OF DIGESTION. 359 



insects,, particularly in those which feed upon vegetable substances, they 

 secrete a peculiar white, frequently perfectly hyaline fluid, which appears 

 to be of an alkaline nature, and becomes intermixed with the food in the 

 mouth itself. This intermixture has a threefold purpose, namely, 



1. The mechanical dilution of the nutriment. This attenuation is 

 the more necessary, particularly in such insects which feed upon hard 

 vegetable substances, from their containing very generally but little 

 moisture, and their comminution in the mouth must necessarily be more 

 difficult than when the food consists of soft animal substances. Thus 

 by manducation, and being mixed with the saliva, it becomes changed 

 into a thick pap, upon which the stomach can more easily act. The 

 grasshoppers, Grylli, larvae of the Capricorns, the wood borers, and the 

 caterpillars of the Cossus, appear especially to require this mechanical 

 attenuation of the food, from its generally consisting of hard wood. 



2. The chemical effect of the saliva upon the nutriment is still more 

 apparent. The saliva, by its very constitution, is a poison which as it 

 were kills the food, depriving it of its natural living quality, and 

 thereby transforming it into a scalded state. This is proved by the bite 

 of poisonous serpents, whose poison is nothing else than the saliva 

 secreted by peculiar glands. According to Humboldt * the saliva of 

 serpents alone suffices to change the flesh of recently killed animals 

 into a gelatinous substance, and they therefore lick their prey all over 

 before they swallow it. The saliva of insects has a similar effect. 

 Immediately after swallowing and the intermixture with the saliva in 

 the mouth, the green leaves upon which caterpillars feed lose their 

 bright colour and acquire by degrees a darker dirty colour, resembling 

 that of boiled vegetables. The puncture also of blood-sucking insects 

 convinces us, most distinctly, by the pain of the wound, of the corrosive 

 effects of the saliva, and the inflammation attendant upon it, of its 

 transforming power. 



3. The dynamical effect of the saliva, under which we understand its 

 faculty of changing the food into that state that the requisite nutri- 

 mental substances can be separated from it. It therefore requires no 

 further proof, for it is evinced by too many experiments that the saliva 

 does not always act in the same way, but that its effects are different ac- 

 cording to the differences of individuals ; consequently a variety of insects 

 may feed upon the same materials and yet produce very different effects 



* Ansicht del 1 Nalur. torn. i. p. 141. 



