124 The Life-History of Insects 



buried underground — those of Crane-flies for ex- 

 ample — work their way to the surface and raise them- 

 selves half out of the soil before the fly emerges. 

 The pupse of Caddis-flies and of the most primitive 

 Moths bite an opening out of their cocoons by means 

 of powerful mandibles. Some Moths enclosed in 

 a very firm and dense cocoon, secrete from the 

 hind-gut a solvent which weakens the cocoon suffi- 

 ciently to allow the insect to break its way out (78 a). 

 The pupa of the Gnat lives in water, but floats with 

 the thorax just beneath the surface, so that the imago 

 can emerge directly into the upper air. Pupae of 

 certain midges {Simu/ium), on the other hand, live in 

 cocoons attached to stones or to the stems of water 

 plants ; the imagos, on emerging from these, rise to 

 the surface enclosed in a bubble of air, the air having 

 been absorbed from the water by the gill-filaments 

 of the pupa and allowed to accumulate beneath the 

 skin (170). 



Sub-imago. — Insects, as a rule, undergo no further 

 moult after attaining the winged state. Mayflies, 

 however, furnish an exception. After a long larval 

 and nymphal life passed in the water and lasting 

 several years, the winged insect appears as a sub-Imago, 

 which quickly casts a delicate skin revealing the true 

 imago. The sub-imago of the Mayfly is comparable to 

 the pupa of insects with a complete metamorphosis, 

 and very possibly represents the survival of a moult 

 during the winged state which was originally uni- 

 versal among insects. 



" Division of Labour " between Larva and 

 Imago. — While the larva and nymphal life of the 

 Mayfly lasts for several years, the imago survives'but 

 a few hours, or at most a few days ; the shortness of 

 these insects' lives in the winged state has passed into 

 a proverb, and their name is associated with all that 



