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The Rusty Tussock moth, a task splitter

The story of a caterpillar at a morphological crossroad


Picture yourself hidden in a crevice on an birch tree trunk, in a forest. You are building your cocoon to prepare your metamorphosis.

You are a Rusty Tussock moth, ready to pupate. You have eaten as much as you needed to embark on your journey to adulthood, and when you will have finished weaving your cocoon, your transformation will start. You stand at a development crossroad, as your species displays extreme sexual dimorphism!

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Spewing silken threads around yourself, you are putting together a nice chamber to have a tranquil metamorphosis. Once this protective cocoon will be finished, you will molt into a pupa, or chrysalis. Leaving your old caterpillar skin, with all its flamboyant hair, you will take the shape of a sober sarcophagus. It is inside this envelop that the wonder of sexual polyphenism will happen.

Depending on the sexual chromosomes you were dealt, you will develop one metamorphosed body, also called imago, or a completely different other. A male, or a female. Very much like the egg and the sperm, one will be very mobile and light, while the other will be heavy and static.

With two similar sex chromosomes, you would turn into a male. Despite looking like a common moth, you would fly during the day, in zigzag across wind to pick up the scent of your goal: a mate. No time to feed, no time for anything else: Your species has life well compartmentalized: caterpillar feed, adults mate. No double tasking. To excel at this task, the males have very developped antennae to detect the faintest smell, and big eyes to navigate the air.

But if you were given two different sex chromosomes, then, you would develop into a female! Once again, you would be much unlike the typical moth. Tiny wings, short legs, and a gigantic abdomen. You would barely be able to walk, let alone to fly! But you would let the wind carry your olfactive signal, the sex pheromone sought after by your male counterpart. Even your eyes would be different from theirs. Instead of the eyes of a day flyer, you would have eyes suited for your lifestyle: small and adaptable, to compensate for your lack in mobility.

Your species has life well compartmentalized: females call, males come. No double tasking.


Mishra, M.; Meyer-Rochow, V. B. Eyes of Male and Female Orgyia Antiqua (Lepidoptera; Lymantriidae) React Differently to an Exposure with UV-A. Micron 2008, 39 (4), 471–480. https://doi.org/10.1016/j.micron.2007.02.006.

Lau, T. F. (Stanley); Meyer-Rochow, V. B. The Compound Eye of Orgyia Antiqua (Lepidoptera: Lymantriidae): Sexual Dimorphism and Light/Dark Adaptational Changes. Eur. J. Entomol. 2007, 104 (2), 247–258. https://doi.org/10.14411/eje.2007.039.