SILK. 
487 
thick silk. The bruchid Caryborus gonagra prepares a cocoon of coarse 
gummy threads that almost come within our definition of silk but which 
are also very little removed from the typical gummy secretion of other 
Coleoptera. 
The production of silk as described above is general throughout the 
Hesperiidce and all Heterocera during larval life. In many Micro-Lepi- 
doptera, the silk is of vital importance in larval life, being used to fasten 
together the habitations of leaves or other matter. So also in Hesperiidce 
and Psychidce. In other groups the larvae use silk in very early 
life only (e.g., Caradrina and other Noctuids) or use it to let themselves 
down from leaf to leaf ; practically all can use it to make a rough surface 
to walk on if they are placed on too smooth a surface ; and finally, nearly 
all use it in the preparation of the cocoon or of the covering of leaves, 
etc., which shelters them during the pupal period. 
Among Diptera silk is produced in the normal manner by the larva 
of Simulium , which lives in hill streams and fastens a network of threads 
across, along which it moves; some other Diptera which live in soil under 
bark, etc., also prepare silken shelters. 
Coccidce are the last family in which silk can be said to be formed, 
and this silk is the product of the numerous glands on the pygidium; 
the scale of the Diaspince is formed of felted threads of silk produced 
from these glands with the cast skins of the previous instars. In this 
family as in Aleurodidce the glands producing waxy thread-like excre¬ 
tions are very abundant and can, strictly speaking, be hardly called silk 
glands ; in the Diaspinae alone is the secretion used as silk is. 
The composition of silk* —The main constituents of silk fibre are 
fibroin coated with a glue-like substance called Seriein or Silk-Albumen, 
and coloured with ceraceous matter. Lombardy silk (Bombyx mori) 
yields about 70% fibroin and 30% seriein and even technically purified 
silk contains about 5% glue. Fibroin is insoluble in superheated water, 
in dilute acids or alkalies; it can be heated to 120° C. for hours without 
any change taking place. 
Seriein is soluble in weak alkaline solutions and in hot water, from 
which it can be precipitated as a white powder by alcohol. The yellow 
colouring matter of raw silk consists of chlorophyll (the green colouring 
matter of plants) more or less altered and deprived of its blue constituent. 
From the cocoon unaltered chlorophyll may be extracted. 
Fibroin is obtained by exhausting the silk with boiling water, alcohol 
(to dissolve and precipitate seriein), ether and acetic acid successively ; 
analysis shows a very high nitrogen and a very low carbon content. It 
differs in its constitution very considerably from other albumins, as it 
contains more than 50% Glycocoll, 10% of tyrosin and only a small amount 
of leucin. Glutaminic and Aspartic acids are absent, and the base 
radicles are slight in amount. It gives the biuret and Millon’s reactions, 
* Kindly {-repared by J. H. Barnes, Esq., Agricultural Chemist, Fumjab. 
