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Word of the Week: Instar

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Welcome to Word of the Week! Stay tuned for a new word each Friday to amp up your nature vocabulary!

Instar [IN-stahr] (noun): A phase between two periods of molting in the development of an insect larva or other invertebrate animal.

July is an exciting month for Monarchs– you can see eggs, caterpillars, pupae, and adults all at the same time!

As caterpillars, they go through five major distinct stages of growth, and after each one it molts. Each caterpillar, or instar, is larger than the previous after molting, as it eats and stores energy. Instars can last about 3-5 days, depending on different factors like temperature and the availability of food.

The first instar caterpillar that emerges from the egg is pale green and translucent. They don’t have any coloured bands or tentacles yet. The larvae or caterpillar eats its egg case and begins to feed on milkweed. It is during this stage of growth that the caterpillar begins to store cardenolides in its body; toxins that cause predators to vomit, should they ingest the Monarch. The first instar is usually between 2 and 6 mm long.

The second instar caterpillar develops that characteristic pattern of white, yellow, and black bands. It is no longer translucent but is covered in short bristles. Pairs of black tentacles will also begin to grow. Like the first instar, second-instar larvae eat holes in the middle of the milkweed leaf, rather than at the edges. The second instar is usually between 6 mm and 1 cm long.

The third instar caterpillar has more distinct bands and the two pairs of tentacles become longer. These third-stage caterpillars will start to eat along the leaf edges rather than the middle of the milkweed leaf. The third instar is usually between 1 and 1.5 cm long.

The fourth instar caterpillar has a different banding pattern. It develops white spots on the prolegs near the back of the caterpillar. It is usually between 1.5 and 2.5 cm long.

The fifth instar has a more complex banding pattern and white dots on the prolegs, with front legs that are small and very close to the head. A caterpillar at this stage has an enormous appetite, being able to consume a large milkweed leaf in a day.

On your next summer visit to High Park, keep an eye out for Monarchs!