LEAVES FROM MY NOTE-BOOK. 



135 



its cell in a similar way, though here the spines cover the whole 

 of the back. 



With the exception of the stag beetle, Hydrophilus is the 

 largest British beetle ; it is not uncommon in stagnant water near 

 London, and in the southern counties. Both in Dytiscus and 

 Hydrophilus a large part of the surface of the body is adapted to 

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



Fig. 3. — Pupa of Hydrophilus piceus, x 2. 



hairs are the 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, upstanding hairs play the same part in many aquatic 

 insects.] The spiracles open into the spaces which the protecting 



