Thursday, May 15, 2014

Salticid Snapshots

Saltation – from Latin, saltus, meaning "leap", and since jumping spiders lack extensor muscles at their leg joints they accomplish their amazing leaps by suddenly increasing the blood pressure to their fourth pair of legs. The precision of their predatory pounces is guided by their large AME – Anterior Median Eyes – the acuity of which are ten times better than the huge compound eyes of a dragonfly.

This pretty little female spider goes by the name of Marpissa formosa. She was found on the doorframe of a car and had probably been hitchhiking there for the past hour, hanging on for dear life. The reflective paint made a poor background, as did some red granite, but she obligingly jumped up and posed on my hand.

Male spiders are often much smaller than females; that's not the case here, both sexes were about 10 mm give or take. But they certainly are colored and patterned differently, and without a field guide or seeing a mating pair of spiders one would never guess that the male Marpissa formosa in the next two pictures is the same species as the female above.

Phidippus princeps (from Latin, meaning "first, chief, sovereign") is also roughly 10 mm long but much more robust. The salticid habit of always turning to face danger with those AME makes it difficult to acquire shots from different angles. In this case it presents this male's field marks quite nicely – white scaling on the palps, white fringes on the legs and blue-green chelicerae. The abdomen can vary between individuals and instars and isn't reliable for identifying this spider.

Like its cousin Phidippus audax a.k.a. the Bold Jumper this spider was totally fearless. He leaped at the camera, jumped on my hand (but of course didn't bite), and his last leap took him ... in my shoe? inside the leg of my pants? or he missed me and went into hiding? ... I'm not sure which.

A couple of Phidippus princeps females – again, there's not much difference in size between the sexes, but even so the females tend to be more robust than males. A good rule of thumb for sexing arthropods in general is "Eggs are bigger than sperm." In addition the males carry their sperm packets in specially modified pedipalps, the profile of which is unique to each species. The palps of females are unmodified, bristly and resemble short legs.

Phidippus princeps is as big as our local jumping spiders get, but the Regal Jumper (Phidippus regius) can attain a length of about 20 mm – large enough to tackle more than the standard menu of insects and other arthropods.

At the other extreme in terms of size is the diminutive Peppered Jumper (Pelegrina galathea). Attaining a length of only about 3 mm this mating pair of spiders was at the limit of the camera's macro.

The pair separated amicably after mating. The female fled the scene immediately but the male hung around for a minute and I was able to get a few photos for future reference.

A male Eris militaris a.k.a. Bronze Lake Jumper looks superficially similar to male Peppered Jumper, but it lacks the strong annulations on the legs, the branched white markings on the cephalothorax and the spots near the tip of the abdomen. In addition the Bronze Lake Jumper is about twice the length of the Peppered Jumper.

The vision of this small arthropod is amazing in proportion to its size. The male recognized a female of his species from a good 30 cm away and started signaling to her by wagging his abdomen and waving his modified front legs, making her aware of his intentions from a relatively safe distance. A necessary prelude to mating, because with their hair-trigger killer instincts jumping spiders will tackle anything that moves and looks small enough to overpower and eat.

The female is a few millimeters longer than the male and much heftier. She did have dinner on her mind, albeit a midge rather than her prospective mate, wasn't receptive to his advances, and no mating took place. So although the markings are correct for an Eris militaris female there are other jumper look-alikes and it's not possible to conclusively say these two spiders were the same species.

The attractive spider in the next group of photos is a female Dimorphic Jumper (Maevia inclemens), she's about 8 mm long.

This looked like a dramatic opportunity to acquire a dramatic shot of the spider catching dinner but it turned out to be something better. The spider homed in on the ant and started to leap but then aborted it, and when I looked at the image on the computer screen I saw why. It looks like the ant was threatening to (or maybe did) spray formic acid or some other nsty chemical at the spider.

Male Dimorphic Jumpers are two to three millimeters smaller than the females. Its small size combined with cryptic markings made this one almost impossible to see against the background of lichen encrusted stone. The males come in two color forms, here's an image of the other variation at BugGuide.Net.

Always look twice – I jumped to the conclusion that these itsy-bisty mottled brown spiders, only about 3 mm in length, were the immature instars of a species I was already familiar with, perhaps Salticus scenicus.

Not so ... one of the spiders became agitated at my presence and waved its palps around, whereupon it became obvious that this was a fully mature male.

Sometimes immigrant species die out because conditions simply aren't right for them in their new habitat. They may – like the ubiquitous primate Homo sapiens – find themselves in an favorable environment with a plentitude of food and lacking the limiting factor of predation, overpopulate their habitat and outcompete the native species, in which case they get bad press and have epithets like "invasive" hurled at them. Or, like the Asian Jumping Spider (Sitticus fasciger), they can blend into the background, fit right in, and live their little lives, doing what spiders do best ...

"If it is not small enough to eat nor large enough to eat you, and doesn't put up a squawk about it, mate with it." ~ (David L. Jameson, Systematic Zoology, 1955) ... that pretty much sums up the lifestyle of jumping spiders, and the rest of nature as well.