264 MEANS OF DEFENCE OF INSECTS. 
state are secured from their enemies, there is none more 
ingenious that than to which the may-flies (Phryganea, 
L.) have recourse for this purpose. You have heard be- 
fore that these insects are at first aquatic, and inhabit 
curious cases made of a variety of materials, which are 
usually open at each end?. Since they must reside in 
these cases, when they are become pupa, till the time of 
their final change approaches, if they are left open, how 
are the animals, now become torpid, to keep out their 
enemies? Or, if they are wholly closed, how is the water, 
which is necessasy to their respiration and life, to be in- 
troduced? These sagacious creatures know how to com- 
pass both these ends at once. They fix a grate or port- 
cullis to each extremity of their fortress, which at the 
same time keeps out intruders and admits the water. 
These grates they weave with silk spun from their anus 
into strong threads, which cross each other, and are not 
soluble in water. One of them, described by De Geer, 
is very remarkable. It consists of a small, thickish, circu- 
lar lamina of brown silk, becoming as hard as gum, which 
exactly fits the aperture of the case, and is fixed a little 
within the margin. It is pierced all over with holes dis- 
posed in concentric circles, and separated by ridges which 
go from the centre to the circumference, but often not 
quite so regularly as the radii of a circle or the spokes 
of a wheel. These radii are traversed again by other 
ridges, which follow the direction of the circles of holes; 
so that the two kinds of ridges crossing each other form 
compartments, in the centre of each of which is a hole®. 
4 Vor. I. 4th Ed. 467-- 
» Reaum, iii, 170. De Geer, ii. 519. 545, Prarr XVII. Fic, 11, 
