138 NATURAL HISTORY OF AQUATIC INSECTS CH. 



sort of internal pocket. The floor of the pocket forms 

 two longitudinal folds, which prolong the folds origin- 

 ating in the 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. 



The details of the formation of the head-parts of 

 the fly are too complex for description here. 1 The 

 last stage of all, in which the paired folds, together 

 with the intervening tract, become drawn forwards 

 and moulded into a new head of simple convex 

 shape, can, however, be observed without serious 

 difficulty. Larvae about to undergo pupation are 

 easily distinguished by the thickened thorax. A 

 number of such larvae must be placed in saucers of 

 water, and observed continuously. The epidermis 

 and soft parts will be seen to b withdrawn from the 

 fore-feet, and about a minute later from the append- 

 ages near the tail. Then a bulge appears just behind 

 the head on the dorsal surface. The larval head, 

 suddenly emptied of its contents, slips round to the 

 lower surface. The deep infoldings described above 

 are everted, as when a sleeve is turned inside out. 

 The surface bearing the facets of the compound eyes, 

 which was internal, and the antennse of the fly, which 

 were deeply buried, now project from the exterior of 

 the newly protruded head. In the course of two or 

 three minutes, the head of the fly, lately sunk into 

 the body and forming a series of elaborate folds, is 

 everted, and takes the form usual in Dipterous flies. 



1 See Miall and Hammond on the Development of the imago 

 of Chironomus. Linn, Trans., Vol. V. (1892). 



