MEANS OF DEFENCE OF INSECTS. 261 



latter by its strength and tension prevents it from being 

 hurt by any external pressure. 



But of all the contrivances by which insects in this 

 state are secured from their enemies, there is none more 

 ingenious than that to which the may-flies ( TricJwptera) 

 have recourse for this purpose. You have heard before 

 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 pupae, till the time of their 

 final change approaches, if they are left open, how are 

 the animals, now become torpid, to keep out their ene- 

 mies ? Or, if they are wholly closed, how is the water, 

 which is necessary 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, cir- 

 cular 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 disposed 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 

 tile 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 kintls of ridges crossing each 

 ' Vol. I. 464— 



