NATURAL HISTORY OF BHOMPSTON POND. 219 



periodically vibrated structures and movements which 

 suggest the origin of insects' wings. 



The well-known Corixa Geoff royi (fig. 60), the largest and 

 commonest species of the genus, is present in all parts of the 

 pond, rapidly swimming with its oar-like posterior pair of 

 legs (a), the tibiae and tarsi of which are profusely furnished 

 with setae. Stiff hairs are also present on the anterior pair 

 of legs, as these appendages are used for swimming as well as 

 for prehension ; two slender claws terminate each of the 

 second pair of legs. The gnathites are modified for sucking 

 the juices of the animals on which it lives. The mandibles 

 and first maxillae are represented by slender piercing styles 

 enclosed within the tubular rostrum (labium). The head is 

 yellowish and bears two large blackish eyes. The thorax 

 is transversely marked with black and yellow and the scutellum 

 is covered by the protothorax. Except at the extremities 

 the elytra are horny and yellowish with black markings, while 

 the delicate hind wings are transparent and irridescent. 

 This insect is almost flat and not keeled, and swims with its 

 dorsal surface uppermost. When it wishes to take a fresh 

 supply of air it rises to the surface until its head and thorax 

 is just above the film, the air then passes round the neck 

 region and communicates with the stigmata, which are situated 

 just above the junction of thorax and legs. 



The larva of Phryganea (plate 3, fig. 62), well-known as 

 the " caddis-worm," is also present, and, like the ephemeral 

 larva, possesses tracheal gills. It lives in a tubular case 

 covered with sand, &c., from which it can protrude its head 

 and thorax, and by means of its six legs crawls about the 

 bottom of the pond in search of food. When mature the 

 larva, bars the entrance to the case with strong silk, which, 

 however, allows the entrance of water for respiration. The 

 pupa, when mature, gnaws through the obstruction and leaves 

 the water. 



Another larva to be found in the pond is that of Dytiscus 

 (fig. 64). In this insect the stigmata, two in number, occur 

 on the terminal (eleventh) segment (a) of its brownish body, 



