128 INTERNAL ANATOMY OF INSECTS. 
or infiltration, through the pores of the skin a ; a cir- 
cumstance which seems to indicate a certain conforma- 
tion of the pores both as to size and figure, so as to 
enable them to admit only one peculiar product. 
In treating on this subject, I shall first consider the 
organs of secretion, and next their products. 
I. Organs of Secretion. In general, these are mem- 
branous vessels that float in the blood or nutritive fluid, 
and secrete from it a peculiar substance. They may be 
denominated according to their products — Silk-secretors, 
Saliva-secretors, Varnish-secrctor, Jelly or Gluten-sccre- 
tor, Poison-sea-etor, and Scent-secrctors. 
i. Silk-Secretors (Serictcria). These organs are most 
remarkable in the caterpillars of the nocturnal Lcpido- 
ptera or moths, especially in that tribe called Bombyccs, 
to which the silkworm belongs : but this faculty is not 
confined to these insects, but is shared by many other 
larvce'xw different Orders; and in one instance at least, by 
the imago. In general, the outlet of the silk-secretors is 
at the mouth ; sometimes, however, as in the larva of 
Myrmeleon and the imago of Hydrophzlus, its exit is at 
the amis. The first is the organ which in the silk-worm 
provides for us that beautiful substance from which the 
animal takes its name. There are always txvo of these 
vessels, which are long floating tubes, growing slender 
towards the head of the insect, where they unite to form 
the spinneret (fusidus) before described b , which renders 
the silk. Their lower extremity also is commonly more 
slender than the middle, and is closed at the end. These 
organs are usually very much convoluted and twisted c . 
a Cuv. Anat. Comp. iv. 103— . b Vol. III. p. 124—. 
Malpigh. De Bombyc. t. y.f. 2. Swamm./. xxxiv./ 5. Lyonet, 
t.v.f.l. 
