THE INSECT WORLD. 



63 



In some species the female when impregnated crawls under 

 water, down the stem of some aqiuitic j)lant, and lays her eggs 

 on stones, plants, or even in the mud. By folding the wings 

 backward, close to the body, a film of air is gathered which keeps 

 the insect dry and supplies it with a sufficient amount of oxygen 

 for a temporary return to its old medium. 



The order contains no injurious species, becau.se all are pre- 

 daceous ; but, on the other hand, none are markedly beneficial, 

 because, though they devour an enormous number of other 

 insects, these are not usually species destructive to farm crojjs. 



Order PLECOPTERA. 



The ordinal term' signifies "plaited wing," and is used because 

 the hind wings are longitudinally folded or plaited beneath the 



Fig. 34. 



Stone-fl\ , Prrla bicaudata, ami its larva. 



anterior pair, which rest fiat upon the back. The insects them- 

 selves are called "stone-flies," and are loosely jointed, flattened. 



