Saturday, June 11, 2011

Shedding and Pupating

Monarch caterpillars shed their skins four times as they grow larger and larger. Each stage between shedding skins is called an "instar." When they've just  hatched from their egg, they're called a "first instar." After they've shed their skin once, they're called a "second instar," and so on.

When a caterpillar is ready to shed, it will often wander a short distance from the milkweed plant, and just sit there for a day. You may think it's dead, but it's not. Be careful not to disturb it -- even though it's not moving, it's alive!
After the caterpillar has shed its skin four times (when it's called a "fifth instar"), it keeps growing until it's ready to pupate. It will probably crawl off somewhere, and then sit still, just like the other times when it sheds its skin.

But the fifth instar does something different. After sitting still for a day or so, it will emit some sticky stuff, and glue its back legs onto whatever it was sitting on. Then, it will let go with its other legs, and hang upside down in a "J" shape.

As it hangs, it will gradually bunch up, and get shorter and fatter, until it sheds its skin one last time -- but now, instead of coming out from its skin as a larger caterpillar, it comes out as a pupa!

The pupa is this smooth, green capsule. It has a few shiny dots on it, and if you look carefully, you can see the tracings of what look like a butterfly wing inside. We'll watch carefully over the next ten or so days, and see what happens with this pupa!

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