152 



HOW INSECTS BREATHE. 



and the surface is of course equal to the size multiplied into 

 twice the number of the laminag ; but in air all the lamina? 

 fall together, and the surface at which the breathing is 

 carried on being very much diminished, the fish dies literally 

 for want of air. Eels, however, and some others, have the 

 gills contained in pouches, opening by small holes at the 

 side of the head ; and, as they thus carry a supply of water 

 about with them, can live for some time out of their natural 

 element. 



In the Freshwater Snail (Lymnceas) we have gills as in 

 the fish, but the animal not being able to move rapidly, other 

 means are necessary to insure a constant supply of water. 

 This is effected by means of ciliae or little hairs, which thickly 

 clothe the whole organ, and being in constant vibration, in- 

 dependently of the control of the snail, cause currents which 

 bring a regular supply of water to the gills. 

 ' In insects I have already mentioned several methods by 

 wdiich the object is effected. One more I must allude to — 

 that of the common large freshwater beetle, Dyt'icus. The 

 spiracles of this insect are placed on the back under the wing 

 cases or elytra. The two first pairs of legs are rather small 

 and used for walking ; the hind pair are strong, flattened and 

 provided with a brush of hairs, and form an excellent pair of 

 oars, propelling the beetle of course forwards. The whole 

 animal is somewhat lighter than water, and the head and 

 thorax are the heaviest parts ; therefore, when it ceases rowing, 

 it will of course rise tail-first to the surface. No one who 

 has ever watched a still pond in summer can fail to have 

 noticed the beetles thus coming up to the surface for a fresh 

 supply of air. The back is covered with a velvet-like 

 down, which repels the water. When, therefore, the insect is 

 suspended as it were by its tail from the surface of the water, 

 it has only to lift up a little the elytra and the space between 



