Critical period of sensitivity to juvenile hormone for the invagination of tracheoblasts into the developing flight muscle fibers of Locusta migratoria

The effect of juvenile hormone (JH) on the process of tracheolization of the dorsolongitudinal flight muscles of the migratory locust is studied during larval flight muscle development. The ratio of the number of interfibrillar tracheoles in a muscle fiber to the total area of this muscle fiber is used as a parameter for quantitative assessment of the invagination of tracheoblasts into the muscle fibers. Elevation of the hemolymph JH titer following implantation of active corpora allata (CA) during the period of Day 5 of the fourth larval stage up to and including (except in one case) Day 2 of the fifth larval instar results in a complete prevention of the invagination of tracheoblasts into the muscle fibers at the end of the fifth larval stage, whereas implantation of CA on Day 3 and Day 5 of the fifth larval instar results in a partial and decreasing inhibition of the invagination process. The amount of JH III per locust recovered 24 hr after CA implantation on Day 5 of the fifth instar (0.70 ng) is higher than after CA implantation on Day 1 of this instar (0.54 ng), pointing to the absence of a substantial increase in JH-specific esterases. Due to a rise in hemolymph volume, however, JH titer after CA implantation on Day 5 is lower; yet the lowering of the titer is not likely to be responsible for the time-dependent decrease in effect of CA implantation. Rather, because ecdysone titer fluctuates during the last larval instar, it would appear that exogenous JH must be administered before the initial increase in ecdysone titer in order to completely prevent invagination of tracheoblasts into the muscle fibers at the end of the fifth larval instar. It is suggested that JH acts by preventing the development of the tracheoblasts to an ecdysone-receptive state.