An admirable invasion: Pisaura mirabilis

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Often, a spider is named after a physical feature. Sometimes a spider is named as a tribute to a person, a naturalist or a celebrity. Other times a spider is named after its behaviour. The origin of the today’s spider name falls into this final category. It’s, you may have recognized it, Pisaura mirabilis, the nursery web spider. Where does its specific epithet of mirabilis, meaning “admirable” come from? Two “admirable” behaviours explain this. Firstly, the female doesn’t act like an “unworthy mother”; it doesn’t abandon its offspring in a vulgar silk cocoon. It will take care of it by carrying its cocoon a little like Lycosidae do, but with the difference that it will hold it by its chelicerae. Then when the hatching time comes, it will weave a bell-shaped web, drop the cocoon underneath this web, and watch over it as well as the freshly hatched babies. Contrary to what one would think, this “maternal instinct” is not unique to this species and many spiders monitor their offspring, to more or less important degrees. The other behaviour that earned it the qualifier “mirabilis” is the male’s relatively exceptional attitude during the nuptial parade and mating: it can bring a gift to the female, gift taking the form of a carefully wrapped prey in a silk packet. Sometimes the male tricks the female and offers an… empty packet! By the time the female unpacked the alleged gift and noticed it’s empty, the male did his business and left. Very often the silk layer surrounding the gift prey is thick and the resultant package is spherical. Some authors argued that this spherical form reminded the female of a cocoon and that would trap her by triggering some kind of maternal instinct, but it would seem rather than the thickness of silk, which therefore requires long unboxing, prevents the female from leaving with the prey without copulating with the male, and that a form other than spherical would hinder the male in his attempt at mating.

Pisaura mirabilis is a very common species (at least here in France), which can easily be found in high vegetation or sunbathing. It hunts on sight, so without a web, although juveniles can weave chaotic webs. For a connoisseur it is identifiable at first glance, but this can be difficult for a novice because of the variety of dresses it can display: ranging from uniform grey to more complex patterns using a wider colour palette. However, it will almost always have a white longitudinal stripe on its cephalothorax, exceeding it in a tuft. And in frontal view, it is unconfoundable, with white cheeks falling to 45° on both sides of its “face”. The plurality of dresses could be explained by the fact that under this species is actually hiding several species. For several years I’ve heard that work is underway to determine whether it is the case or not, but still no change. Anyway, in the south of France around the Mediterranean there is another species, Pisaura quadrilineata, which a priori can’t be distinguished from P. mirabilis.

Each of these 6 photos shows a different individual. Actually where I was, a corner of grass near a road behind my campus, there was plenty of nursery web spiders, almost one by blade of grass! This surprised me a bit because although I knew they could be there en masse, I had only found isolated individuals until then. People were passing by these tall grasses without seeing or even knowing that there was this proliferation of Pisaura mirabilis. Neither did I at first sight: at the beginning I actually spotted only one and took some pictures. Staying motionless to photograph it allowed me to realize it was far from being alone (as often, being motionless allows you to spot many arthropods you wouldn’t have seen if you were just walking around). In fact there are plenty of spiders around us, and even more in the unconstructed lands. But most of the time we are not aware of their presence nor see them. It is not without echoing the alleged spider invasions that there would be right now in Australia. The spiders seen in the pictures don’t have magically appeared in a snap. They’ve always been there. However the floods made them more visible. A colleague wrote an article on this subject, I invite you to read it (if you can read in French). In addition, a spider invasion is not possible: if there are too many spiders compared to the number of available prey, they will eat each other and therefore will self-regulate. Kinda like the heart of a star that is nothing but a thermonuclear system self-regulated by the balance of gravitational forces and energy released by the fusion of elements. Oops, I feel like I’m going astray!

 

 


Last update: March 27, 2021

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