MOLTING FROM FOURTH TO FIFTH INSTAR

The monarch caterpillars on the right in the image below appear to be preparing to molt that is to shed their skins (cuticles) to become 5th and final instars (photo from the International Space Station). 

The following information from the 11-page Monarchs in Space guide (available for download from the project home page) is relevant at this time.

"When the fourth instar caterpillar is ready to molt to become the 5th and last caterpillar stage, it seeks out a vertical or horizontal surface and spins a bed of silk. When the bed is completed, the caterpillar anchors itself to the bed with the crochets on the abdominal and anal prolegs. Once the development of the new skin is completed beneath the old, the caterpillar forces fluid forward causing the skin behind the head to split. The new 5th instar caterpillar then 'walks' out of the old skin and after resting for some time, it turns around and eats this skin. Shortly thereafter, the caterpillar will resume feeding on the artificial diet and will do so until it has completed development in 4-6 days."

Students should be able to observe this process in the classroom. And, if they can, such observations may give rise to a number of questions such as: how long does the molting process take (i.e. from the time they make the silk bed until they shed their skin), how long does it take a caterpillar to “walk” out of the old skin, how long does it take a new 5th instar to mature so that it can turn around to eat the old skin, why do they eat the old skin anyway, how long does this eating process take and finally how long is it from shedding to when they return to feed on the diet? Believe it or not scientists do not have data on any of these questions.

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