Mar. 23, 1888.] 



SCIENTIFIC NE^ATS. 



275 



Natural 1t(0iorf|. 



WILD SILKS. 

 The secretion of silk is certainly one of the most 

 curious processes which take place in the animal 

 economy. We find it elaborated in larger or smaller 

 quantities by the vast majority of true insects, by most 

 of the spider-group, and even by some molluscs, or, as 

 they are popularly called, shell-fish. It is singular that such 

 a multitude of creatures, differing in their structure, their 

 habits, and their diet, should all agree in producing a 

 material substantially identical in its nature, though sub- 

 serving very different purposes. Nor is it easy to see 

 why if silk-secreting extends so far in the animal kingdom 

 it is not met with also in other groups. 



In the body of the animal silk exists as a semi-trans- 

 parent gummy fluid, which, in the silk-worm and its near 



est stage in the Lepidoptera, which are generally 

 regarded as the most modern group. Hence we may 

 fairly infer that the spinning faculty has been graduallj' 

 developed since the first appearance of insects upon the 

 earth. 



It further appears that, with two or three exceptions, 

 the secretion of silk in insects is confined to their im- 

 mature, or larval stage, and that this material serves 

 chiefly for enfolding and securing the chrysalis until it 

 reaches maturity. 



In spiders we find some important differences. The 

 organs for secreting and emitting the silk are placed in 

 the posterior part of the body. The elaboration of silk 

 takes place during the whole life-time, and its purpose is 

 chiefly to serve in the capture of prey, though the 

 females of some species enclose their eggs in a strong 

 silken bag which they carry about. 



Whenever silk is referred to in a technical or com- 

 mercial point of view, we understand the product of 



Antherea Mylitta. 



allies is contained in two long bags, each provided with 

 an exit duct terminating in apertures near the mouth 

 from which the silk can be emitted at will and attached 

 to any substance. In its composition silk approaches to 

 white of egg and to gelatine. But, unlike the former, it 

 contains no sulphur. From the latter it differs by a 

 physico-chemical property not fully explained. Every 

 one knows that if glue or gelatine be exposed to damp air 

 it softens ; if soaked in water it swells up, and if heat be 

 applied it dissolves. If it be allowed to stand for any 

 length of time, especially in a damp, warm place, it 

 becomes mouldy and is decomposed, with an exceedingly 

 offensive smell. The behaviour of silk is quite different. 

 When once it has issued from the body of the insect and 

 dried, it can no longer be brought back to its original 

 liquid state. It does not soften or swell up in moist air, 

 and it may be pronounced incapable of putrefaction. 



It is remarkable that in the group Orthoptera, the 

 locust and grasshopper tribe, to which the most ancient 

 insects known belong, the power of secreting silk does 

 not exist. On the other hand, this power is at its high- 



certain moths of the genus Bombyx, or of certain nearly 

 allied groups. These insects alone yield silk in such 

 quantity as to be capable of industrial application. 

 Spiders do not produce their thread in abundance ^at 

 once so as to admit of ready collection. Nor can they 

 be reared in numbers together on account of their 

 quarrelsome disposition and their disposition to stray. 



The best known and most valued silk-insect is Bombyx 

 tnori, which feeds on the mulberry, and which, though 

 a native of China, has for a long time been reared in a 

 domestic condition in France, Italy, and other of thewarmer 

 regions of Europe. Though small, this species possesses 

 several advantages over its larger allies. Its cocoons can be 

 easily unwound or " reeled," so as to yield an unbroken 

 thread. The silk is easily freed from colouring matters, 

 and from the so-called gum, and brought to a fine, clean, 

 white condition ready for the manufacturer and the 

 dyer. 



It is very possible that this silk-worm would have con- 

 tinued to supply the wants of European industry, so that 

 the so-called wild silks, the produce of other and larger 



