Because when the manti first emerge from the egg case, evidently they chill initially, to let their exoskeletons harden. I guess it is a little like an egg. You boil an egg, then peel it. Only this time, when the egg is peeled, the egg expands, and forms another shell.
There has to be a better analogy.
But I get home from work, too late to do anything particularly meaningful in regards to the dissertation and other insundries -- so I tend to the sunflowers, and start checking in on the new manti -- and last but not least, there is Grishnak -- but wait -- something is different.
Here you can see Grishnak, upper left, and the exoskeleton -- and he was still connected to it. The pictures were a little hard to get right, but you get the idea.

A different attempt, with a bit of scale added in:
