Plateau’s researches upon _Periplaneta americana_,[132] modified by subsequent experiments upon _P. orientalis_,[133] and by still more recent observations, lead him to the following conclusions[134]:-- [132] Bull. Acad. Roy. de Belgique, 1876. [133] Ib., 1877. [134] We are indebted to Prof. Plateau for the statement of his views given in the text. 1.--The saliva of the Cockroach changes starch into glucose; but the saliva is not acid, it is either neutral (_P. orientalis_) or alkaline (_P. americana_). Any decided acidity found in the crop is due to the ingestion of acid food; but a very faint acidity may occur, which results from the presence in the crop of a fluid secreted by the cæcal diverticula of the mesenteron. 2.--The glucose thus formed is absorbed in the crop, and no more is formed in the succeeding parts of the digestive tube. 3.--The function of the gizzard is that of a grating or strainer. It has no power of trituration. If the animal consumes vegetable food rich in cellulose, a substance not capable of digestion in the crop, the fragments are found unaltered as to form and size in the mesenteron. If it is supplied with plenty of farinaceous food, such as meal or flour, the saliva is not adequate to the complete solution and transformation of the starch, and the intestine is found full of uninjured starch granules, which must have traversed the gizzard without crushing. 4.--The cæcal diverticula secrete a feebly acid fluid. To demonstrate its acidity an extremely sensitive litmus solution, capable of indicating one part in twenty thousand of hydrochloric acid, must be used. The fluid secreted by the cæca emulsifies fats, and converts albuminoids into peptones. In all Insects digestion is effected in the following way (which is particularly easy of demonstration in _Carabus_ and _Dytiscus_). The crop is filled with food coarsely divided by the mandibles, and the gizzard being shut to prevent further passage, the fluid secretion of the cæca ascends to the crop, and there acts upon the food. Digestion is effected in the crop, and not beyond it. This is clear beyond doubt. In Decapod Crustacea also it is very easy to prove that the fluid secreted by the so-called liver ascends into the stomach (which corresponds to the crop, together with the gizzard of the Insect). To satisfy ourselves on this point we have only to open a Crayfish during active digestion. When digestion in the crop is finished, the gizzard relaxes, and the contents of the crop, now in a semi-fluid condition, pass into the mesenteron, which is devoid of chitinous lining, and particularly fitted for absorption.