Cvi . Digestion in the Cockroach

THE most recent account of the digestive processes of the cockroach is that given by Swingle [1925], who employed the Common Cockroach (Blatta orientalis) in his experiments and whose conclusions are briefly as follows. The salivary glands produce only an amylase. The hepatic caeca of the midgut produce an invertase, maltase, lipase and protease. The remainder of the mid-gut (stomach) secretes the same enzymes though less abundantly. The protease he divides into a " trypsin " and an " erepsin "; but the tests employed for erepsin were (1) the formaldehyde titration of a peptone digest, and (2) the production in this digest of nitrogenous compounds not precipitated by phosphotungstic acid; both of which tests would be given equally well by trypsin alone. The hind-gut does not secrete digestive enzymes. The greater part, if not the whole, of digestion takes place in the crop, the enzymes passing backwards from the salivary glands and forwards from the mid-gut. The crop is lined by a membrane of chitin and is generally regarded as playing no part in the secretion of digestive ferments; but Swingle claims that extracts of washed crops contain traces of invertase, maltase and lipase, and the same claim is made by Sandford [1918] for lipase. Swingle does not decide from his evidence whether these enzymes are formed in the cells of the crop or absorbed by them from the digestive cavity. It may be mentioned here that the results of Swingle have been confirmed, but that, in addition, an invertase has been found in the salivary glands of the German Cockroach (Blatella germanica), though not in the American Cockroach (Periplaneta americana) and that small quantities of lactase have been demonstrated in extracts of the mid-gut. With regard to the enzymes of the crop, the results of Swingle have been repeated by his method. The reliability of these results depends, however, upon the successful washing of the crops, a procedure which is not easily carried out, since the crops contain earthy material which is often very difficult to remove; and it has been shown that if the gut is washed out thoroughly in the living animal by giving it water but no food for a fortnight or three weeks, extracts of the crop are free from maltase and invertase, while the other parts of the digestive system retain