FORMATION OF THE IMAGO OF CHIRONOMUS 671 



the larval antenna, and its base is accordingly telescoped into the head, while 

 the shaft becomes irregularly folded. 1 Culex, though more modified than 

 Chironomus in many respects, e.g. in the mouth-parts, is relatively primitive 

 with respect to the formation of the imaginal head, and shows a mode of 

 development of the eye and antenna which we may suppose to have character- 

 ized a remote and comparatively unspecialized progenitor of Chironomus." 



Formation of the imago in Chironomus. The development of the 

 head of the imago of Chironomus dorsalis has been discussed by 

 Miall and Hammond. The imaginations which give rise to the head 

 of the fly could not be discovered even in a rudimentary state until 

 after the last larval moult. 



" Weismann has given reasons for supposing that invaginated imaginal rudi- 

 ments could not come into existence before the last larval moult in an insect 

 whose life-history resembles that of Corethra or Chironomus. If the epidermis 

 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 with incomplete metamorphosis. In Muscidae the invaginations 

 for the head of the imago have been traced back to the embryo within the egg, 2 

 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." 



The pupal and imaginal cuticles do not follow at all closely the 

 larval skin, but, says Miall, become at particular places folded far 

 into the interior. " The folds which give rise to the head of the fly 

 are two in number and paired. They begin at the larval antenna on 

 each side of the head, and gradually extend further and further 

 backwards. The object of the folds is to provide an extended sur- 

 face which can be moulded, without pressure from surrounding ob- 

 jects, into the form of the future head. On one part of each fold 

 the facets of the large compound eyes are developed ; another part 

 gives rise to the future antenna, a large and elaborate organ, which 

 springs from the bottom of the fold, and whose tip just enters the 

 very short antenna of the larva. The folds for the head ultimately 

 become so large that the larval head cannot contain them, and they 

 extend far into the prothorax. Here a difficulty occurs. If the 

 generating cuticle of the prothorax were also to be folded inwards, 

 the future prothorax would take a corresponding shape. But the 

 prothorax of the fly has a form dictated to it by the limbs which it 

 bears and by the muscles to which it gives attachment. These call 

 for a great reduction in its length, and a peculiar shape, which it is 



1 C. Herbert Hurst, The Pupal Stages of Culex. 



2 Lowne on the Blow-fly, new edit., pp. 2, 41, Fig. 7. 



