672 



TEXT-BOOK OF ENTOMOLOGY 



not here necessary to describe. It will be enough to realize that 

 the epidermis of the future prothorax cannot be sacrificed to the 

 folds which are to give rise to the head of the fly. All interference 

 between the two developing structures is obviated by the provision 

 of a transverse fold, which pushes into the prothorax from the neck, 

 and forms a sort of internal pocket. The floor of the pocket forms 

 two longitudinal folds, which prolong the folds originating in the 



FIG. 625. Process of formation of the parts of the head of the fly in the larva of Chironomus 

 (male) : A, the new epidermis thrown into complicated folds which have been cut away in places to 

 show the parts within. 7?, the same parts in horizontal section ; Ic, larval cuticle ; if, transverse 

 fold ; if, upper wall of the same ; m, cut edge of new epidermis ; tint, larval antenna : an, nerve 

 to the same ; ant', antenna of fly ; If. longitudinal fold ; o, eye of fly ; on, optic nerve ; an', root of 

 antennary nerve; br, brain; <#s, O3sophagus; 6, bulb of antenna of fly ; s, s, ', blood-spaces. 

 After Miall. 



larval head. The roof of the pocket shrinks up and forms the 

 connection between the head and thorax of the fly. Ultimately 

 the head-part is drawn out, leaving the prothoracic structures 

 unaffected." l 



The development of the head of the fly of Chironomus appears, as 

 Miall and Hammond state, to be intermediate between the groups 

 Adiscota and Discota of Weismann; i.e. "between the types in 



1 Miall, Natural History of Aquatic Insects, pp. 136-138. Also Trans. Linn. Soc. 

 London, V, Sept., 1892. 



