Studies on the Nutrition of Blow-Fly Larvae

Abstract

1. The mid-gut in Lucilia larvae can be divided into three distinct regions (termed anterior, middle and posterior segments). 2. Histologically the anterior and posterior segments are similar. In a feeding larva the cells are highly vacuolated and contain fat; in a starved larva the cells are deeply staining and non-vacuolated. In the middle segment the cells are always deeply staining and free from vacuoles and fat, whatever the state of nutrition. 3. The hydrogen-ion concentration varies along the gut and with the nature of the food. With liquefied meat as food, the pH is 7.5-8.0 in the crop, 7.5 in the anterior segment, 3.0-3.5 in the middle segment, 7.5-8.3 in the posterior segment, and 8.0-8.5 in the hind-gut. With fresh gelatine (pH 7.0) as food, the values are the same except in the crop and anterior segment, for which the figures are respectively pH 7.0 and 6.5. 4. It has been suggested that the acidity in the middle segment may be due to an acid secretion, the most likely component being phosphoric acid. The alkaline reaction in other parts of the gut is probably caused by ammonia, which is present in the gut-contents and excreta. 5. Tryptase, peptidase and lipase are present in the mid-gut, the enzymes being concentrated in the anterior and posterior segments. The proteolytic enzymes persist in the excreta and some extra-intestinal digestion, therefore, can occur without the aid of micro-organisms. Carbohydrate-splitting enzymes are absent except for a feeble secretion of amylase in the salivary gland. 6. By combining the evidence from various sources, I have attempted to obtain a complete picture of the process of digestion, which I suggest is as follows: The food is stored unchanged in the crop and, passing into the mid-gut, is rapidly forced into the middle segment. Some absorption of water and simple products of bacterial action occurs along the anterior segment, the concentration of the food being completed in the middle segment where the acidity prevents digestion. The food, by now of a pasty constituency, passes into the posterior segment, dissolves in the alkaline fluid and is digested and absorbed. The digestive enzymes are secreted in the anterior and posterior segments, but digestion does not progress far in the anterior segment owing to the rapid passage of the food.