Monday 13 July 2015

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
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.

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