72 SUPPLEMENT ON 



their materials, on which to operate each in its own peculiar 

 mode, for the performance of the secretions. 



Such is the mode in which, in the actual state of anatomical 

 and physiological science, we must suppose that nutrition 

 takes place in insects. It is by the porosity of the intestinal 

 tube, that the materials most proper for alimentation are 

 separated from the indigested mass. Their division is such 

 that they then form a sort of vapour, the molecules of which 

 are, probably, absorbed in the form of gas, which would then 

 be contained by the numerous trachae with which the intesti- 

 nal tube is covered. 



There are other secretions, however, which take place in 

 insects, but the mode of this separation of humours is very 

 little known. We know, for example, that the acid produced 

 by the ants, and which is analogous to that of vinegar, is dis- 

 gorged by the insects ; that many other animals of the same 

 class, eject or disgorge, at will, certain matters, either foetid 

 or hurtful, at the moment when they believe themselves in 

 danger. 



The secretions of insects, according to Mr. Kirby, are as 

 follow : — sill: ; saliva ; varnish, or gum ; jelly ; oils ; milk ; 

 honey ; wax ; poisons and acids ; odorous Jluids or vapours, 

 and luminous matter. We must dismiss the consideration of 

 these very briefly here, reserving for the future portions of 

 the work, a more detailed account of them. Many insects 

 discharge from their mouth a fluid resembling the saliva of 

 larger animals. The use of this seems, generally, to be to 

 dilute the food. Several flesh-eating, and herbivorous coleop- 

 tera, under the influence of fear, emit a coloured acrid fluid 

 from the mouth. This does not appear to be secreted by the 

 usual salivary vessels, nor must we suppose that every fluid 

 ejected from the mouth of insects is saliva. The eggs of 

 many insects, when excluded from the oviduct, are covered 

 with a sort of varnish or gum, that they may adhere to the 



