28 TRANSFORMATIONS OF INSECTS. 



and evolve the fluid from their cellular structures. But the 

 glands of the herbivorous insects are not visible on the outside 

 of the stomach, and are contained in its walls instead. The 

 secretion of the gastric juice is determined by the stomach being 

 more or less filled, just as in man, and when this is the case 

 the little glands give forth their cellular and fluid contents in 

 great abundance. When the stomach is empty, the digestive 

 fluid is very scanty, loses its acidity, and is even occasionally 

 alkaline. Now this repeated secretion of a special acid fluid 

 which continues during the life of the caterpillar — for it is con- 

 stantly eating, except when moulting, or perhaps when in the 

 dark — is discontinued during the chrysalis state, and the glandular 

 structures become atrophied and often lost altogether, especially 

 in those perfect insects which do not take food. 



The intestine follows the stomach, and its commencement is 

 indicated by a constriction and by the attachment of the liver 

 canals ; the constriction has a fold internally which prevents the 

 too rapid passage of the food out of the stomach into the intestine. 

 It is very remarkable that the variation in the length and shape 

 of the intestine should not depend upon the food or upon the 

 habits of the insect, and that it should not difl"er much in these 

 respects in the larva,, the chrysalis, and the perfect insect. The 

 intestinal canal ends in pocket-shaped enlargements or in simple 

 odd-shaped swellings, one of which is shown in the case of 

 Dytisais, 



The salivary glands are situated on each side of the oesophagus, 

 and look like twisted tubes, or sacs with cellular walls. Usually 

 there are two or three pairs of glands, and they are of two kinds. 

 For instance, in the grasshopper, whose salivary glands are large, 

 they consist of one pair of bunches of bag-like swellings, and of 

 another pair of elongated tubes. ' Both kinds secrete special 

 fluids, and their admixture is effected in the canal which is 

 common to the two sorts of glands, and the saliva flows straight 

 into the mouth. The saliva is slightly alkaline, and not only 

 lubricates the morsel during deglutition, but assists in the diges- 

 tion of it also. In the Lcpidoptcra, or butterfly tribe, the salivary 

 glands are simple elongated tubes, which are largely developed in 

 the caterpillar, and more or less atrophied in the chrysalis and 



