SOME AQUATIC INSECTS 79 



of the pond, and is quite a placid, inoffensive 

 insect as compared with the Dragon-fly and 

 Dytiscus. Although the Great Water Beetle is 

 nearly twice the size of the Dytiscus, it fre- 

 quently falls a victim to that bold and ferocious 

 insect. It is easily distinguished from its foe by 

 its greater size, its more convex form, deep 

 glossy olive-black colour, less energetic motions 

 in the water, for it is by no means so powerful a 

 swimmer as Dytiscus, and in the way it works its 

 legs alternately instead of simultaneously, so that 

 it swims with a jerky motion. 



The method by which the Great Water Beetle 

 takes in its supply of air from the surface of the 

 pond is quite different to that of the Dytiscus, 

 which protudes its back above the surface of the 

 water when it wishes to obtain a fresh supply of 

 air. The Great Water Beetle rises head first to 

 the surface where it inclines its body slightly to 

 one side, so as to bring the angle between the head 

 and the prothorax on one side of the body to the 

 surface, and at the same time the antenna on that 

 side is curved outwards and downwards in such 

 a manner as to prevent the water entering the 

 cleft between the head and prothorax. Through 

 the opening so formed the air is drawn down by 

 rhythmic movements of the body and wing-cases, 

 and the beautiful silvery air film on the ventral 

 surface of the body may be seen to alternately 

 dilate and contract. Having by this means 

 obtained a fresh supply of air, the Beetle once 

 more dives beneath the surface. 



The female Beetle is of considerable interest, 

 for she possesses special silk glands or spinnerets 



