Silk and its Secretors. 
[June, 
34 6 
We said that “ in one point only ” silk differs chemically 
from wool, &c., and that one point is very interesting. It 
contains, like gelatine, no sulphur. But between gelatine 
and silk the difference is wide. Gelatine dissolves in hot 
water and softens even in damp air. Silk, as we have just 
seen, is not adted upon by water. It exists, indeed, in 
solution in the secretory reservoirs of the spider, the silk- 
worm, and other creatures. But when once it has been 
emitted and has dried and hardened, which it quickly does, 
it shows no disposition to resume a liquid state. 
As regards the successive steps of the process by which 
silk is elaborated from the fluids of certain animals, and, of 
course, ultimately from their food, there is a sad lack of 
precise knowledge. One point is certain, that the diet 
of the silk-spinner has no diredt influence upon the com- 
position of the produdt. We find it yielded, though in 
varying quantities, by creatures feeding upon every kind of 
matter which is eatable at all, — upon the juices of living 
animals, the remains of dead ones, upon wood, leaves, 
fruits, and the pollen of plants. But all, we believe, eat 
heartily. The elaboration of a stock of highly nitrogenous 
matter over and above what is required for growth, for 
maintenance, and for the reproduction of the species, necessi- 
tates an abundance of nutriment. 
We come now to- the question what animals are silk- 
secretors, and we may be surprised to find that perhaps half 
the existing species of animals secrete silk, though in very 
varying quantities. It may probably be said that all insedts 
which pass through a metamorphosis, that is such as on 
issuing from the egg are unlike their parents and have to 
pass through the successive states of a larva and a pupa, 
will be found to possess glands and reservoirs for the pro- 
duction and storage of this secretion. This would include 
all the Lepidoptera, probably all the Hymenoptera, a 
majority of the Diptera and Coleoptera, and some Neuroptera. 
The Orthoptera, Homoptera, and Heteroptera, which are 
hatched in forms closely resembling those of the perfect 
insects, save that the wings are generally absent, do not 
appear to secrete silk. It will be seen at once that among 
such a vast multitude of species not all can have been ex- 
amined for the possession of silk-glands, and the rules we 
lay down must be regarded as merely tentative. 
It is interesting to note that in the genus — so-called — to 
which the most ancient insedts hitherto discovered belong, 
i.e., the Orthoptera, the locust and grasshopper tribe, the 
power of secreting silk is not known to exist. Hence we 
