142 HOW INSECTS BREATHE. 



Diptera (flies) and Nenroptera (dragon-flies, may-flies, &c), 

 and a few Lepidoptera (butterflies, moths, &c), are aquatic 

 when in the larval state, yet the perfect insects of these 

 orders live altogether in the air. 



The Hemiptera (fieldbugs, &c), on the contrary, present 

 us with several aquatic insects ; many of these however live 

 on water rather than in it. Every one must have noticed 

 Velia currenSy a dark brown, long, narrow insect, which may 

 be seen running on the surface of almost any pond. An allied 

 species, Halobates Streatjieldiana, has been found coursing 

 over the ocean midway between Asia and Africa.* 



The genera Nepa and Ranatra live actually submersed, 

 and are provided at # the posterior part of the body with two 

 tubes, varying in length according to the species, and which 

 the animal projects out of the water and thus obtains the 

 necessary supply of air. 



Among the Coleoptera (beetles) three families are essen- 

 tially aquatic — the Gyrinidce, Dyticidce and Hydrophilidce. 

 Besides these many little beetles live on the seashore, and are 

 covered by the water for hours together. The Gyrinidce or 

 whirligigs must be familiar to all lovers of nature, as they 

 may be seen during summer, in little troops coursing round 

 and round in circles on almost every smooth surface of water, 

 but they are entirely surrounded with air, for the lower part 

 of their body is covered by a thin layer. 



The Hydrophilidce have clubbed antennae, while in the 

 Dyticidce they are filiform (threadlike). The former swim 

 with difficulty, and usually creep along the bottom, or on 

 aquatic plants. They are obliged to come to the surface to 

 breathe, and are enabled to carry down with them a supply of 



* Trans. Ent. Soc. of London, vol. 1, p. 230. 



