MEANS OF DEFENCE OF INSECTS. 263 
assailants. But even here they are taught to conceal 
themselves from their enemies by various and singular 
contrivances. Some seek for safety by burying them- 
selves, previously to the assumption of the pupa, at a 
considerable depth under the earth; others bore into the 
heart of trees, or into pieces of timber; some take their 
residence in the hollow stalks of plants; and many are 
concealed under leaves, or suspend themselves in dark 
places, were they cannot readily be seen. But in this 
state they are not only defended from harm by the situa- 
tion they select, but also by the covering in which num- 
bers 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 ar- 
ticles of commerce, the silk that gives lustre to the beauty 
of our females. These shrouds are sometimes double. 
Thus the larvee 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 withstands pressure like a racket?. 
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 
4 Reaum. v. 100. 
