INTERNAL ANATOMY OF INSECTS. 137 
but in the most active solvents, and even heat has no ef- 
fect upon it to melt or soften it : indeed, without these 
qualities it would be of no use to us\ As soon as it 
leaves the spinneret it becomes the thread we call silk, 
which being drawn through two orifices is necessarily 
'double through its whole length. This thread varies 
considerably in colour and texture, as has been before 
stated 5 , and sometimes resembles cotton or wool rather 
than silk. In spiders it is of a much softer and more 
tender texture than that of other spinning insects ; and 
Mr. Murray seems to have proved that it is imbued, in 
the case of the gossamer, with negative electricity : in 
the sericterium the fluid that produces it is sometimes 
white or grey, and at others yellow . A remarkable 
gnat (Ceroplatus tipuloides), living on an agaric, carpets 
its station of repose and its paths with something be- 
tween silk and varnish, which it spins, not in a thread, 
but in a broad riband d . 
ii. Saliva. Many insects have the power of discharg- 
ing from their mouth a fluid which seems in some degree 
analogous to the saliva of larger animals. Thus many, 
as Lepidoptera, Hemiptera, Diptera, &c, can dilute 
their food, and render it fitter for deglutition. I have 
seen a common fly when not employed in eating, emit a 
globule of fluid as big as a grain of mustard-seed from 
its proboscis, and retract it again. On a former occa- 
■ N. Diet. d'Hist. Nat. vi. 305. b Vol. III. p. 221. 
e Treviran. Arachnid. 44. In Paraguay a spider is found which 
makes spherical cocoons of yellow silk, which are spun because of 
the permanence of the colour. This operation occasions a flow of 
water from the eyes and nose of the spinners. Azara Voyag. 212. 
See also Murray in Werner. Trans. 1823. 8—. d Rcaum. v. 24. 
