Some effects of dietary ascorbic acid on locusts

Abstract

In growth experiments using synthetic diets ascorbic acid was shown to be necessary for normal development to the adult stage in two locusts, Schistocerca gregaria (Forsk.) and Locusta migratoria L. Without ascorbic acid growth was noticeably retarded from the third instar in Schistocerca and no nymphs survived to the adult stage, heaviest mortality occurring at the moult from the fourth to the fifth instar. In Locusta a marked retardation in growth occurred in the last nymphal instar and mortality was particularly associated with the final moult; although some adults were obtained, they were small, malformed and short-lived. For both species 1$\cdot $4 mg ascorbic acid/g of diet gave apparently optimal growth. Attempts to demonstrate ascorbic acid histologically and in extracts of the midgut tissue of Schistocerca were unsuccessful, but it was possible to measure the 'ascorbic acid equivalent' of acidified whole haemolymph by titration with 2,6-dichlorophenol indophenol. In fourth and fifth instar nymphs reared normally on grass this value was very low immediately after the moult, steadily increased up to the middle of the instar and declined slightly in the premoult condition when blood volume increases. Newly moulted adults, like newly moulted nymphs, gave very low values. Using hoppers reared on synthetic diets it was found that no 'ascorbic acid equivalent' could be detected in the haemolymph when ascorbic acid was absent from the diet. Only traces were found in hoppers reared on diets containing 1$\cdot $4 mg ascorbic acid/g, but values of the order found in hoppers reared on grass were found for hoppers reared on synthetic diets containing 7$\cdot $0 mg ascorbic acid/g. It is concluded that in locusts ascorbic acid plays some part in the processes occurring at the moult, and the relevance of these results to the biochemical mechanisms involved in the hardening and melanization of cuticle are discussed.