II FLIES WITH AQUATIC LARV^ 197 



The structure of the antennae is greatly relied upon 

 by systcmatists as a mark of the leading groups of 

 Diptera, and the antennas of Stratiomys are con- 

 sidered as intermediate between the many-jointed 

 antennae of the Nemocera (Culex, Chironomus, Tipula, 

 &c.) and the highly specialised and peculiar antennae 

 of a Blow-fly or House-fly. This intermediate 

 position of Stratiomys gives a special interest to the 

 manner of its transformation. It is the simplest in 

 structure and life-history, and the most like a Nemo- 

 ceran, of all the Diptera which retain the larval skin 

 as the outermost covering of the pupa. 1 Here we 

 find the artifice in its simplest form ; it is protective 

 and useful, but does not as yet involve serious altera- 

 tion in the enclosed pupa. When the larval skin of 

 Stratiomys bursts to allow of the escape of the fly, it 

 bursts after the Nemoceran manner, that is, by split- 

 ting longitudinally, though there is also transverse 

 fission. In the Muscidae the anterior segments of the 

 larval skin are detached by a transverse fissure only. 

 The protection afforded by the hardened larval skin 

 seems to have led in the Muscidae to a very peculiar 

 and interesting process of destruction and regenera- 

 tion of all the tissues of the body. The old organs 

 break up, and the new organs are developed apparently 

 anew, but really from rudiments developed during the 

 larval stage. (See Chironomus, p. 135.) This pro- 

 Some species of Ceratopogon do not free themselves from 

 the larval skin at the time of pupation, but allow it to encumber 

 the tail of the pupa. The pupa of a certain species of Subula 

 pushes out at the head end of the larval skin and remains 

 sticking there. 



