Summary 1. The appearance of larval fat body as seen under the light or electron microscope depends on the nutritional state of the larva and on the stage of larval development at which the fat body is observed. 2. Early in the last larval instar the cells usually possess a well‐developed endo‐plasmic reticulum rich in ribosomes, numerous mitochondria, glycogen granules, a Golgi complex and fat droplets, while later in the instar the endoplasmic reticulum is much reduced and mitochondria are few, but glycogen and fat droplets are present in greater amount together with the appearance of large numbers of proteinaceous spheres. 3. Early in the last instar the fat body synthesizes proteins and exports them into the blood, while later in the instar proteins are sequestered from the blood into the fat body. 4. The rate of protein synthesis by the fat body is high in the early to mid part of the last instar, but then falls off rapidly to a low level, at which it remains until the larva pupates. In diapausing pupae, protein synthesis remains at this low level. 5. The similarity between the electrophoretic patterns of proteins from the fat body and those from the blood provides strong evidence that the fat body is the site of synthesis of many of the blood proteins. 6. Some of the blood proteins have been shown to possess enzymic properties, while others are thought to play a role in the transportation of various types of compounds. 7. Ecdysone and juvenile hormone both stimulate the rate of protein synthesis by larval fat body. Protein synthesis in fat body from diapausing pupae is stimulated after injury to the pupae. 8. The appearance of adult fat body and the amount of protein it contains is often closely linked with the nutritional and reproductive states of the insect. 9. An important role of the fat body in the adult female insect is the synthesis of yolk proteins, which are released into the blood and then taken up by the developing oocytes. This synthesis and uptake are under the control of hormones secreted by the corpora allata and by the median neurosecretory cells of the pars intercerebralis. 10. The RNA content of fat body in final‐instar larvae is not constant throughout the instar. In some larvae it is at its highest level early in the instar, falling to a low level as the instar progresses, while in other larvae (e.g. Calliphora) the level of RNA in fat body does not decrease as the instar progresses. 11. In some dipterous insects the base composition of total RNA is DNA‐like in that the guanine + cytosine content is low, accounting for 40 % of the bases. A similar composition is seen in rapidly labelled RNA isolated from insects of other orders (Coleoptera and Lepidoptera), but the base content of total RNA from these latter insects resembles ribosomal RNA from vertebrate tissues in that it has a high (ca. 60 %) guanine + cytosine content. 12. The RNA/DNA ratios in blowfly larval tissues are high compared with those found in any vertebrate tissue. 13. In larval fat body, RNA synthesis is low at the time...

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Adult and immature individuals of many insect species can survive anaerobic conditions for periods ranging from 4 hr. to 4 days, and even in a few cases for longer periods up to 56 days (see Brand, 1946, for numerous references). The present paper describes an investigation into some effects of anoxia on adult Musca domestica. A preliminary account of part of this work has been published (Heslop, Price & Ray, 1962). MATERIALS AND METHODS Experimental animals. Houseflies were reared at this Laboratory. Puparia were placed in cheese-cloth sleeves on wire frames for periods not exceeding 24 hr. and then removed. The adults that had emerged were fed on glucose and water only. Male flies were removed for experiment 3-6 days after emergence. Groups of 10 or 20 flies were subjected to chill-coma, placed in tared glass cages with wire-mesh ends and weighed. When flies were to be subjected to anoxia the cages were dropped into vessels previously flushed with oxygen-free nitrogen. The flow of nitrogen was maintained for at least 10 min. after introduction of the cage and then the vessel was re-sealed. Commercial oxygen-free nitrogen was further purified by bubbling it through either alkaline pyrogallol or a solution of 4-5 g. of sodium dithionite plus 0 5 g. of sodium anthraquinone-fl-sulphonate in 25 ml. of N-potassium hydroxide.