Natural History 509 



Through these spiracles the air is driven out by movements 

 of contraction ; fresh air passes in passively as the body expands. 

 As in birds, so in insects, expiration is the active part of the 

 breathing process. The air-tubes fork and re-fork, sending side- 

 branches to every corner of the body, even to the tips of the 

 feelers, so that the whole body is thoroughly aerated. The ex- 

 tensiveness of the air-tube system compensates for the relatively 

 poor blood-system. In aquatic forms various devices are adopted 

 to secure a supply of oxygen. Some water-insects come to the 

 surface to breathe, others, like young may-flies, have special 

 structures tracheal gills of different types. The Water-Beetle 

 (Dytiscus) has its spiracles on its back, and when it dives under 

 water it carries with it, in an air-tight compartment between its 

 back and its hard wing-covers, enough air to last for several 

 minutes. The bubble-of-air method is another plan, adopted 

 by Whirligig Beetles and some water-bugs, whose covering of 

 fine hairs entraps bubbles of air, ensuring a sufficient supply of 

 air about the body for a short time under water. 



In addition to the respiratory system there are inside the 

 body of the insect all the usual organs food-canal and associated 

 parts, a heart, excretory organs, reproductive organs, and so on. 

 Some insects are so small that they can creep through the eye 

 of a needle, and it is difficult to believe that in such minute 

 dimensions all the ordinary organs are packed away. 



Locomotion 



Insects are essentially active, and they exhibit various kinds 

 of locomotion. Many grubs and maggots are quite passive, but 

 even limbless larvae, though naturally not so active as the legged 

 types, have their ways of getting about. They may jerk them- 

 selves along with the aid of bristles or jaws, they may make 

 relatively enormous leaps into the air by taking their tails in 

 their mouths and suddenly letting go, or they may swing them- 

 selves from place to place by paying out silken lines from their 



