OF DIGESTION. 361 



of a particular species of fly cannot be considered as the cause of this 

 malady. 



These three different qualities of the saliva do not present themselves 

 separately, but more or less contemporaneously. The vegetable fibres 

 are by its admixture softened and loosened, then chemically changed 

 and made tender, or, as it were, scalded, and, lastly, by its intimate 

 incorporation it is rendered fit for assimilation and digestion. After 

 this preliminary change a second comminution takes place in the crop 

 when this organ exists. We consequently find among the mandibulate 

 insects salivary glands only in such species, genera, and families,, which 

 are more or less strictly herbivorous, for example, the grasshoppers, 

 Gryl/i, Termites, and they are entirely deficient in the carnivorous 

 ones. In them the larger quantity of gastric juice that is secreted 

 supplants the function of the saliva, whence it is that their intestine 

 beyond the crop is beset with a multitude of blind, doubtlessly gland- 

 ular, appendages ; and even if such appendages are found in the herbi- 

 vora, for example, in the grasshoppers and others, they are fewer in 

 number and smaller in size. Where both salivary vessels and these 

 appendages are wanting the long stomach is then entirely covered 

 with glands, as in Hydrophilus. In haustellate insects the saliva 

 attenuates the imbibed juices and becomes intermixed with it in the 

 process of sucking. Thus in the bees the salivary duct opens into the 

 same duct through which the honey is sucked ; in the Lcpidoptera, 

 through the central canal which is formed by the union of the two 

 probosces, and it drops down out of this channel whilst the insect is 

 sucking. Reaumur and Treviranus have both seen it fall in drops. 

 In the Hemiptera and flies it also opens into the proboscis, probably 

 here also, as in general, beneath the tongue ; by means of it the hard 

 setae are kept constantly lubricated, which facilitate their reciprocal 

 motion. It is also intermixed with the imbibed nutriment in the 

 mouth, it kills and scalds it, and thus prepares it for digestion, which 

 then next takes place in the long or subdivided stomach. In the 

 Cicada and bugs, the majority of which imbibe crude vegetable juices, 

 this preparation for digestion is of considerable importance, and we 

 therefore find in them very large salivary glands. 



221. 



The remaining function of digestion, subsequent to manducation and 

 the intermixture of the saliva, is exhibited less uniformly in insects than 



