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portia fimbriata

portia fimbriata, sometimes called the fringed jumping spider, is a jumping spider (family salticidae) found in australia and southeast asia. it is often difficult to find p. fimbriata in the wild, as its shape and movements are well disguised.

portia is a genus which feeds on other spiders (araneophagic). they are remarkable for their hunting behaviour which suggests they are capable of learning and problem solving, traits normally attributed to much larger animals. the genus portia have been called "eight-legged cats", as their hunting tactics are as versatile and adaptable as a lion's. all members of portia have instinctive hunting tactics for their most common prey, but often can improvise by trial and error against unfamiliar prey or in unfamiliar situations, and then remember the new approach. they can also make detours to find the best attack angle against dangerous prey, even when the best detour takes a portia out of visual contact with the prey, and sometimes the planned route leads to abseiling down a silk thread and biting the prey from behind. such detours may take up to an hour, and a portia usually picks the best route even if it needs to walk past an incorrect route.

when not hunting for prey or a mate, portia species adopt a special posture, called the "cryptic rest posture", pulling their legs in close to the body and their palps back beside the chelicerae ("jaws"), which obscures the outlines of these appendages. when walking, all portia species have a slow, "choppy" gait (sometimes called "robotlike") that preserves their concealment: pausing often and at irregular intervals; waving their legs continuously and their palps jerkily up and down; and moving each appendage out of time with the others and continuously varying the speed and timing. a portia′s walk is unlike that of any other spider, and this gait and the spider's fringes gives the appearance of light flickering through the forest canopy and reflecting from piece of detritus. in queensland, p. fimbriata walks and waves more jerkily and about twice as slowly as other portias, including p. fimbriata in other areas.

jumping spiders have eight eyes, the two large ones in the center-and-front position (the anterior-median eyes, also called "principal eyes" housed in tubes in the head and providing acute vision. the other six are secondary eyes, positioned along the sides of the carapace and acting mainly as movement detectors. in most jumping spiders, the middle pair of secondary eyes are very small and have no known function, but those of portias are relatively large, and function as well as those of the other secondary eyes. the main eyes focus accurately on an object at distances from approximately 2 centimetres to infinity, and in practice can see up to about 75 centimetres. portia fimbriata can distinguish prey and conspecifics up to 280 millimetres (47 times its own body length). however, a portia takes a relatively long time to see objects, possibly because getting a good image out of such tiny eyes is a complex process and needs a lot of scanning. this makes a portia vulnerable to much larger predators such as birds, frogs and mantises, which a portia often cannot identify because of the other predator's size.

while most jumping spiders prey mainly on insects and by active hunting, females of portia also build webs to catch prey directly. males of portia do not build capture webs. a web spider's web is an extension of the web spider's senses, informing the spider of vibrations that signal the arrival of prey and predators. if the intruder is another web spider, these vibrations vary widely depending on the new web spider's species, sex and experience. a portia can pluck another spider's web with a virtually unlimited range of signals, either to lure the prey out into the open or calming the prey by monotonously repeating the same signal while the portia walks slowly close enough to bite it. such tactics enable portias to take web spiders up to twice a portia′s size, and portias hunt in all types of webs. a few web spiders run far away when they sense the un-rhythmical gait of a portia entering the web - a reaction researchers wilcox and jackson call "portia panic".

when hunting in another spider's web, a portia′s slow, choppy movements and the flaps on its legs make it resemble leaf detritus caught in the web. portias use breezes and other disturbances as “smokescreens" with which these predators can approach web spiders more quickly, and revert to a more cautious approach when the disturbance disappears. females are more effective predators than males. all types of prey spiders occasionally counter-attack, but all portias have very good defences, starting with especially tough skin. portias' venom is unusually powerful against spiders.

p. fimbriata in queensland is an outstanding predator of other jumping spiders and of web spiders, but poor against insects. the queensland variant use a unique "cryptic stalking" technique which confuses most other jumping spiders so thoroughly that most cannot identify this p. fimbriata as a predator, or even as an animal at all. queensland p. fimbriata′s cryptic stalking may be a regional adaptation to the abundant but dangerous salticid prey, especially jacksonoides queenslandicus, in the local rainforest. when encountering j. queenlandicus, p. fimbriata often first notices chemical cues on j. queenlandicus′ silken safety lines and then looks for its prey. the smell makes p. fimbriata to quicker to see the prey, possibly by lowering thresholds in the visual system. sometimes p. fimbriata cannot see j. queenlandicus through the prey's camouflage, and "hunts by speculation", jumping high in the air, so that j. queenlandicus betrays itself by turning and looking for the disturbance. p. fimbriata then turns toward j. queenslandicus and waves its palps. it appears that only p. fimbriatas from queensland behave this way while portias from other areas do not, and that p. fimbriata from queensland reacts this way only to j. queenslandicus, and that j. queenslandicus perceives no chemical warnings that p. fimbriata is around.

when catching an insect outside a web, a portia sometimes lunges and sometimes uses a "pick up", in which it moves its fangs slowly into contact with the prey. in some pick ups, portia first slowly uses its forelegs to manipulate the prey before biting.

a test in 2001 showed that four jumping species, including p. fimbriata, take nectar, either by sucking it from the surface of flowers or biting the flowers with their fangs. the authors suggest that, in the wild, nectar may be a frequent, convenient way to get some nutrients.

females of many spider species, including p. fimbriata, emit volatile pheromones into the air, and these generally attract males from a distance. the silk draglines of female jumping spiders also contain pheromones, which stimulate males to court females and may give information about each female's status, for example whether the female is juvenile, subadult, or mature. pheromones may help to find jumping spiders' nests, which are usually hidden under rocks or in rolled leaves, making them difficult to be seen. portia exhibits a different mating behavior and strategy compared to other jumping spiders. in most jumping spiders, males mount females to mate. in portia the female drops a dragline after the male mounts her, mating in mid-air. before this happens the male shows off his legs and extends them stiffly and shakes them to attract the female. the female then drums on the web. unlike in other portias, females of p. fimbriata do not eat their mates during courting, nor during or after copulation.

for moulting, all portias spin a horizontal web whose diameter is about twice the spider's body length and is suspended only 1 to 4 millimetres below a leaf. the spider lies head down, and often slides down 20 to 30 millimetres during moulting. portias spin a similar temporary web for resting. p. fimbriata in queensland can be very sedentary, in some cases remaining in the same web for over 48 days during a series of moults.
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