THE ANIMALS OF THE FRESHWATER AQUARIUM 123 



pools. It can swim actively through the water, but on fine 

 summer days is often seen sunning itself at the surface, either 

 lying on the back in the ordinary position when swimming, or 

 half out of water and with the wings partially unfolded. Where- 

 as the water-beetles cannot fly out of the water unless they find 

 some supporting point from which to make the start, the water- 

 boatman can jerk itself out of the water with a sudden move- 

 ment of its powerful swimming legs, and then, unfolding its 

 wings, take its flight. In addition to these points, captive speci- 

 mens will show the interesting method 

 of obtaining food, which consists of other 

 aquatic insects. These are seized by the 

 prehensile fore-limbs, and the juices sucked 

 by means of the beak, which can be driven 

 deeply into the body of the prey. In addi- 

 tion to serving to seize the prey the anterior 

 legs, like the second pair, are used to 

 anchor the creature when it is at rest at the bottom ; for, 

 being lighter than the water, it tends to float upwards to the 

 surface unless it grasps some solid body. The lightness is 

 due to the large amount of air entangled beneath its wings, 

 and in the hairs at the sides of the body. This film of air 

 serves for respiratory purposes. The third pair of legs are 

 very strong, and are the organs by which the creature propels 

 itself vigorously along while lying on its back. Here, as in water- 

 beetles, the legs are flattened and furnished with bristles, which 

 change in position during swimming, offering the maximum 

 resistance to the water in the effective stroke, and the minimum 

 in the non-effective stroke. The eggs are laid on water-plants, 

 and the larvae resemble the adults save in the absence of 

 wings. 



Related to Notonecta are the species of Corixa, which closely 

 resemble the water-boatman, but are smaller, have flattened 

 instead of keeled dorsal surfaces, and swim with the under surface 

 downward instead of on their backs like Notonecta. The fore- 

 limbs also are not so markedly prehensile, and in natural con- 

 ditions Corixa prefers to keep near the bottom, in place of swim- 

 ming actively through the water like Notonecta. 



