264 MEANS OF DEFENCE OF INSECTS. 



we owe one of the most valuable articles of commerce, 

 the silk that gives lustre to the beauty of our females. 

 These shrouds are sometimes double. Tiius the larvae 

 of certain saw-flies spin for themselves a cocoon of a 

 soft, flexible, and close texture, which they surround 

 with an exterior one composed of a strong- kind of net- 

 work, which witJistands pressure like a racket '. Here 

 nature has provided that tise inclosed aniir.al shall be 

 protected by tiie interior cocoon from tie injury it 

 might be exposed to from the harshness of the exterior, 

 while the latter by its strength and tensi n 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 {Phyganea, 

 L.) have recourse for this purpose. You have Iieard 

 before that these insects are at first aquatic, and inha- 

 bit curious cases made of a variety of materials, wliich 

 are usually open at each end *•. Since they must re- 

 side 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 necessary to their 

 respiration and life, to be introduced ? These saga- 

 cious creatures know how to compass botli these ends 

 at once. They fix a grate or portcullis to each extre- 

 mity 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. 



" Rcaum. V. ICO. " Vol. I. 2d Ed. 467 — 



