Pirate spider eyespots?

Wolf Pirate Spiders (likely Pirata piraticus, but there is a similar species, P. tenuitarsis) are one of my favourite spiders, their existence in the boundary of water and land, and their ability to walk and run on water makes them fascinating. A few days ago at the wildlife garden, I discovered a curious fact about them while looking at a photo of a female carrying an egg sac. The female was lodged at an angle on some leaves and I did a double take: it gave me the impression that the abdomen of the spider was in fact, its face. This impression was due to two paired white spots on the female's abdomen looking like eyes. Instead of a spider facing away with her egg sac attached to its abdomen, it looked like a spider looking forward, while carrying its egg sac on its jaws, in the way of nursery web spiders. Pirate spider eyespots? Wolf pirate spiders have paired line of white spots on their abdomen, so I wondered if it was just this particular individual that casually had the spots in the shape and area to give this impression. I checked my photos of this spider and found at least two others with the same spots, one also carrying an egg sac. I also found other photos on the web of this species with the same abdominal spots.

Another Pirata sp with egg sac and 'eyespots' 22/07/2014, same habitat

For comparison, a female nursery web spider carrying her egg sac with her chelicerae.

 Only photos taken in a particular angle, of the spider facing away show this, and I guess everybody prefers a front shot of a spider, so I think this pattern is probably found in the species at least regularly, and not only in the females, this side view of the spider allows to see the larger spot towards the end of the line of spots in a male

24/05/2014

A frontal view of the same male showing his enlarge palps:

Have these spots evolved due to them giving an impression of eyes to visual predators, as the eyespots found in some caterpillars and butterflies? In some cases eyespots function to startle a predator giving the impression of a different, larger animal (like the elephant moth caterpillar, the peacock butterfly or this amazing tropical caterpillar), instead, in the wolf pirate spider the eyespots would work like those found at the rear end of some fishes and butterflies, whose function might be to trick or confuse a predator as to where the head of the animal actually is. In fact, eyespots are not unheard of in spiders.

 Why would this be beneficial to this spider? What follows is only speculation, but apparently, wolf pirate spiders live in retreats in moss at the shore of ponds, and females carrying egg sacs often expose the egg sac to the sun at the retreat entrance, while the female remains inside. This could put the female in danger of predation, as she would be less able to detect danger, so the eyespots could afford some protection to the mother and its offspring to be, but I am not aware of any research into the evolution of these eyespots.

With thanks to Catherine Scott (@cataranea) for discussing this on twitter before I wrote the post.

Elephant hawkmoth and caterpillar eyespots

While visiting a farm in the Peak District, we stumbled with this awesome mini beast: a fully grown Elephant Hawkmoth caterpillar. I had been wanting to see this caterpillar for a long time. Knowing that they like willowherbs I had often look for them when I saw a drift of rosebay willowherb. But not this time, somehow, its dark colour made it stand up against the green foliage. She was hanging upside down from the underside of a rosebay willowherb leaf, on a plant growing by a hedge.
 While handling it it adopted all the postures this caterpillar is known for, suggesting either an elephant, when the caterpillar is fully extended, or a snake, when the caterpillar is disturbed and it raises its front, while retracting its head.
 A 'snake' resemblance is a recurrent theme in various large caterpillars from several families, and it has been suggested that this way the caterpillars gain protention from birds, wary of snakes, which are startled when the caterpillar moves its head and the eyes are exposed on the thickened anterior end.
 A Canadian team formed by Thomas Hossie and Thomas Sherrat carried out an interesting set of experiments using pastry caterpillars, which they exposed to natural predation by placing them in branches in the wild. They used pastry caterpillars coloured with food dye with or without eyespots with or without defensive posture ('snake') and with or without countershading.Their results suggested that the presence of eyespots and the raised position might deter birds from eating the caterpillars and countershading and the position of the eyespots in the thickened anterior end has a protective effect too. But don't rely on me telling you, as you can read it from Hossie's himself in his blog Caterpillar Eyespots.

A close up of the eyespots.

The 'elephant' pose.

and the 'snake' pose

another angle of the snake pose

And of the elephant pose.

The happy caterpillar munching away on its new home.

Scary peacock butterflies

Butterflies have high predation rates by birds. I am sure most of you have come across a butterfly with damaged wings suggestive of a beak 'bite' mark. Long-lived butterfly species often rely on camouflage (crypsis) to avoid being attacked in the first place. The Comma, the Small Tortoiseshell and the Peacock are some of these, mimicking shriveled leaves. They remain immobile if resting, with their wings closed. The Peacock (Inachis io) has a second defense mechanism. If discovered, they suddenly flash their wings open, exposing four large eyespots they also may flick their wings repeatedly and at the same time they make a hissing noise and a series of inaudible clicks by rubbing two wing veins together, during this display, they continually adjust the tilt of their bodies to face the potential attacker.

This Peacock was feeding upside down on a Buddleia showing its eyespots

 This intimidating display was described over a century ago and its effect on predators noted, but experimental support for its effects on the survival of the butterflies themselves was lacking until very recently. Adrian Vallin and collaborators tested the effect of eyespots, stridulation and both combined by modifying captive-reared peacock butterflies experimentally. They removed the eyespots by painting over them with a black marker pen and removed the stridulation ability of the butterflies by cutting out the veins responsible for making the hissing noise. They also tested the effect of the combined factors, that is removing both the eyespots and the noise-making wing veins. Unmodified controls, and controls in which a similar area of wing was painted black without touching the eyespots or cut without affecting the ability to make noise were also tested (bottom row below).

The researchers recorded the effect these various treatments had on survival of peacocks upon exposure to a potential predator, blue tits (Cyanistes caeruleus), a small insectivorous bird. The results show conclusively that butterflies without eyespots are more likely to be predated by the blue tits. All except one butterfly with intact eyespots survived by scaring the blue tit away, showing how effective the eyespots are in intimidating the bird. The effects of the sound or the combined effect were not significant. The predated butterflies were readily eaten by the birds, indicating that the Peacock is not distasteful and supporting the view that, in the words of the researchers, 'a harmless prey can increase its fitness by survival through the adoption of intimidation by bluffing'.
References: 
Vallin, A., S. Jakobsson, J. Lind & C. Wiklund (2005) Prey survival by predator intimidation: an experimental study of peacock butterfly defence against blue tits. Proc. R. Soc. B. 272:1203-1207.

Ruxton G.D. (2005) Intimidating butterflies. Trends in Ecology and Evolution 20: 276-278.