366 LAKES AND STREAMS 



that on land the whirligig-beetle is awkward in its movements, although, like all 

 aquatic insects, it can fly from one stretch of water to another. Respiration is 

 performed like that of ordinary beetles, but the larvae possess so-called air-gills, 

 which are peculiar respiratory organs, by means of which oxygen is extracted from 

 the water, the gills being intersected by air-canals connected with those in the rest 

 of the body. The larvae, which, like the beetle itself, are carnivorous, leave the 

 water as soon as their metamorphosis is about to take place. This group is repre- 

 sented in Europe by about a dozen different species, the best known being 

 Gyrinus natator, which is a quarter of an inch long, ovate, glossy bluish black in 

 colour, with a reddish mouth and legs, the elytra being also reddish on their 

 reflexed edges. 

 Black water- The large black water-beetle (Hydrophilus piceus) belongs to a 



Beetle. totally different group from the one containing the common brown 

 species. This beetle is obliged to come to the surface of the water for the purpose 

 of breathing, but, unlike the brown species, holds its head and not its back out of 

 the water, and during this process bends it on one side in such a way that a hairy 

 spot lying near the hollow of the eye touches the water. At the same it lays the 

 hollow side of its hairy club-shaped antennae in such a way against the head that 

 a tube-like air-canal is formed towards the hairy under part of the body. It then 

 begins to perform a curious pumping motion, alternately lifting and dropping its 

 body, the air being conducted in this manner through the silk-like hairs which 

 extend over the whole breadth of the body on the under side. The wing-cases 

 also contribute towards the retaining of the air on the lower surface of the body ; 

 which, on account of the covering of air, glistens like silver. When swimming, the 

 great black water-beetle employs not only its hind -legs, but also the middle pair, 

 using them alternately so as to waddle in the water. Nevertheless, it swims much 

 less vigorously than the brown species. The males are characterised by a hatchet- 

 shaped broadening of the last joint on the front-legs, at the base of the claw : the 

 females, like those of the brown species, are of two types, some having a ridge-like 

 projection on the front part of the edge of the elytra, while others have none. 

 Unlike the brown species, which is carnivorous, the black water-beetle is omnivorous, 

 for, though it is generally said to live on plants alone, it also eats dead insects and 

 such living animals as it can kill. From a substance issuing in threads from tube- 

 like projections in the rear of the body, this beetle and its relatives weave a white, 

 circular web of considerable size, with a continuation on one side in the shape of a 

 chimney ; in this the large oval eggs are laid, the larvae which emerge from them 

 spending the first part of their life in the web, and going on land when the meta- 

 morphosis takes place. This web is attached to the under side of a leaf, and seen 

 from the bank looks like a piece of paper dropped into the water. The larvae, like 

 those of the brown species, feed on water-insects, whose juices they suck with the 

 orifice of the mouth, after first crushing their prey. These larvae carry their 

 captures to land, and there raise their heads in such a way that the opening of the 

 mouth and the food within are held vertically, and in this peculiar position, 

 which prevents its juices escaping, they devour the prey. If they fed in the water 

 a large portion of the nourishment would be lost, and a quantity of water would get 

 into the alimentary canal 



