METAMORPHOSIS 169 



of the silkworm suggest the direction in which knowledge may 

 be found, for they show that the physiological conditions of the 

 later larval life are different from those of the earlier life, possibly 

 as the direct result of the mere aggregation of matter, and the 

 consequent different relations of the parts of the organism to 

 atmospheric and aqueous conditions. 



If we wish to understand metamorphosis, we must supplement 

 the old opinion that ecdysis is merely an occurrence to facilitate 

 expansion, by the more modern conception that it is also an 

 important physiological process. That shedding the skin is done 

 solely to permit of enlargement of size is a view rendered unten- 

 able by many considerations. The integument can increase and 

 stretch to an enormous extent without the aid of moulting ; wit- 

 ness the queen-termite, and the honey-bearers of the Myrmeco- 

 cystus ants. Many moults are made when increase of size does 

 not demand them, and the shedding of the skin at the time of 

 pupation is accompanied by a decrease in size. And if moulting 

 be merely connected with increase of size, it is impossible to see 

 why Cloeon should require two dozen moults, \vhile Campodecu 

 can do with one, or why a collembolon should go on moulting 

 during the period of life subsequent to the cessation of growth. 



The attention of entomologists has been chiefly directed to 

 the ecdyses connected with the disclosure of the pupal and 

 imaginal instars. Various important transformations may, how- 

 ever, occur previous to this, and when they do so it is always 

 in connexion with ecdyses. Caterpillars frequently assume a 

 different appearance and change their habits or character at a 

 particular ecdysis ; and in Orthoptera each ecdysis is accom- 

 panied by a change of form of the thoracic segments ; this 

 change is very considerable at one of the intermediate ecdyses. 



The assumption of the pupa state is the concomitant of an 

 ecdysis, and so also is the appearance of the imago ; but the 

 commencement of each of these two stages precedes the ecdysis, 

 which is merely the outward mark of the physiological processes. 

 The ecdysis by which the pupa is revealed occurs after the 

 completion of growth and when great changes in the internal 

 organs have occurred and are still taking place ; the ecdysis by 

 which the imago appears comes after development has been quite 

 or nearly completed. 



Although the existence of a pupa is to the eye the most 



