THE MARGINED WATER BEETLE. 75 



they are by no means adapted for terrestrial progres- 

 sion. The tarsi are always composed of five joints'^, 

 and the antennse, as in the preceding tribe, are filiform 

 (or thread-like) organs. 



One of the largest, most formidable, and most 

 abundant of the British species of this group, is the 

 Dyticus marginalis, or Margined Water Beetle, speci- 

 mens of which may be obtained in almost any piece 

 of water. This insect measures from an inch to an- 

 inch and a quarter in length, and is of a broad oval 

 form ; the whole upper surface is blackish olive, except 

 the margins of the prothorax, and the outer margins of 

 the elytra, which are of a dull yellow colour, as is also 

 the lower surface of the body. The whole structure 

 of this insect is admirably adapted to its aquatic resi- 

 dence ; its thorax is as wide at the base as the elytra, 

 so that the form of the insect is a complete oval, its 

 body is thickest in front, and there is not a projection 

 of any kind upon its surface to impede its motion 

 through the water. The hind legs form a pair of 

 long, broad, and powerful paddles, the surface of 

 which is increased by a friitge of stifi" hairs running 

 down the inner margin of the flattened tarsi, and by 

 the action of these the Dyticus is enabled to swim 

 through the water at a rapid rate. But although 

 thus fitted for an aquatic life, the beetle is never- 

 theless under the necessity of rising frequently to the 

 surface for a supply of air, and this operation is 

 eff'ected in a very simple fashion. It is to be borne 

 in mind that the body of the insect is considerably 



* In some small species the last joint but one of the tarsi is 

 of very minute size, and completely concealed within the pre- 

 ceding one, so that the tarsi appear to be composed of only four 

 joints. 



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