Formation of Nezv Mouth-parts 131 



Simultaneously with, tlie formation of the compound Formation 

 eyes and the imaginal antennae, new mouth-parts are mcutii- 

 developed. As in Corethra. they develop within those ^'^'^ ^" 

 of the larva. On either side of the salivary ducts and 

 their common opening into the mouth, the epidermis of 

 the larval head becomes infolded, and the pouches ulti- 

 mately extend backwards to the back of the head. From 

 the inner side of each pouch, and close to its hinder 

 extremity, a secondary invagination pushes forwards 

 and downwards, and this ultimately gives rise to the 

 labella ^ of the fly. In larvae ready to change into 

 pupae the tips of the labellae are bent inwards, towards 

 each other, at a right angle. The invagination for the 

 maxillary palp forms on the side of the larval head. 

 The mouth of the primary fold is at first nearly equi- 

 distant from the larval maxillae and the occiput. The 

 secondary forward-directed fold is long and narrow, and 

 extends from the back of the head into the larval maxilla. 

 As it lengthens it becomes coiled, and much resembles 

 one of the developing imaginal legs. The new parts 

 thus formed are those of the pupa, and the imaginal 

 rudiments are enclosed within them. The pupal integu- 

 ment of the head, like that of some other parts of the 

 body, recedes considerably from the larval cuticle, and 

 the imaginal integument recedes again from that of the 

 pupa, so that in sections of the pupal head a tolerably 

 wide space separates the mouth-parts of the fly from the 

 empty cuticle which represents the corresponding organs 

 of the pupa. 



The history of the invaginations which give rise to Early 



stages of 

 These authors seem to think that part of the larval prothorax is, so to fbldf.^"* 

 speak, annexed by the imnginal head. It is desirable to inquire whether 

 their observations do not admit of the same explanation that we have 

 given in tlie case of Chironomus. 



1 The lateral halves of the labium, which become fice distally. See 

 Meinert, 1881, or Dimmock, 1881. 



K 2 



