The variation in fat and glycogen content of the bot fly (Gastrophilus intestinalis) larva tracheal organ during development.

Numerous analyses on the chemical composition of various insects have shown that, as in mammals, fat and glycogen constitute the principal food reserves. Among holometabolic insects the accumulation of these two substances during the larval instars is, to a varying extent, utilized during the process of pupation. Most of the data on the chemical changes during metamorphosis are summarized by Needham (1942). However, as pointed out by Wigglesworth (1939), analyses of the body as a whole show only the gross alterations, and almost nothing is known concerning the composition of the separate organs. The only information available concerns the glycogen content of the isolated insect fat body; thus, Babers (1941) found that in the mature larva of Prodenia eridania (Lep.) the glycogen formed 23-3 % of the fat body dry weight, while Yokoyama (1934) estimated the glycogen content of the silkworm fat body to vary from 2 to 17% of the dry weight according to age. No figures have been published for the fat content of the fat body, but histological evidence (Pardi, 1939; Wigglesworth, 1942) clearly demonstrates that fat is also present in high concentration. In the present work the changes in fat and glycogen occurring in the tracheal organ cells of Gastrophilus intestinalis de Geer during certain stages of development have been followed quantitatively. A description of the life cycle of Gastrophilus is given in earlier papers (Dinulescu,-1932; Keilin, 1944; Levenbook, 1950a), but it may briefly be mentioned that late 2nd instar and 3rd instar larvae up to the time of pupation live attached to the gastric mucosa of the horse's stomach. The duration of the 3rd instar lasts some 7-8 months, from November to June, and the greater part of this period is spent in a diapausing condition. From July onwards the larvae leave the alimentary tract and pupate in the soil. The tracheal organ consists of a bunch of very large cells occupying the posterior portion of the body. The structure of these cells, and their relation to the respiratory system, may be found in the works of Enderlein (1899), Prenant (1900), Portier (1911), Radu (1932) and Keilin (1944). As first shown by Dinulescu (1932) and confirmed by the present author, the cells are developed from undifferentiated fat body cells during the first larval instar. Apart from the possession of haemoglobin (Keilin & Wang, 1946) and intracellular tracheoles, the tracheal cells histologically appear similar to the normal fat body cells present in the more anterior part of the body,