By James Duncan
Woods Mill Engagement Officer
The world of invertebrates is a strange one indeed. Full of illusion and deception, things are often not as they seem and a prime example of this is mimicry. In the weird and wonderful kingdom of the arthropods, it can be hugely beneficial to look dangerous - a matter of life and death in fact. There are of course a myriad of genuine biting and stinging insects in the UK, though for most mimics it's outright trickery - simply a way to avoid detection by predators. It's one thing to mimic a harmless object like a leaf, a twig, a flower, a bird dropping or an eye but quite another to mimic something a whole lot more fearsome. From flies mimicking bees, to moths mimicking birds, to spiders mimicking ants, to caterpillars mimicking snakes - there's countless examples of 'defensive mimicry' in the natural world. But what if you're already a predator - is there a potential benefit?
A creature exemplifying this is the fabulously striking wasp spider, Argiope bruennichi. This orb-web spider is a creature being spotted regularly at Woods Mill. It displays conspicuous 'aposematic' colouration - black and yellow. These warning colours generally depict the undesirability of an invertebrate to any likely predator, indicated through either aggressive nature, toxicity or perhaps a foul taste or smell. There's no doubt the colours do a mean impression of a wasp. The spider is of course an effective hunter itself, rapidly immobilising its insect prey within a silk cocoon and with a paralysing bite, incorporating a protein dissolving enzyme. However, despite its colourful advertising signal, and unlike its namesake, it's quite harmless to humans.
However, many mimics also use the promise of nourishment as a way of attracting their own prey, so called 'aggressive mimicry.' This seems relatively common amongst orb-web spiders. It seems the aposematic colouration can ironically serve as an attraction device! The colours may well lure the spiders' insect prey (mainly Orthoptera - crickets and grasshoppers) closer to the vicinity of the web. Though bright colours (such as yellow) serve as a useful warning, they also regularly indicate the presence of a food resource such as a flower or new plant growth. The wasp spider may very well get the best of both worlds.
Wasp Spider showing 'stabilimentum' © James Duncan
An additional attraction device is a feature built vertically into the web of the wasp spider. It's a zig-zag of silk, the 'stabilimentum.' Though its function is not clearly understood, it's thought that it may reflect UV light and therefore mimic a pattern often depicted in flowers - so called 'nectar guides.' Interestingly a spider may regularly change its web pattern owing to an ability amongst some insects to remember these patterns through spatial location. The size difference between the male and female Wasp Spider (4.5mm vs 15-17mm) generally allows the male to enter the female's orb without being recognised as a prey item. Unfortunately this doesn't necessarily help the males as the majority are eaten during copulation. They'll generally wait until the female undergoes her final moult and reaches sexual maturity - at which point her jaws will be softer - though this is no guarantee of survival success.
The wasp spider was first recorded in Rye in 1922, and is originally a native of Southern Europe. Warming climate has allowed the movement of the species into the UK, where a few months of warm weather and a mild winter are required for their breeding success. They're gradually spreading further and further North.