302 Proceedings of the Royal Society of Edinburgh. [Sess. 
XXX. — Hydrachnidae collected by the Lake Survey. By Mr Wm. 
Williamson. Communicated by Sir John Murray, K.C.B. 
(MS. received July 2, 1907. Read July 15, 1907.) 
The systematic study of the Hydrachnidae, or water-mites, of Scotland 
appears to have been entirely neglected until within the last few years, 
and, so far as I am aware, the only communications dealing with 
Hydrachnids found in Scotland are two noted in the list of literature 
cited. 
Of the life-history of Hydrachnids we have only a limited knowledge, 
and that may in time be modified as further information is acquired. At 
present the life-cycle appears to be in four definite stages — the egg, the 
larva, the nymph, and the imago. When an egg is hatched, a six-legged 
larva emerges, and apparently its main function is to find a suitable host 
to which it may attach itself. I have hatched out at various times the 
larvae of different species, but never succeeded in keeping them alive more 
than a few days, doubtless because a host was not present in the hatching 
tube. Species of Dytiscus, Notonecta, Corixa, Ranatra, and even dragon- 
flies have been found with these larvae firmly attached. During the period 
of attachment the larva undergoes a change, and finally emerges as an 
eight-legged nymph. Although I have been able to keep nymphs alive 
for several months, I have not yet succeeded in keeping them alive 
during the whole of their existence as nymphs, and have not therefore 
been able to determine the duration of the nymphal period. At the 
end of this period the creature passes into another stage in which it 
undergoes further development, and this is followed by the emergence of 
the imago, of which I have been able to keep specimens alive for eleven 
months. 
While the Lake Survey investigations were being carried on at Locli 
Rannoch in May 1902 a trout was caught, in the stomach of which, 
on examination by Dr T. N. Johnston, a great number of living Hydrach- 
nid larvae were found. A short account of these was given by Mr 
Soar in the Quekett Journal, with figures of the larvae, which were most 
probably swallowed by the trout just before it was caught. 
For the nomenclature followed herein, reference is made to “ Hydrach- 
nidae und Halacaridae,” Piersig und Lohman (Das Tierreich, Lief. 13). As 
