H. viridicollis ' ' 



214 ANNALS OF SCOTTISH NATURAL HISTORY 



Associates of H. gyllenhalii Associates of D. griseostriatus 



(continued]. (continued}. 



SKYE and EIGG. SKYE and EIGG. 



per cent. per cent. 



H. incognitas ~\ G. minutus 



A. arcticus 

 I. senescens 

 D. punctulatus 

 D. lapponicus 

 G. sulcatus 

 G. minutus 

 H. picicrus 

 P. nigroasneus 

 A. limbata 

 H. aquaticus 



From these lists it is seen that H. obscurus and A. 

 bipustiilatus are practically indifferent as regards loch or 

 pool habitat, while others such as H. pubesccns and tristis, 

 A. globuhis, etc., A. arcticus ; A. sulcatus, etc., are almost 

 confined to the one or the other. The composition of 

 either association will vary in different parts of the country, 

 new species coming in and others disappearing, and as 

 showing the subtle influence of climatic factors in giving 

 advantage to one species in one place and another in another 

 place, the cases of Hydroporus morio and Agabus arcticus 

 are very good illustrations. The former is undoubtedly a 

 member of the pool association, and so far as I know occurs 

 nowhere as a loch species. The scarcity of this species in 

 Eigg and in Skye, so far as my collecting went, was 

 probably due to insufficient elevation. The height above 

 sea-level, at which the species becomes common, no doubt 

 varies considerably in different localities, but is, I think, 

 usually somewhere near the 1000 ft. contour in the southern 

 part of Scotland. 



I have already pointed out that it replaces H. gyllenlialii 

 at the higher elevations. An interesting case of this change 

 occurred in Arran, where I was collecting in June 1906. 

 On Goatfell, climbing by way of Brodick Castle, one 

 comes to a more or less extensive peat moss at about 

 1000 ft. elevation. Here I collected in a number of pools, 

 and H. gyllenhalii was common ; it was in fact the dominant 



