X WATER-BOATMEN 357 



connected with the peculiar attitude of the Insect. 

 The air is so lodged in and around its body as to 

 make the ventral side the lighter, and this floats 

 upwards. When it reaches the surface, the tips of 

 the expanded oars and the tail press against the 

 surface-film and keep the head submerged. In this 

 position the Notonecta breathes at ease, but is ready 

 to make a sudden spring without suffering any delay 

 from the difficulty of wetting its body. Now and 

 then it swims back upwards for a few strokes, but 

 soon assumes its ordinary position. Corixa is much 

 less buoyant, and can remain as long as it pleases at 

 the bottom of a glass bowl, where there is nothing to 

 cling to. The hindermost abdominal spiracles, we 

 might expect, would be large in Notonecta, consider- 

 ing that when it reposes, only the extremity of the 

 body is in contact with the air. In reality they are 

 very minute. Larger spiracles, well defended by 

 hairs to prevent the accidental entrance of water, are 

 found on the sides of the thorax. To these the air 

 is led by a singular passage. The side of the body 

 which floats uppermost is keeled along the middle 

 line. On each side of the keel and between it and 

 the lateral edge, runs a long row of elastic hairs, 

 while a second and parallel row runs along the edge 

 itself. These rows of hairs enclose a watertight 

 covered way, leading to the thorax, and along it the 

 air is guided to the large thoracic spiracles. Now 

 and then one of the long oars is drawn over the 

 abdomen, as if to urge the air more rapidly along 

 the passage.^ Corixa breathes in a different fashion. 

 1 Schmidt- Schwedt, in Tier- und Pflanzenwelt des Siisswassers. 



