DEGREES OF METAMORPHOSIS 



597 



Latreille (1831) called insects with an incomplete metamorphosis homotenous 

 (which means similar to the end of life), and those with a complete metamor- 

 phosis, polymorphous. For the different degrees of metamorphosis of insects he 

 employed two terms : for the incomplete degree, metamorphosis dimidia, and for 

 the total or pupal, metamorphosis perfecta. 



VVestwood in his Introduction to the Modern Classification of Insects (1839), 

 taking into account the relation of the larva with the imago, divided insects into 

 two divisions : the Heteromorpha, or those in which there is no resemblance 

 between the parent and its offspring, and Homomorpha, in which the larva 

 resembles the imago, except in the absence of wings. 



From the point of view of the degree of metamorphosis, insects 

 have been divided into Heterometabola and Metabola. 



I. Heterometabola. This group may be divided as follows : 



1. Manometabola, 1 embracing those forms with a slight or gradual 



metamorphosis, but which are active in all the stages, without any 



FIG. 559. Manometabolous metamorphosis of the cockroach (Phyllodromia germanica) with 

 its four nymphal stadia a-d ; <>, h, adult ; /, female with egg-case ; g, egg-case. From Riley. 



resting stage. The orders passing through this degree of meta- 

 morphosis are the following: Orthoptera, Dennaptera, Platyptera, 

 Thysanoptera, and Hemiptera (Coccidse excepted). 



In all these groups, the only external differences of importance 

 between the freshly hatched nymph and the adult is the presence of 

 wings. The chief difference internally is the complete development 

 of the sexual glands. 



It should be observed, however, that in the last nymph stage of 

 the Thysanoptera the articulations of the limbs are enveloped by a 

 membrane and the wings enclosed in short fixed sheaths; the 

 antennae are turned back on the head, and the insect, though it 

 moves about, is much more sluggish than in the other state. 

 (Haliday.) Hence here we have a close approach to the following 

 degree. 



U. Ileremetabola, 2 including those forms with a gradual though 



iFrom the Greek n^vo*, scanty; /nerapoAjj, change. 

 2 Greek, "ip^a, quiet ; fiera/SoAr;, change. 



