The Mysterious Work of Molting Hormones

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It’s common knowledge that insects molt--but have you ever wondered how it happens? How does an insect know when to start recycling its old exoskeleton for a new one? Or to spin a cocoon, or form a chrysalis? As you might expect, these processes do not just happen randomly; on the contrary, the mechanisms are very complex and timed to occur at precise moments during development. So, what is behind all of this intricate coordination? Hormones! Hormones—chemical messengers used throughout an organism’s body—cause larvae to molt into the next instar and adults to burst out of their pupae (Meyer, 2016b). Depending on the stage of development, though, different hormones will be in the main spotlight. Molting is divided into three main steps: apolysis, forming a new exoskeleton below the old one; ecdysis, shedding the old exoskeleton; and sclerotization, the hardening of the new exoskeleton (Meyer, 2016a).

Photo Credit: Missoula Butterfly House and Insectarium

For hemimetabolous insects, the nymph repeats apolysis, ecdysis, and sclerotization several times until reaching the adult state. The number of molts varies for each insect species. For example, stoneflies (Plecoptera) can require 21 to 32 molts to go from larva to adult, while earwigs (Dermoptera) only need three to five molts (Nijhout, 1981). Although the external appearance of molting seems simple, the internal workings are quite complex and require over six different hormones. The diagram below shows the levels of these hormones during the molting process. Molting begins when sensory neurons signal to the brain that more space is needed for growth. At these signals, the brain’s neurosecretory cells begin producing brain hormone, indicated by the far left peak in the diagram. Brain hormone in turn activates organs called the corpora cardiaca which produce prothoracicotropic hormone (Meyer, 2016a). These organs amplify the molting signal, “sending out a big pulse of hormone to the body in response to a small message from the brain” (Meyer, 2016b). The last major step is the production of ecdysteroids by the prothoracic glands. Ecdysteroids are also called molting hormones and are shown by the broad peak on the diagram.

Figure credit: John Meyer (2016)

Up to this point, the insect has used brain hormone, prothoracicotropic hormone, and molting hormone. The insect next synthesizes a new exoskeleton below the old one in response to high ecdysteroid levels. Once the new exoskeleton is in place, ecdysis begins. The insect takes in water or air to expand its body and uses muscle contractions to push out of the exoskeleton at ecdysal lines (Daly et. al., 1998). Ecdysis involves eclosion hormone and bursicon, which promote the sclerotization of the new exoskeleton. At the end of a hemimetabolous molt, the insect has a larger but otherwise very similar exoskeleton. The nymphal molts are similar, but the molt into the adult form is a bit different. When an immature insect molts into an adult, the primary hormone involved is called juvenile hormone. Juvenile hormone is secreted by the corpora allata in immature insects and prevents adult features, like wings and reproductive organs, from developing (Meyer, 2016). During the final molt these organs atrophy and can no longer secrete juvenile hormone. The lack of juvenile hormone allows the new molt to develop into a mature adult. With the final molt complete, the insect can begin its short life as an adult. Its offspring will continue the cycle of apolysis, ecdysis, and sclerotization as they grow from eggs to adults, testifying of the secret but crucial work of molting hormones.

Photo credit: Edith Smith (monarchbutterflyusa.com)

Works Cited

Daly, H. V., Doyen, J. T., & Purcell, A. H. (1998). Introduction to insect biology and diversity (2nd ed.). Oxford University Press.

Meyer, J. R. (2016). Hormonal Control of Molting & Metamorphosis. General Entomology. Retrieved May 26, 2022, from projects.ncsu.edu/cals/course/ent425/library/tutorials/internal_anatomy/molting.html

Meyer, J. R. (2016). The Endocrine System. General Entomology. Retrieved May 26, 2022, from projects.ncsu.edu/cals/course/ent425/library/tutorials/internal_anatomy/molting.html

Nijhout, H. F. (1981). Physiological control of molting in insects. American Zoologist, 21(3), 631–640. https://doi.org/10.1093/icb/21.3.631

            Photo Credits

  Missoula Butterfly House and Insectarium. (n.d.). Notes from the lab: Everyone Molts. Retrieved May 27, 2022, from www.missoulabutterflyhouse.org/notes-from-the-lab-metamorphosis/

The munching caterpillar. Monarch Butterfly USA. (2019, April 15). Retrieved May 27, 2022, from monarchbutterflyusa.com/monarch-life-cycle/the-munching-caterpillar/

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