136 INTERNAL ANATOMY OF INSECTS. 



emit scents from their anus, and from various other parts 

 of the body, of which having before given you a very 

 full account a , 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 powdery Aphi- 

 des, Chermes, &c., the substance secreted appears to be 

 a transpiration through* the pores of the body, a kind of 

 excretion from the superabundance of its fluid contents b . 

 In many, however, this secretion transpires through ap- 

 propriate orifices : thus in Chermes Abietis, which pro- 

 duces those curious galls resembling the cone of a fir % 

 the flocoons of seeming cotton that cover it proceed from 

 little oval concavities on its back, four of which are ar- 

 ranged 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 flocoons are almost an inch long e . 



The secretions of insects may'be considered under the 

 following heads Silk ; Saliva ,- Famish or Gum ; Jelly ; 

 Oils; Milk ; Honey ; Wax ; Poisons and Acids ; Odorous 

 fluids anql Vapours ; and Luminous matter. 



i. Silk. This valuable product of insects, while in the 

 silk-secretor, assumes in the Lepidoptera 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 



z VOL. II. p. 241. III. p. 147. b De Geer Hi. 41. 



e VOL. I. p. 451, where by mistake it is represented as the work 

 of Aphis Abietis. d De Geer iii. 111. 



c Reaum. iii. t. xxvi. /. 4 6. 



