COCKROACHES, ETC. 121 



quickly taken in and quickly expired, none being 

 stored up or allowed to accumulate. As a natural 

 result, when insects so constructed are deprived of 

 their normal supply of air they are soon asphyxiated. 



Such a purely aquatic insect as the little Water- 

 Boatman, Notonecta glauca, common in English ponds 

 and streams, has limbs well adapted for swimming ; it 

 carries a supply of air about with it under the wing- 

 covers, and is in consequence very buoyant, so much 

 so that it is only by powerful strokes of the oar-like 

 hind-legs that it can force itself below the surface of 

 the water, and directly it stops swimming it comes 

 bobbing up to the top like a cork. But even these 

 insects die very soon if their air-supply is cut off, so 

 it is evident that they do not carry a great quantity 

 of air inside their tracheae. The limbs of the Water- 

 Cockroach are not specially adapted for swimming, but 

 since the tracheae are always deflated, and since it 

 carries about no adventitious supply of air, it is not 

 at all buoyant, but can swim easily in mid-water, and 

 can lie at the bottom of a pool without clinging to 

 some sodden leaf or stone as a Water-Beetle, such as 

 Dytiscus, is forced to do. As a matter of fact the 

 Water-Cockroach spends very little of its life entirely 

 submerged ; it rests amongst decaying vegetation at 

 the side of a pool or stream, with the greater part 

 of its body under water, but always with the tip of 

 the abdomen projecting above the surface. If closely 

 watched, it will be seen that the abdomen gently 

 moves up and down with a regular action, and that 

 there appear at the submerged thoracic spiracles at 

 regular intervals bubbles of air, which grow in size 

 and then break away to give place to fresh bubbles. 



