9 6 



INSECT TRANSFORMATION 



structures appear as backwardly-directed, dorso-lateral out- 

 growths of the second and third thoracic segments (Fig. 52 B). 

 They become larger after each succeeding moult, and the young 

 insect may now be termed a nymph. The development of 

 wings in the aquatic mayfly nymph does not differ in any 

 important respect from the same process in dragon-flies and 

 other insects with the typical open type of wing-growth 

 already described. But with the passage from water to air 

 some striking peculiarities are to be noticed. When the 

 nymph leaves the water the moult is often exceedingly rapid, 



d 



FIG. 53. JAWS OF NYMPHAL MAYFLY (Heptagenid}. 



a, mandible (c apical teeth ; m, molar area) ; b, tongue (hy) and maxillulae 

 (mxl) ; c, maxilla (ca, cardo ; st, stipes ; /, lobe ; p, palp) ; d, labium (m, 

 mentum, ; I, lacinia ; g, galea ; p, palp), x 10. After Vayssie're, Ann. Sci. 

 Nat. Zool. XIII. 



so that the winged insect may fly away less than half a minute 

 after the nymph-cuticle has begun to split ; naturally the 

 cuticle of the newly-emerged mayfly is very thin and delicate. 

 But now ensues a feature which makes the life-history of 

 mayflies different from that of all other insects. The winged 

 creature which has left the old nymph-husk resting on the 

 surface of the water is not the adult. It is a sub-imago, which 

 after flying about for a few hours,|'or perhaps for a shorter 

 period, casts off its delicate cuticle that clothes the wings as 

 well as the body and legs, and thus becomes transformed into 



