192 THE BIOLOGY OF INSECTS 



instar is quiescent, and in the transformation of the last- 

 named family we notice that the wing-rudiments of the 

 male are formed beneath the preceding larval cuticle. These 

 conditions suggest an approach on the part of certain 

 Exopter^'gota towards the pupa of metamorphic insects. 

 The Mayflies (Ephemeroptera) are of especial interest in 

 this connection because they combine the open method of 

 wing-growth with a very wide divergence between larva 

 and imago. The larval mayfly (Fig. 52, A) might almost be 

 described as a bristle-tail adapted for aquatic life, since in 

 its long feelers and tail-cercopods and its crustacean man- 

 dibles, it closely resembles a thysanure, while its paired 

 abdominal tracheal gills must be compared, as R. Heymons 

 (1896) and C. Borner (1909) have shown, with a bristle-tail's 

 short abdominal limbs. There is a long aquatic larval life 

 with very many moults, outward wing- rudiments becoming 

 conspicuous in the later stages. The mayfly has little 

 obvious likeness to its larva except in its elongate abdomen 

 bearing terminal cercopds, for the feelers are very short 

 and the mouth parts reduced to mere vestiges, so that the 

 insect in its winged state cannot feed and its very name 

 implies the rapid passing of its life. Its aerial existence 

 nevertheless presents a feature of very great and exceptional 

 interest. When the ripe nymph has come out of the water 

 and shed its cuticle, the instar revealed, though possessing 

 developed wings, is not the true adult, but a " sub-imago " 

 which has to undergo another moult before the insect 

 reaches the imaginal state and becomes capable of repro- 

 duction. In no other group of insects does a moult occur 

 after the power of flight has been acquired. The existence 

 of the majrfly's sub-imago suggests the fascinating idea 

 that a moult after the development of functional wings was 

 possibly of general occurrence among the primitive winged 

 insects of past ages. In connection with this view it is 

 worthy of notice that in many of the less specialised meta- 

 morphic insects of to-day — Coleoptera and Neuroptera, for 

 example — the pupal wings are relatively of large size, and 

 that in the transformation of several Hymenoptera, including 



