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).