Nick Baker’s hidden Britain
The spider that sports an ‘eye horn’
Eight legs, eight eyes and a body bristling with all manner of hairs and knobbly bits – so far, so normal for a spider. But, if it wasn’t for Walckenaeria acuminata’s diminutive size (less than 3mm) we would all have heard of this miniature monstrosity. A word used in the best possible and original taste, for this spider is of no harm to anything bigger than a thrip.
It is a spider that doesn’t conform to anything you might have seen anywhere else in the spider world. Look a spider in the ‘face’ and, while you’ll struggle to see features that you can relate to, most spiders at least have eyes (usually eight) and a couple of these are where eyes might be expected to be positioned. An adult male Walckenaeria is very different.
It belongs to the dwarf or money spider group, of which there are 270 species in the UK – that’s 40 per cent of all British spider species.
The double challenge of this group is not just their tiny size, but also that they are very difficult to tell apart, without considerable knowledge and a powerful microscope. However, this spider – well, the adult male anyway – bucks the trend somewhat and knowing they are common and widespread is enough to make you look closely at every money spider you see from now on.
Overall, it resembles other spiders of its family, with a dark, shiny body and orange-red legs, but lean in a little closer and you’ll notice something strange. Slap bang in the middle of its head is a bizarre protuberance. Looking a bit like a unicorn’s horn and twice as high as the rest of the spider, this turret can only amaze and instigate an internal monologue of questions in those lucky enough to meet one. How does it work? And what does it do?
A closer look still and it will be noted that towards the very top, looking like a pin head on the top of a very bent pin, is a swelling, which houses two pairs of glassy eyes. The remaining four of its optical contingent are clustered around another bulbous swelling about halfway up.
So, what is it used for? There have been many theories, such as enabling it to see its prey over obstacles, or using it like a stag might use its antlers to display to females, or as a club in combat. None of these really stack up in my view, both because these eyes don’t see very well and only males have the eye stalk.
The clues to its possible purpose may come from studies on another money spider species, Hypomma bituberculatum, which is nearly as odd. The males have a couple of glossy, bean-shaped swellings on the top of the front part of their body and it seems that these are surrounded by glands that exude a liquid the female finds attractive. So, when in the grips of passion, she holds onto the bumps with her fangs while supping up these mysterious secretions.
It is possible that the cephalic protuberance of Walckenaeria may act in a similar way – an extreme form of ‘love handle’, perhaps? Or it could serve to aid in the dissemination of pheromones. Still, nobody has actually witnessed the mating of this tiny, widespread spider, so for now the purpose of Walckenaeria’s ‘eye horn’ remains a secret that is yet to be revealed.