76 NATURAL HISTORY OF AQUATIC INSECTS CH. 



retain a pellicle or flattish bubble of air. Close-set 

 hairs are the simple means employed to prevent the 

 wetting of these particular tracts. Wrap a strip of 

 velvet round a stick, and dip it into water, or sprinkle 

 a few drops of water on a scrap of velvet. You will 

 see with what difficulty the water penetrates the 

 narrow spaces between the threads which form the 

 pile of the velvet. Close, outstanding hairs play the 

 same part in many an aquatic Insect. 



The air-space beneath the wing-covers of Dytiscus 

 is described in connection with that Beetle. Hydro- 

 philus has a similar reservoir of air upon its back, but 

 it has in addition a peculiar and extensive reservoir 

 on its ventral surface. 



On the under side of a Hydrophilus we can readily 

 see the hairy tracts which retain the film of air. 

 They cover the thorax and fore part of the abdomen 

 on each side. A prominent ridge along the middle 

 line, \vhich ends in the spine mentioned above, is free 

 of hairs ; so arc the greater part of the abdomen and 

 the thighs. The rest of the ventral surface is over- 

 spread in the living and submerged Beetle with a film 

 of air, which glistens like silver. The superficial air- 

 tracts extend also along the sides of the thorax and 

 abdomen, being bounded above by the overhanging 

 edges of the prothorax and wing-covers, and join the 

 reservoir of air on the back. The air-tracts appear 

 as transverse silvery streaks behind the head and 

 across the base of the wing-covers. 



The spiracles open into these air-spaces, and the 

 submerged Hydrophilus replenishes from this source 

 the air within its body by a gentle rhythmical move- 



