Insects and spiders can’t grow steadily like we do because their skeletons are on the outside and serve simultaneously as skin, skeleton and armour. It doesn’t grow or stretch once it has hardened so the animal has to grow a new skin underneath the old one, crack the old one open and crawl out, and wait nervously until the new one toughens. I have seen different parts of the process in different insects recently so I thought I would put a group together.
Dragonflies: Juvenile dragonflies are so different from the adult it is hard to believe they are even related. They are water-dwelling predators which fishermen know as ‘mud-eyes’ and use as bait. After several moults in this form they climb up out of the water and split open to emerge as the winged adult dragonflies we know and love. I have yet to see it happen but here is a lovely photo from SE Qld.
Cicadas: Cicadas also undergo a radical change, since the nymphs live underground, emerging as stumpy-looking bugs with strong burrowing front legs and splitting open to emerge as a winged adult. I haven’t seen one emerging but here is a cast-off skin (known as an ‘exuvia‘), and here and here are some adults
Cockroaches: We have a resident population in our compost bin and I caught this photo recently of a just-moulted cocky resting next to its old skin. Like many (perhaps most) insects, fresh-moulted cockroaches are nearly colourless as well as soft; they darken as they harden.
Mantises: I found this cast-off skin eighteen months ago but I didn’t see one in the process of emerging until yesterday. It is one of the family I have been watching recently and described here. In terms of that post, this is a nymph with wing-buds emerging from one without. One photo of it is here and another is below.
In all cases, the amount of change from one stage (‘instar’) to the next is hard to believe. Even when they don’t change from swimmers or diggers to fliers, the difference in size makes you wonder how on earth that big bug fitted inside that little skin.
In human terms the changes would be (roughly) like changing from a one-year-old to a three-year-old overnight, then to a six-year-old, to an eleven-year-old and finally to an eighteen-year-old. What a difference that would make to our lives!