HYDRADEPHAGA. 65 



their allies delight in running streams, and these latter 

 are usually more metallic and variegated with light spots 

 (Agabus maculatus, Plate III, Fig. 2) . 



The larva of the common D. marginalis (Fig. 1, p. 10) 

 is, when full-grown, about two inches long, dirty brown 

 in colour, and tapering to the tail, which is provided 

 with two thin appendages fringed with hairs. The 

 larva suspends itself in the water, head upwards, with 

 these appendages at the surface, and thus obtains air ; 

 for the ordinary breathing-holes on the sides are closed 

 up, and the air-vessels are reached by means of two 

 spiracles at the apex of the eleventh segment near the 

 fringed extremity. Its head is large and oval, with com- 

 posite eyes (Fig. 1 a, p. 10), rudimentary antennae, and 

 very long, sickle-shaped, pointed jaws, which are chan- 

 nelled so as to allow the larva to extract the juices of 

 its prey, there being no opening at the mouth. It is 

 very rapacious, and is often called "Fresh-water Shrimp." 



Cybister, reputed British, has its wing-cases widened 

 behind, and not furrowed in the female. Acilius sul- 

 catus, a moderately large, common, flat, grey species, 

 has been observed to make a considerable humming 

 noise, apparently produced by the action of the air upon 

 the alulae or winglets, two small circular membraneous 

 plates at the base of the elytra (Fig. 8 h, p. 25). 



The genera Laccophilus and Noterus, which have no 

 visible scutellum, appear to point towards the next 

 family, their hind legs being much widened. Noterus, 

 also, has the antennae short, and much thickened ; being, 

 moreover, of a somewhat similar build to Gyrinus, and 

 having similar "jerky" habits. Species of both genera, 

 also, occur in brackish water, a habitat of some of the 

 Gyrini. 



F 



