AQUATIC COLEOPTERA OF THE OUTER HEBRIDES 91 



district, and in such an area as Lewis a newly dug pond 

 should be first and permanently occupied by the peaty pool 

 species, especially seeing that most of the species of this 

 community freely occupy ponds in other parts of the British 

 islands. Yet although we find members of this community 

 well represented in such a pond, species from the lochs also 

 occur there. In the ballast-holes dug in the boulder clay 

 along the roadsides one could trace the gradual disappear- 

 ance of these latter as the Sphagnum and other bog-plants 

 invaded and ultimately completely occupied the holes. In 

 the newest holes, dug only a few weeks, one or more of four 

 species seemed to be always the first arrivals — H. gyllenhaln, 

 H. pubesce?is, A. bipiistulatus, H. viridicollis — all species 

 typical of the peaty pool community. 



Altogether 32 species occurred in the twenty-three ballast- 

 holes examined, which were in all stages of degeneration 

 from a weedless clayey pit to a Sphagnum pool. Naturally, 

 the most abundant species on the island had the best chance 

 of overrunning these pits, so that in the accompanying table 

 showing the percentage of occurrence of all the species in 

 the pits these are to be found highly placed. If, however, 

 we compare the number of occurrences of these species in 

 the pits with the total number of their occurrences in the 

 island, we find that several of the loch species are high in the 

 list — vide column 2 of the accompanying table : — 



The Water-Beetles of the Ballast-holes. 



Agabus bipustulatus 

 Hydroporiis pubescens 

 Hydroporus gyllenhalii 

 Helophorus viridicollis 

 Hydrobius picicrits 

 Philhydrus fuscipennis 

 Limnebiiis truncatellus 

 Hydroporus obscuriis 

 Haliplus lineatocollis 

 Hydroporus nigrita 

 Agabus sturmii 

 Agabus chalconotus 

 Rhantus bistrialiis . 



23 

 20 



}■■ 



10 



8 



Hydroporus iiicogtiiius 

 Laccobius minutus 

 Agabus sturmii . 

 Haliplus welmckei 

 Dytiscus punctulatus 

 Acilius sulcatus . 

 Haliplus ruficollis 

 Colymbeles fuscus 

 Hydrobius picicrus 

 Ilybius a-nescens . 

 Limnebius truncatellus 

 Helophorus brevipalpis 

 Helophorus viridicollis 



% 

 - 100 



1 



50 



43 



42 



41 



40 

 38 

 37 

 36 



