132 Development zvitJiin the Larva 



the head of the fly can be followed in a series of larvae 

 of different ages. They are not to be discovered even in 

 a rudimentary state until after the last larval moult ^ 

 Weismann - has given reasons for supposing that inva- 

 ginated imaginal rudiments could not come into existence 

 before the last larval moult in an insect whose life-history 

 resembles that of Corethra or Chironomus. If the epi- 

 dermis were invaginated in any stage before the ante- 

 pupal one, the new cuticle, moulded closely upon the 

 epidermis, would become invaginated also, and would 

 appear at the next moult with projecting appendages 

 like those of a pupa or imago. This is actually the way 

 in which the wings are developed in some larval insects 

 Avith incomplete metamorphosis. In Muscidae the inva- 

 ginations for the head of the imago have been traced 

 back to the embryo within the egg. but the almost total 

 subsequent separation of the disks from the epidermis 

 renders their development independent of the growth 

 of the larval cuticle and of the moults that probably take 

 place therein '^ 



Very soon after the last larval moult, when the 

 Chironomus-larva is about half an inch long, the first 

 indications of the invaginations can be discovered by 

 means of sections. They form rapidly, and among larvae 

 quite similar in size and outward appearance some are 

 found to exhibit tolerably advanced invaginations, while 

 others do not possess even the rudiments of such struc- 

 tures. In an early stage the invaginations are restricted 



' There are probably four larval moults in Chironomus, as in Corethra, 

 but the burrowing habits of the insect render it difficult to be quite 

 certain of the exact number. 



"^ 1866, p. 115. 



' Leuckart and Weismann have inferred the occurrence of at least two 

 moults in the larva of the blow-fly, from the changes observed in the 

 stigmata and the hooks. Weismann (1863) suspects that as many as four 

 moults may take place (p. 104). 



