Spiders, like other creatures with exoskeletons, can’t just grow steadily bigger as we do. Instead, they have to moult: they grow a new (but still soft) ‘skin’ under their old one, then burst open the old one to step out of it and wait (preferably somewhere safe) for their new skin to harden.
The cast-off exoskeleton (technically called the exuvia or exuviae) is sometimes eaten – it is valuable protein, after all – by its previous inhabitant or a passing predator. Some insects’ exuviae are typically left untouched, however, and may remain for months; cicadas’ and dragonflies’ exuviae are often seen hanging on a twig or grass stem like this.
My photo here shows an attractive little jumping spider, Cytaea plumbeiventris, on a leaf beside a cast-off skin of the same species. I didn’t see it emerge but my guess is that the skin was its own. (If you click on the photo to see it at full size you may be able to see the green of the leaf through the eye-lenses of the exuviae.)
Any animal with a rigid skin has a problem with growth: the skin doesn’t grow once it has formed, so it somehow has to be entirely replaced. Moulting, the mechanism which has evolved to achieve this, isn’t quite as mind-bogglingly mysterious as caterpillars’ transformation into butterflies but it is still impressive.
Animals with a rigid skin are technically “arthropods.” The group includes insects, spiders and crustaceans, all of which seem to have descended from a common ancestor which evolved in the sea more than 500 million years ago. They were early colonisers of land and have been successful in all sorts of environments.
I saw this Giant Grasshopper, Valanga irregularis, resting on our lemongrass last week, and then spotted something dangling beneath it – the skin it had just shed, as it turned out. A couple of points are worth noting:
• This was the grasshopper’s last moult, since it is now an adult. The fully developed wings (still damp and soft when I took the photos) are now longer than the abdomen and will enable it to fly considerable distances. In its previous stage it had only the stumpy little wings visible on the shed skin (you will see them better at full size – just click on the photo as usual).
• The adult seems much larger than the skin it has just vacated, doesn’t it? It is: grasshoppers can double their weight between moults. That’s quite a trick, since the big new skin has to form inside the small old one and must be half-empty when it hardens to provide enough internal space for the individual to put on weight later. Wikipedia to the rescue: wikipedia.org/wiki/Ecdysis details the strategies used to achieve this.
Grasshoppers don’t change their body-plan much when they moult – they just get bigger until, in the last stages, the wings develop – but some other insects change quite significantly. Dragonflies, for instance, are ugly little underwater predators until they crawl out of the water, split open and fly off as the gorgeous aerial predators we know so well. Finding an abandoned shell clinging to a reed or post beside a pond is not unusual but seeing the adult’s emergence is rare. Here is an amazing photo taken by Steve Passlow, someone I know only as a fellow contributor to the Flickr group Field Guide to the Insects of Australia (a great resource, by the way, for anyone wanting to know more about the bugs around us).
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!