HYDRACHNA. 257 



Ije called rather littoral, whicli frequent the margm and the shelter of the 

 coverts cast by shade, or otherwise, over the places they inhabit, and 

 run along in search of such minute particles as are to be found near the 

 bottom. 



Many swim with evident exertion, advancing slowly and heavily 

 amidst their element, from the disproportionate powers and dimensions of 

 their slender hairy liml:)s as natatory organs, compared with the size and 

 density of the body. Their speed is precisely in the ratio of the pre- 

 valent temperature. Hence they are most active in summer. 



HydrachntB, like all other insects, are extremely sensitive of the 

 influence of heat or cold. During the genial season of the year, they 

 are fuU of life and action ; but retreating to their haunts below, on the 

 approach of winter, they probably remain torpid until the return of 

 spring. 



They are subject, besides, to a kind of torpor or lethargy, also like 

 insects, during the finer parts of the season. Specimens become suddenly 

 motionless, without any obvious external cause ; and after remaining as 

 dead at the bottom of their vessel, gradually revive to activity. 



Some years ago, while occupied with the effect of freezing mixtures, 

 I subjected several HydrachnsB to them, during the heat of summer. 

 When the preparation had sunk to thirty-eight degrees, six fell to the 

 bottom, and there remained motionless. On this occasion the cold was 

 resisted longest by the species denominated Pajnllator of Miiller. In 

 other experiments during winter, two HydrachniB became languid when 

 the water fell to thirty-six degrees ; and when the heat was reduced to 

 thirty-two degrees only one or two could move their legs. Next day the 

 wonted activity of all was resumed. 



Possibly the habits of this animal are modified by situation. In the 

 close of July I took four from a small pool lying in the course from the 

 high road to Canty Bay, in East Lothian, opposite to the Bass islet. All 

 were of the species Cruenta, plump and vigorous. Unhke the ordinary 

 habits of their kind, they burrowed in the mud, and frequently ascended 

 from the bottom of the vessel, totally disguised by what had adhered to 



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