AQUATIC INSECTS 



changed notably in those structuies which have to do with 

 reproduction. It has transformed from a nymph to a mature 

 adult. After this it does not grow larger or ever shed its skin 



Fig. 143. — Eggs of mayflies with structures, 

 threads and floats, by which they are suspended in 

 the water: i, Tricorythus; 2, Ephemerella rotunda. 



again. In mayflies there is a second molt after the winged 

 form has been attained, a change from the nearly adult sub- 

 imago to the fully mature imago (PI. XI) but this is an excep- 

 tion among all insects. The changes of form which the 

 mayfly or stonefly undergoes during its life are known as 

 incomplete metamorphosis (Fig. 144). 



The change of form between the young insect and the adult 

 may be far greater than this. A caterpillar hatches out of a 

 moth egg; it shows no hint of wings, has a wormlike shape, a 

 great appetite, and is known as a larva. After a period of 

 eating and growing, it loses its appetite, stops moving about, 

 moves into a secluded spot, and finally sheds its larval skin. 

 This discloses a very differently shaped insect with wings and 

 antennas outlined on its skin which suggest the adult moth. 

 The moth is now in the pupal stage during which it is made 

 over, inside and outside. At the end of the pupal stage the 

 fully formed adult moth comes forth from the pupal skin. 

 The changes which it has gone through are called complete 

 metamorphosis (Fig. 145). Incomplete and complete meta- 

 morphosis both occur in aquatic insects. 



187 



