THE YOUNG INSECT 25 



certain pupae retain limited powers of movement and loco- 

 motion. Subterranean pupae wriggle their way to the 

 surface of the soil, often by the aid of spines winch arise 

 from the abdominal segments. The grotesquely shaped 

 pupa of a gnat, though it usually floats passively at the 

 surface of the water, dives with the utmost promptitude 

 and alertness to escape danger. The pupa of a snake-fly 

 becomes active just prior to its final moult and creeps 

 from under the bark where it has lain hidden. The most 

 remarkable motile pupa?, however, are found among the 

 well-known caddis flies. The full-grown larva, or " worm," 

 turns to a pupa within its case ; but just before the final 

 transformation, the pupa bites its way out and swims 

 to the surface of the water, using its middle pair of 

 legs, which are developed like oars for the purpose. The 

 skin of the pupa then splits down the back, and the 

 perfect caddis-fly emerges. 



Insects usually undergo no further moult after they 

 attain the winged state. May-flies, however, furnish a 

 remarkable exception to this rule. When the nymph is 

 full grown, it ascends a plant stem to the surface of the 

 water, where its skin splits, and a winged insect emerges. 

 This process occupies a very short time, it may be only a 

 few seconds. The winged form, however, is known as 

 a sub-imago. It is still completely enveloped in a delicate 

 skin, which is thrown off either at once, or after an 

 interval of several hours, and the insect is then in its 

 perfect state. The cast skins, looking like ghosts of 

 departed may-flies, may often be seen in myriads attached 

 to fences, tree trunks, and vegetation in the vicinity of 

 streams and rivers. 



So far we have considered the young insect mainly in 

 its capacity as a forerunner of the adult, liut we have 

 seen that it lives a life apart, and is chiefly concerned with 



