320 THE MICROSCOPE. 



and are concealed beneath the eyes. Of these, the Notonectida are 

 distinguished by their broad, rounded head, which occupies the whole 

 width of the front of the body. They swim rapidly about in the 

 water, with their bellies directed upwards, rowing themselves along 

 by means of their flattened hinder legs, which are extended on each 

 side of them like oars. Hence the Notonecta is generally known as 

 the hoat-fly. They carry the air required for their respiration in a 

 space left for this purpose between the wings and the back. They 

 are very active and predaceous animals, and when captured some of 

 them often inflict a painful wound with their powerful rostrum. Se- 

 veral species may be met with in almost any piece of water. In the 

 second group, the Nepina, the head is small and triangular, and gene- 

 rally considerably narrower than the thorax. Their legs are generally 

 less distinctly formed for swimming than in the preceding group j but 

 the anterior pair are converted into powerful raptorial organs. 



The Nepa cinerea is a British example of this group, to be met with 

 in every pond. These insects respire by means of the filaments of the 

 caudal extremity, which they place at the surface of the water, the 

 only available stigmata being situated at the base of these filaments. 



The Dytiscus, the principal genus of this tribe, is common in fresh 

 and placid waters. Its larva, represented in fig. 92, feeds upon other 

 aquatic larvse, such as those of gnats, dragon-flies, &c. 



In the Gyrinus, Whirligigs, we have combined contrivances to 

 facilitate the creature's movements in the element it frequents. The 

 hinder legs are converted into a pair of oars of remarkable efiiciency, 

 the point of their connection with the body being adapted with great 

 precision to insure the most efiectual application of the propelling 

 power ; and as they are struck out behind in the act of swimming, 

 there is a membranous expansion, which enables the insect to move 

 about with great rapidity ; while, on their being drawn back towards 

 the body, the membrane closes up again, and thus ofiers but a small 

 resistance to the water (fig. 152). If the insect wishes to remain below 

 the surface of the water, he employs the small hooks at the extremity 

 of the leg, for the purpose of adhering to the roots or stem of some 

 aquatic plant. The eyes are not the least curious part of the merry 

 little creature ; the upper portions of them, being fitted for seeing in the 

 air, are placed on the upper part of the head ; whilst the other portions, 

 fitted for seeing in water, are placed in the lower part, a thin division 

 separating the two. 



To the Orthoptffra belong the Locustina, Gfryllina, and the Achetina, 

 all herbivorous insects. The first is represented by our well-known grass- 



