DEVELOPMENT. 7 



and form in their greatest perfection. Growth and development 

 now cease as far as the individual is concerned, to give place to 

 the generative function. 



Every degree of metamorphosis exists amongst insects, from 

 that in which the larva, nymph, and imago, closely resemble each 

 other, where the successive changes are merely those of ordinary 

 development, as in the cockroach, to that in which the change 

 is so complete that it might almost be doubted whether the 

 larva and imago should be considered the same individual at 

 all, so closely does the process resemble an alternation of gene- 

 ration. This is the case in the fly. 



Dr. Weismann, in Germany, as long ago as 1865* pointed out 

 that the development of the fly differs remarkably from that of 

 most insects, in an elaborate memoir upon the subject ; and two 

 years after, in another upon the development of a species of gnat, 

 Corethra plumicornis, proposed the division of insects into two 

 classes, according to the presence or absence of certain structures 

 in the larva which he calls Imaginal discs, f This distinguished 

 naturalist asserted in his former paper, that the head and thorax 

 of the fly does not depend for its development upon the corres- 

 ponding larval segments, but that these parts are developed 

 from a series of discs (Imagined discs) firmly adherent to the 

 nerves and tracheae of the anterior extremity of the larva ; and 

 he goes on to add, " I believe in all those insects in which the 

 anterior larval segments are unprovided with appendages (legs) 

 the head and thorax of the Imago are entirely redeveloped, 

 whilst in those in which the larva is furnished with legs, these 

 parts depend for their formation upon the anterior larval 

 segments." 



Startling as the assertions of that writer appear, I have been 

 fortunate enough to confirm them myself, and not only to confirm 



* K'jlliker aud Siebold Zeitschrift, band xiv-xvi. f Imaginal Scheibcn, 



