214 



MEANS OF DEFENCE OF INSECTS. 



be seen. But in this state they are not only defended from 

 harm by the situation they select, but also by the covering 

 in which numbers envelop themselves ; for, besides the 

 leathery case that defends the yet tender and unformed 

 imago, many of these animals know how to weave for it a 

 costly shroud of the finest materials, through which few of 

 its enemies can make their way ; — and to this curious 

 instinct, as I long since observed, 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. Thus the larvae of certain saw-flies spin for them- 

 selves 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 withstands pressure like a racket. 1 

 Here nature has provided that the inclosed animal shall 

 be protected by the interior cocoon from the injury it 

 might be exposed to from the harshness of the exterior, 

 while the 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 ( Trichoptera) 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 enemies ? Or, if they are wholly closed, how 

 is the water, which is necessary to their respiration and life, 

 to be introduced ? These sagacious creatures know how to 

 compass 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 



Reaum. v. 100. 



