Moulting
Part 3: Juvenile Hormone
Play a role in many aspects of reproduction and development including metamorphosis, regulation of behaviour in honeybee colonies, polyphenisms of locusts and aphids, larval and adult diapause, ovarian development, among others.
Juvenile hormones are terpenoids derived from farnesenic acid. Five structurally related forms are known in insects.
JH got its name through the results of parabiosis experiments in which last instar larvae, parabiosed to earlier instar larvae, moult into an intermediate adult/immature form. These experiments led to the conclusion that a blood borne juvenilising factor is responsible.
CA implant had the same effect, leading to the conclusion that CA is the origin of JH. CA removal causes premature metamorphosis in many insects. JH is produced by intrinsic secretory cells of the CA. The rate of secretion is determined by the rate of production, i.e. it is not stored.
In most insects, the CA come under nervous control.
Neurosecretory cells in brain send axons to CA that release neurohormones which:
- inhibit JH production (allatostatins)
- or promote JH production (allatotropins)
Upon release into hemolymph, JH binds to JH binding proteins to:
- enhance solubility,
- protect from breakdown by enzymes
- help to enter cells
Juvenile hormones and metamorphosis
Holometabola and hemimetabola have different JH profiles (see diagram)
JH-sensitive periods were determined by applying artificial JH analogs to larvae of different ages and determining the time when they were sensitive to the added JH. Perhaps this is the “window” of time when the cells (e.g. epidermis) have the appropriate receptors.
General rule: if JH is present, maintain the current state (hence its juvenilising effect): if JH is absent, switch developmental pathways.
Typical juvenile hormone titer profiles during the end of larval life of hemimetabolous and holometabolous insects. Hypothetical JHsensitive periods are indicated by gray bars. In the Hemimetabola there is a JHsensitive period near the end of each larval instar, during which the commitment to larval versus adult development is controlled. In the Holometabola there is a JHsensitive period for larval versus pupal determination near the end of each larval instar, except in the final larval instar. The two JHsensitive periods in the final larval instar are based on the findings in Manduca sexta. There is an additional JHsensitive period for pupal versus adult determination early in the pupal stage, at the outset of adult development. (From Nijhout, 1994.)
Go on to part 4: Eclosion & ecdysis triggering hormone