1885.] 
Silk and its Secretovs. 
347 
may not unfairly infer that the secretion of silk is a function 
which has gradually been evolved since the first appearance 
of inseCts upon the earth. We find also that silk-spinning 
reaches its maximum development among the Lepidoptera, 
which are generally recognised as the most modern group. 
We say its maximum because there is not one butterfly or 
moth known which does not spin to a greater or less extent, 
whilst the silk-worm and its near allies produce silk in a 
greater proportion to their own weight than does any other 
animal. 
_ All the species already noticed, broadly speaking, secrete 
silk in their larval state only, though indeed it is said that 
two inserts are capable of spinning also when mature. 
Further observation is here wanting. 
But among another Arthropod group, the spiders, the case 
is very different. Here a metamorphosis does not exist at 
all, and silk is of course secreted, if at all, by the mature 
animal. 
However, the power of spinning is here less universal 
than in certain insert genera. Not merely the scorpions and 
other outlying groups, but many true spiders do not con- 
struct webs. Among those which do spin there is a 
wide range in the quantity of material at the creature’s 
disposal and in the completeness of the work. Yet a certain 
writer has professed an inability to recognise here the 
workings of evolution. This is strange, if we reflect that 
amongst spiders we can trace successive stages, from the 
single thread used by some roving species to secure them- 
selves against a fall when leaping upon their prey, from the 
few straggling threads laid upon the very surface of a wall 
or a tree-trunk, in which a creeping inseCt may entangle its 
feet, the shelf-like horizontal webs of the Tegenaria, which 
are soon clogged and rendered conspicuous by dust settling 
upon them, to the geometrical net of the Epeira extended in 
a vertical position between two trees, and which, in some 
lights, approaching inseCts seem unable to see. Surely, we 
repeat, we have here a series of phenomena not opposed to, 
but in harmony with, the main principles of evolution. 
An interesting faCt is that the silk-organs in the spider 
and in the inseCt, though subserving the same purpose, 
cannot be pronounced as respectively homologous. The 
caterpillar and all other spinning inseCts give out their 
thread from two organs situate on the upper lip, while 
the spinnerets of the spider are placed at the posterior 
extremity of the body and are consequently no modification 
of the parts of the mouth. 
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