DIPTERA. 365 



watery fluid, whereb}' the thin transparent outer skin or cuticle 

 is elevated like a Aesicle or blister, which occupies about 

 a fourth of the length of tlie worm on its under side, but 

 is much shorter on its back. Tlie insect is now in its em- 

 br3'^o-pupa state, having lost its larva form and having not 

 yet assumed its pupa form. In the fluid contained in tliis vesi- 

 cle, the wings, legs and autennje of the future fly now begin 

 to be developed, Avhereby the sheaths of the wings at length 

 come to be discerned immediately under the skin. This skin 

 is exceedingly thin, delicate and transparent, like tiie tunica 

 arachnoides of the human brain, a mere film, as thin as a spi- 

 der's web. Eventually the insect, by gently writhing, ruptures 

 this fllm at its anterior end, and gradually crowds it off down- 

 wards to the lower end of the vesicle, carrying the minute 

 l)lack jaws of the larva with it. It there remains, becoming 

 dry and torn into shreds which flake and fall oft" by the con- 

 thiued motions of the insect. At the same time from the 

 remainder of the surface not occupied by this vesicle, a still 

 more slight and delicate film, appearing as though the worm 

 had been wet in milk which had dried upon it, forming an ex- 

 ceedingly thin pellicle or scurf, becomes separated by the same 

 motions of the insect and drops off in minute scales scai'cely 

 to be perceived with a magnifying glass. And now the insect 

 has acquired its perfect pupa form." 



Frederic Brauer has proposed in his "Monographie dor 

 O^^striden," a division of the Diptera into two large groups. 

 This division is much more natural than the old one into those 

 with coarctate and obtected puptB. The first group is the Dip- 

 tera ortJiorapJia, comprising the Nemocera, or flies with long an- 

 tenufB, together with the Stratiomyidce^ Xylojjhagidm, 

 Tabanidm, Acroceridm (?), Bombylidoi , Asilidm. 

 Leptidm, Therevida'-, Empidce a,nd Dolichoj) idee (pass- 

 ing over some small fiimilies whose metamorphoses are not 

 known). In these families the larva skin at the last moult 

 splits down along tlie middle of the back of the three thoracic 

 rings, while a transverse split on the first thoracic ring makes a 

 T-shaped fissure. Through this the mummy-like pupa with free 

 limbs escapes ; or it remains within the loose envelope formed 

 by the old larval skin, when this author calls it a "false pu- 

 pai'ium." 



