Firstly, once again I find myself apologising for not writing more regularly. I WILL become better at this! I’ve been somewhat engrossed in writing up a report for my dreaded first year PhD appraisal, but on the way I have been fortunate enough to stumble across some truly remarkable literature on spiders and their amazing abilities – plenty of ammunition for future posts!
So what’s this you say? Spiders getting stung by wasps? Well it turns out that when venomous insects get trapped in spiders’ webs, in their struggle to get away they can end up stinging the spider. Although spiders are protected by their outer exoskeleton, if the insect is lucky enough (or cold and calculated enough) to aim their sting for the vulnerable joint membrane between the joints on the spiders’ legs, the sting can penetrate and venom is injected.
In this particular study I was reading (Eisner and Camazine, 1983), the authors saw this happen out in the field, with orb-weaving spiders (Argiope spp.) being stung by a small ambush bug, Phymata fasciata. This bug can give noticeable stings to humans, too. On seeing the struggle between spider and bug come to a halt, they noticed that the spider had been stung – and it immediately autotomised it’s leg.
Autotomy is a really remarkable ability that most spider species possess. Essentially, they are able to voluntarily ‘shed’ or amputate their own legs, as a defence mechanism if they’re grabbed by a predator, or if they get stuck in one of their moult cycles. It’s similar to the way that some lizards can pop their tail off if you grab them by it – sacrificing a limb in order to escape and survive. Sometimes spiders can even grow the legs back – but I think I’ll save that for another post!
So these spiders that had been stung by the ambush bug got rid of the injured leg, presumably to stop the venom spreading to the rest of the body and killing the spider. But, the authors asked, is this because of the physical puncture of the sting, or because of the injected venom? So they went to the lab and set up a series of tests. Firstly, they compared the reactions of spiders that had been stung by the bug, versus those that were punctured in the same part of the leg by a sterilised pin. Turns out that the pin-punctured ones didn’t shed their legs (well, only 1 out of 8 spiders autotomised the leg, compared to 10 out of 10 stung by the bug). It seemed then that it was indeed the venom that caused the spider to do this, so the researchers were curious as to what venoms, or components of venom, induced this reaction.
They selected wasp and honeybee venoms to test on the spiders, and four major components of these kinds of venom: serotonin, histamine, phospholipase A2, and mellitin. They also used a few inactive components of venom, such as dopamine and adrenaline. Along with the bee and wasp venoms themselves, these were all injected into legs of spiders, at concentrations comparable to those found in venom, and compared to control injections of sodium chloride and saline. [Here I have a bit of an issue with the way the experiments were carried out – they didn’t use separate spiders for each venom, they used separate legs, using up to four legs per spider – which I think makes the results less reliable. It doesn’t say what order the substances were injected in, and I have a theory that spiders may be reluctant to shed legs after they’ve lost a certain amount – so I think it would have been better to use separate spiders per injection.]
The results showed that the bee and wasp venoms caused spiders to autotomise their legs in approx. 70+% of cases, as well over 48% of cases in the active venom components. I think this is a really cool adaptation – how many of us have been stung by wasps or bees and just wished we could lop off the offending limb or finger to stop the pain?! And the amazing thing is, spiders shed the leg just seconds after being stung. Spiders really are quite incredible!