320 INSECT TRANSFORMATIONS. 



again brought to the surface, so that the fluid is 

 repelled from its disk, it remains there, without 

 efibrt, as before. Just before the exclusion of the 

 flj (^Chironomus 2Jlitmosi(S, Meigen), the dry part 

 of the thorax is seen to split in the middle. The 

 air enters, and forms a brilliant stratum, resembling 

 quicksilver, between the body of the insect and its 

 puparium; and the former pushing forth its head 

 and fore-legs, like the gnat, rests the latter upon the 

 water, and in a few seconds extricates itself wholly 

 from its envelope.'* 



The grubs of several of the four-winged water 

 flies [Phryganidce, Stephens), popularly called 

 case worms and caddis worms, construct a remark- 

 able apparatus for protecting the pupa during its 

 state of inactivity and helplessness. So long as it 

 remains in the grub state it can withdraw itself within 

 its case of shells, stones, or reed-stems; but as soon as 

 it feels its change approaching, it contrives additional 

 security. It weaves, for this purpose, at the entrance 

 of its gallery, a grating of its singular silk, which 

 hardens in water and remains indissoluble, as was 

 first observed by Vallisnieri. The strong threads 

 are made to cross each other, forming a small 

 thickish circular plate of brown silk, which becomes 

 as hard as gum, fitting exactly into tlie opening, 

 and placed a little within the margin. One of these 

 gratings (described by De Geer) is pierced all over 

 with holes, disposed in concentric circles, separated 

 by ridges running from the centre to the circum- 

 ference, though not quite so regularly as the spokes 

 of a wheel Other ridges, again, are made to tra- 

 verse the concentric rays, following ihc course of the 

 circles of holes, in such a manner as to form com- 

 partments, each having a hole in its centre. t Rfeau- 

 mur found that these holes were for the purpose of 



* Intr., iii.p. 290. t De Geer, Mem., vol. ii, p. 519 — 45. 



