DIRECT BENEFITS DERIVED FROM INSECTS. 335 



De Azara states that in Paraguay a spider, which is 

 found to near the thirtieth degree of latitude, forms a 

 spherical cocoon (for its eggs) an inch in diameter, of a 

 yellow silk, which the inhabitants spin on account of the 

 permanency of the colour*. And according to M. B. de 

 Lozieres, large quantities of a very beautiful silk, of daz- 

 zling whiteness, may be collected from the cocoons even 

 of the Ichneumons that destroy the larvae of some moth 

 in the West Indies which feed upon the indigo and cas- 

 sada b . 



It is probable, too, that other articles besides silk might 

 be obtained from the larvae which usually produce it, par- 

 ticularly cements and varnishes of different kinds, some 

 hard, others elastic, from their gum and silk reservoirs, 

 from which it is said the Chinese procure a fine var- 

 nish, and fabricate what is called by anglers Indian 

 grass c . The diminutive size of the animal will be thought 

 no objection, when we recollect that the very small quan- 

 tity of purple dye afforded by the Purpura of the ancients 

 did not prevent them from collecting it. 



I now conclude this long series of letters on the inju- 

 ries caused by insects to man, and the benefits which he 

 derives from them ; and I think you will readily admit 



a Voyage dans V Artier. Merid. i. 212. It may here be observed as 

 a benefit derived by the higher walks of philosophy from insects— that 

 astronomers employ the strongest thread of spiders, the one namely 

 that supports the web, for the divisions of the micrometer. By its 

 ductility this thread acquires about a fifth of its ordinary length. 

 Nouv. Diet, a" Hist. Nat. ii. 280. 



b American Phil. Trans, v. 325. 



c Anderson's Recreations in Agriculture, &c. iv. 399. 



