Since the discovery that methyl farnesoate (MF), the unepoxidated form of the insect juvenile hormone (JHIII), is produced by mandibular organs of numerous crustaceans, extensive evidence has accumulated that this compound appears to perform similar functions in the Crustacea as JH performs in insects. A major function of MF appears to be in enhancing reproductive maturation. This was first shown by indirect experimentation with eyestalk ablation, which augmented MF production. Subsequently, direct treatments of several species of crustacea with MF showed that reproductive maturation was enhanced.

A second function of MF, similar to that of the JH of insects, is in the maintenance of juvenile morphology. This is especially true in the late larval transformations into juveniles, where MF plays an inhibitory role, as well as during the transformation of juveniles into adults. These results were inferred from eyestalk removal experiments. In the case of the larval-juvenile transition, inhibitory results were also obtained with MF by direct hormone treatments. However, the transition from very early larval stages, such as one nauplius stage proceeding to the next, which in many cases also involves morphogenetic changes, may be occurring in the presence of MF. Indeed, MF appears to be stimulatory to early postembryonic larval stages of Crustacea. Again, this function of MF in Crustacea appears to be similar to functions of JH in early postembryonic insects. However, it should be pointed out that there are many more “early” stages in Crustacea than there are in insects, and very few of these cases have been investigated.

When considering the animal kingdom and larval metamorphosis, the question may be raised whether there are other members of the JH family regulating metamorphosis and reproduction. One plausible example appears to be among certain annelids. The trochophores of Capitella respond to various juvenoids, but are most responsive, within one hour, to MF and eicosatrienoic acid. This latter compound is present also in adult annelids, where it has been named “Sperm Maturation Factor,” since it seems to function in the maturation of sperm in Arenicola. Therefore, eicosanoids perform in annelids two functions performed in insects by JHs.

In conclusion, it seems that there are morphogenesis promoting responses to JHs in early larval development in crustaceans, annelids, and possibly other forms, which differ from those MF effects in later larvae of Crustacea where MF retards morphogenesis. Such early responses as noted here have recently also been described for insects. Furthermore, it is clear that the polyunsaturated 8,11,14-eicosatrienoic and aracidonic acids seem to be juvenoids, and appear to function as such in annelids, and may also be functionally active in insects and crustaceans. It seems reasonable to conclude therefore that new and novel juvenoids exist, while others still await discovery.