Abstract-Simultaneous quantitative determination of the three naturally occurring juvenile hormones in insects (JH-I, JH-II and JH-III) was performed on haemolymph samples of both normally developing locusts and iocusts implanted with active corpora allata, using capillary gas chromatography with electron capture detection.In fourth instar female larvae, 24-48 hr after the third ecdysis, as well as in adult females, 18 days after the imaginal ecdysis, only JH-III was detected. In fifth instar female larvae JH-III was present in very low concentrations, if at all.After implantation of four pairs ofcorpora allata taken from young fourth instar fernaL larvae or one pair or corpora allata taken from adult females into fifth instar female larvae O-24 hr after eedysis. an elevation of the JH-III titre was observed. Neither JH-I nor JH-II coufd be detected. The amount of JH-III, already elevated 2 hr after implantation, remained high for several days in comparison to that of control insects. On the third day after the subsequent moult the JH-III level was comparable to that of normally developing fifth instar larvae. Factors involved in the achievement of the haemolymph JH-titre are discussed.

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