and that these varieties, when pro- 

 duced, are specially favoured by natural selection. 



Such observations have been made on few insects except 

 Lepidoptera. There are, however, instances of melanism in Coleop- 

 tera. My friend, Mr. H. K. Gore Cuthbert, informs me that in 

 Irish specimens of the beetles, Badistev bipustidatus, Dromius quadvino- 

 iatus, Nehria complanata, and Agabus guttatus, the pale spots and 

 markings are generally smaller than in English examples, A small 

 ground beetle, Calathus tnelanocephalus, with red pronotum, has, in 

 mountainous regions, the pronotum clouded with black (var. nuhigena). I 

 have lately received this variety (which is considered a rarity in England) 

 from the Aran Isles, in*Galway Bay, but little above sea level, and 

 also from the Faroes. This is a good indication of the similar 

 darkening effects produced in mountain, western, and northern regions.' 



Specimens of the common wheel-web spider, Epcira diadeinafa, 

 which I have received from the Aran Isles, also show the same 

 darkening tendency. The large conspicuous white spots in form of a 

 cross, so characteristic of typical examples of this spider, are reduced 

 in these western individuals to small dots. A similar, but less 

 marked, variation is to be noted in specimens from co. Donegal. 



Observations on this spider have furnished me also with a few 

 facts bearing on the subject discussed in the earlier part of this 

 review. The ground colour of the abdomen is well-known to vary 

 from light brown through various shades of brown and red to a nearly 

 black hue. Among granite rocks I have always found the latter 

 variety ; as it crouches in its retreat in a crevice of the rock, 

 its black and white markings harmonise admirably with the mica 

 and white felspar of the granite ; but among herbage, the red and 

 brown varieties are the more common. Whether this correspondence 



^ On the other hand, the black carrion beetle, Silpha atrata, is represented in 

 Ireland by the var. subrotundata, which is generally of a brown colour. 



