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What is the larva doing — and why?

Eating its Shed Exoskeleton
This monarch larva just shed its exoskeleton and is now eating the remaining portion, called the exuvia. Like all insects, a monarch must shed its exoskeleton at various stages to make room for growth. (Each stage of growth is called an instar. The process of shedding is called molting.)

Why Eat the Exuvia?
The most likely reason for eating the exuvia is to recycle the nurtients it contains, especially hard-to-get nutrients like nitrogen. Dr. Alex Mira studied this behavior in cockroaches. He found that animals raised on low-protein diets ate the exuvia most often. Females also ate the exuvia more than males, probably because of the need for nitrogen in egg-production. (Female monarchs are known to get nitrogen from males during mating, in a packet called the spermatophore.)

Nitrogen is a building block for proteins and genetic material, so it is essential for life. Nitrogen is abundant in the earth's atmosphere but is often a scarce resource in the food web.

Image © Bud Hensley

The exuvia contains valuable nutrients. It is made of protein chains and chitin. (Yum, yum!)