130 INTERNAL ANATOMY OF INSECTS. 
and genera emit scents from their anus, and from various 
other parts of the body, of which having before given you 
a very full account*, I shall proceed to the consideration 
of the secretions themselves: but first I must observe, 
that in many cases, as in some of the cottony and pow- 
dery Aphides, Chermes, &c., the substance secreted ap- 
pears to be a transpiration through the pores of the body, 
a kind of excretion from the superabundance of its fluid 
contents’. In many, however; this secretion transpires 
through appropriate orifices: thus in Aphis Abietis, 
which produces those curious galls resembling the cone 
of a fir °, the flocoons of seem*ng cotton that cover it pro- 
ceed from little oval concavities on its back, four of which 
are arranged in a transverse line on each dorsal segment 
of the abdomen: these concavities have minute tubercles 
probably terminating in a pore’. In Aphis Fagi the 
cottony docoons are almost an inch long ®. 
The secretions of insects may be considered under the 
following heads—Swk; Saliva; Varnish or Gum; Jelly ; 
Oils; Milk; Honey; Waz: Poisons and Acids; Odorous 
fluids and Vapours ; and Luminous matter. 
i. S2lk. ‘This valuable preduct of insects, while in the 
silk-secretor, assumes in the Lepédoptera the appearance 
of a viscid gum, but the moment it is exposed to the air 
it hardens into a silken thread. It is remarkable for the 
following qualities :—it dries the instant it comes in con- 
tact with the air; it is then insoluble not only in water 
but in the most active solvents, and even heat has no ef 
@ Vor. Il. p. 241—. Tl, p. 148—. » De Geer iii. 41. 
© Vor. I, p. 454, where by mistake it is represented as the work 
of A. Pini. ° De Geer iii. 111.) * Reaum. iii. t. xxvi. f 4—6. 
