DYTICUS MARG1NALISA WATER-BEETLE 33 



and the animal descends once more beneath the surface, there 

 to remain until the store of air is exhausted. The respiratory 

 stigmata open into the chamber beneath the elytra, and the 

 most posterior pair is unusually large, so that a plentiful supply 

 may be taken in by them direct during their brief exposure to 

 the air. 



Life-History. The female Dyticus lays her eggs in the leaves 

 and stems of Potamogeton and other water plants during the 

 early spring. Her ovipositor is provided with two sharp plates 

 with which a slit is cut deep into the plant ; a single elongate 

 egg is pushed into each incision. The larvae emerge in about 

 three weeks, and in the course of another five weeks, after several 

 moultings of the skin, attain their full size. The full grown 

 larva is about two inches long, yellowish-brown in colour, and 

 tapering slightly towards the neck, but decidedly towards the 

 tail. The head is flat and somewhat circular in outline. Very 

 conspicuous upon its anterior margin is a pair of powerful sickle- 

 shaped mandibles with which this voracious larva seizes the 

 tadpoles, fish and other animals which form its food. Near 

 the tip and on the inner side of each mandible is an elongate 

 slit fringed by fine bristles and leading into a tube which opens 

 posteriorly at the base of the limb near the mouth. Through 

 these passages the juices of the prey are sucked and passed into 

 the very narrow mouth. Solid particles of food are, however, 

 swallowed occasionally. 



The tip of the abdomen bears a pair of small fans fringed 

 with hairs ; similar fringes being also present on the lateral 

 margins of the two last abdominal segments. These structures 

 enable the larva to dart rapidly through the water by vigorous 

 lashing strokes of the abdomen ; in swimming leisurely the legs, 

 which are rather long and likewise fringed with hairs, suffice 

 as organs of propulsion. 



Unless the larva maintains a hold upon weeds, etc. beneath 

 the water it floats to the surface tail uppermost. On arriving 

 at the top the tail fans are thrust through and suspend 

 the larva to the surface-film. It is in this position that the 

 creature normally rests, the tail attached the surface, the back 



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