The Storage of Protein, Fat, Glycogen and Uric Acid in the Fat Body and other Tissues of Mosquito Larvae

In the newly moulted fourth stage larva of Aedes aegypti the cells of the fat body contain fat droplets and numerous watery vacuoles. Protein is present only in the form of cytoplasmic strands between these inclusions. Some of the watery vacuoles contain glycogen; in, others there is uric acid in solution. Glycogen is present also in the ganglia and connectives of the central nervous system and in great masses in the sarcoplasm enveloping the muscles and between the muscle fibrils. During starvation, fat, glycogen and protein are used up concurrently. In 10-15 days (at 28° C.) all the stainable fat has disappeared and glycogen is absent or present in minute traces in the sarcoplasm only. Nuclei and cytoplasm in all tissues are much wasted. The utilization of protein is marked by a progressive accumulation of uric acid in the aqueous vacuoles of the fat body. In some of these uric acid may crystallize out during life. This accumulation of uric acid can be largely prevented by feeding on casein or on starch. When starved larvae are fed on starch there is a massive deposition of glycogen in the epithelium of the posterior half of the midgut. A little is deposited in the cells of the gastric caeca and occasionally in the cells of the Malpighian tubes. Subsequently glycogen collects in the cells, of the fat body until these are enormously distended with it and around the muscles until the sarcoplasm is practically solid with glycogen. Some appears also in the central nervous system. Rather small fat droplets collect round the nuclei of the fat body and in the oenocytes. After feeding on sugars, glycogen and fat have the same distribution as above. Different sugars vary in their efficiency as precursors of glycogen; from some (raffinose, sorbose, rhamnose, arabinose) no glycogen seems to be formed. After-feeding on olive oil, droplets of fat are limited to the cells of the anterior half of the midgut and to some cells in the gastric caeca. Occasionally droplets appear in the cells of the Malpighian tubes. Much fat collects in the oenocytes and in the fat body, but no glycogen is laid down. After feeding on casein there is a rapid increase in protein in the cytoplasm of all the tissues and the nuclei enlarge. Minute droplets of fat appear in the fat body, chiefly around the nuclei, and later pass outwards and enlarge. Fat accumulates also in the oenocytes. Glycogen is, deposited in large amounts in the cytoplasm and in indefinite vacuoles chiefly at the periphery of the fat body cells. It is laid down in great quantity in the sarcoplasm of the muscles, to a less extent in the central nervous system and later in the epidermis, Malpighian tubes and gut wall. Minute droplets rich in protein appear on the nuclear membrane of the fat body cells; they have the appearance of being discharged from the nuclei; gradually they enlarge and pass outward among the fat droplets. These protein droplets blacken with osmic acid but they do not stain with fat stains. They are basophil when first formed, becoming acidophil as they enlarge. After feeding on alanine or glutaminic acid glycogen is laid down, as after carbohydrates, in the caecal cells, the pyloric half of the midgut, the central nervous system, sarcoplasm and fat body. Small amounts of fat become visible in the fat body.