321 TEXT-BOOK OF ENTOMOLOGY 



b. Digestion in insects 



For the most complete and reliable investigation of the process of 

 digestion, we are indebted to Plateau, whose results we give, besides 

 the conclusions of later authors : 



In mandibulate or biting insects, the food is conducted through 

 the oesophagus by means of the muscular coating of this part of the 

 digestive canal. Suctorial insects draw in their liquid food by the 

 contractions followed by the dilatations of the mid-intestine (chylific 

 stomach). Dragon-flies, Orthoptera, and Lepidoptera swallow some 

 air with their food. 



Where the salivary glands are present, the neutral alkaline fluid 

 secreted by them has the same property as the salivary fluid of 

 vertebrates of rapidly transforming starchy foods into soluble and 

 assimilable glucose. In such forms as have no salivary glands, 

 their place is almost always supplied by an epithelial lining of the 

 oesophagus, or, as in the Hydrophilidse, a fluid is secreted which has 

 the same function as the true salivary fluid. 



Nagel states that the saliva of the larva of Dyticus is powerfully 

 digestive, and has a marked poisonous action, killing other insects, 

 and even tadpoles of twice the size of the attacking larva, very 

 rapidly. The larvae not only suck the blood of their victims, but 

 absorb the proteid substances. Drops of salivary juice seem to 

 paralyze the victim, and to ferment the proteids. The secretion is 

 neutral, the digestion tryptic. Similar extra-oral digestion seems to 

 occur in larvae of ant-lions, etc. (Biol. Centralbl., xvi, 1896, pp. 51-57, 

 103-112 ; Journ. Roy. Micr. Soc., 1896, p. 184.) 



In carnivorous insects and in Orthoptera, the oesophagus dilates 

 into a crop (ingluvies) ended by a narrow, valvular apparatus (or 

 gizzard of authors). The food, more or less divided by the jaws, 

 accumulates in the crop, which is very distensible ; and, when the 

 food is penetrated by the neutral or alkaline liquid, there undergoes 

 an evident digestive action resulting, in carnivorous insects, in the 

 transformation of albuminoid substances into soluble and assimilable 

 matter analogous to peptones, and, in herbivorous insects, an abun- 

 dant production of sugar from starch. This digestion in the crop, a 

 food-reservoir, is very slow, and, until it is ended, the rest of the 

 digestive canal remains empty. 



" Any decided acidity found in the crop is due to the injection of 

 acid food ; but a very faint acidity may occur, which results from 

 the presence in the crop of a fluid secreted by the csecal diverticula 

 of the mesenteron." (Miall and Denny.) 



