76 OF THE SENSES OF 



Although insects appear to have dry, rigid mouths, 

 yet they possess the salivary glands, which are neces- 

 sary for moistening their food, and fitting it for 

 mastication. Professor Rennie has recently made some 

 conclusive experiments on this interesting subject. 

 He says, "one of the circumstances that first awakened 

 our curiosity with regard to insects, was the manner 

 in which a fly contrives to suck up, through its 

 narrow sucker, (or haustellum,} a bit of dry lump 

 sugar ; for the small crystals are not only unfitted to 

 pass, from their angularity, but adhere too firmly 

 together to be separated by any force the insect can 

 exert. Eager to solve the difficulty for there could 

 be no doubt of the fly's sucking the dry sugar we 

 watched its proceedings with no little attention ; but 

 it was not till we fell upon the device of placing some 

 sugar on the outside of a window, while we looked 

 through a magnifying glass on the inside, that we 

 had the satisfaction of repeatedly witnessing a fly let 

 fall a drop of fluid upon the sugar, in order to melt 

 it, and thereby render it fit to be sucked up, or. 

 precisely the same principle that we moisten with 

 saliva, in the process of mastication, a mouthful of 

 dry bread, to fit it for being swallowed, the action 

 of the jaws, by a beautiful contrivance of Providence, 

 pressing the moisture along the channels at the time 

 it is most wanted." 



To the investigations of Swammerdam, we are 

 indebted for our first knowledge of these vessels ; 

 he observed them in the small Nettle Tortoise-shell 

 Butterfly ; but he was unable to trace their termina- 



