DIRECT BENEFITS DERIVED FROM INSECTS. 191 



stronger that twenty threads will carry an ounce more. 1 Don Luis Nee 

 observed on Psidium pomiferum and pyriferttm ovate nests of caterpillars 

 eight inches long, of grey silk, which the inhabitants of Chilpancingo, 

 Tixtala, &c., in America, manufacture into stockings and handkerchiefs. 2 

 Great numbers of similar nests of a dense tissue, resembling Chinese 

 paper, of a brilliant whitenesu, and formed of distinct and separable layers, 

 the interior being the thinnest and extraordinarily transparent, were ob- 

 served by Humboldt in the provinces of Mechoacan and the mountains of 

 Santarosa, at a height of 10,500 feet above the level of the sea, upon the 

 Arbutus Madrono, and other trees. The silk of these nests, which are the 

 work of the social caterpillars of a Bombyx (B. Madrono), was an object 

 of commerce even in the time of Montezuma ; and the ancient Mexicans 

 pasted together the interior layers, which may be written upon without 

 preparation, to form a white glossy pasteboard. Handkerchiefs are still 

 manufactured of it in the Intendency of Oaxaca. 8 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. 4 And according to M. B. de Lozieres, large quantities of a 

 very beautiful silk, of dazzling whiteness, may be collected from the 

 cocoons even of the Ichneumons that destroy the larvas of some moth in 

 the West Indies, which feed upon the indigo and cassada. 5 



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

 from the larvae which usually produce it, particularly 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 varnish, and 

 fabricate what is called by anglers Indian grass. 6 The diminutive size of 

 the animal will be thought no objection, when we recollect that the very 

 small quantity 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 injuries caused by 

 insects to man, and the benefits which he derives from them ; and I think 

 you will readily admit that I have sufficiently made good my position, that 

 the study of agents which perform such important functions in the eco- 

 nomy of nature must be worthy of attention. Our subsequent corre- 

 spondence will be devoted to the most interesting traits in their history 

 as their affection to their young, their food and modes of procuring it, 

 habitations, societies, &c. 



I am, &c. 



1 Pullein in Phil Trans. 1759. 54. 



2 Annals of Botany, ii. 104. 



5 Political Essay on N. Spain, iii. 59. 



4 Voyage dans I'Amer. 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 divi- 

 sions of the micrometer. By its ductility this thread acquires about a fifth of its 

 ordinary length. Nouv. Diet. d'Hist. Nat. ii. 280. 



American Phil. Trans, v. 325. 



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



