METAMORPHOSIS. 649 



only by the incompleteness of their organization, and change 

 gradually by a series of moults into the adult form. 



(ii) Metamorpha, with a definite larva the structure of which 

 is in many ways completely different from that of the adult, and 

 with a more or less complete transformation from young to 

 adult. 



I. The Epimorpha. 



The young resemble the adult on the whole in habit and 

 structure. No special larval organs are developed. This group 

 includes the Collembola and Thysanura, and the Mallophaga, in 

 which the differences between young and adult are only very 

 slight ; consisting, for example, in the young having no external 

 genital structures, and a less number of antenna! joints than the 

 adult. It also includes orders in which the young differs from 

 the adult by the absence of wings, which develop gradually 

 at successive moults. These are the Dermaptera, Orthoptera. 

 Termitidae, Psocidae, and Hemiptera. 



II. The Metamorpha. 



A definite larval stage is present. This category is further 

 divided, according to the degree of the transformation exhibited 

 in changing from larva to adult, into : 



(a) Hemimetabola. Insects without the definite resting stage 



known as the pupa. 



(b) Holometabola. Insects with a definite pupal stage. 



(a) The Hemimetabola include certain orders in which the young 

 differs from the mature insect or imago not only in the absence of 

 wings but in the possession of certain larval organs which later 

 disappear. The habits of larva and adult are often very dif- 

 ferent. To this group belong the Odonata and Plecoptera. * 

 The larvae are in these cases aquatic and breathe by external 

 gills or by tracheal folds of the rectum, which disappear at the 

 last ecdysis. The stage which precedes the imago and in which 

 the process of transformation takes place is frequently called 

 the nymph. Although in this stage the insect often becomes 

 for some time quiescent, yet there is no definite pupa. 



* In the second of Heymoiis' papers referred to previously, he places 

 the Ephemeridae in an independent division (Prometabola) of the Holo- 

 metabola, owing to their having a winged and active sub-imago stage 

 between the pupal and imago stages. 



