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Just inside a doorway in northern Taiwan I found an interesting looking spider flat against the wall, pointed downward about 30 cm above the floor, sitting motionless.

Mostly dark but some patterns appear with the flash photo.

Everything is long; long mouth parts in front, long "pincers" in back, and long legs, except the 3rd pair from the front which were thinner and only about 20% of the front (longest) pair. See photo for an estimated scale bar - roughly 16 mm from mouthparts in front to "pincers" in back, over 80 mm toe-to-toe.

I assume it's just sitting there waiting for something delicious to walk by and eat, but why are the mouth parts and "pincers" so long and the 3rd leg pair so short? Are there specific activities or functions that can help explain these?

Does it always hunt like this or does it also build webs?

Dark, long-legged spider with long mouthparts in front, long "pincers" in the back and very short 3rd pair of legs (northern Taiwan)

above: cropped, below: click for full size, the first uses a flash and is viewed from above-front, 2nd is with ambient light and top-down view.

Dark, long-legged spider with long mouthparts in front, long "pincers" in the back and very short 3rd pair of legs (northern Taiwan) Dark, long-legged spider with long mouthparts in front, long "pincers" in the back and very short 3rd pair of legs (northern Taiwan)

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From a browse through iNaturalist, I believe this is a member of the genus Hersilia also known as long-spinneretted bark spiders or two-tailed spiders. I have no idea which of the species this might be, but a Hersilia striata comes up as being present in Taiwan.

The characteristics are, unsurprisingly, the long spinnarets and long legs. I believe these will be largely ambush spiders similar to wolf spiders, using camouflage to hide and deceive prey. I don't know if the long spinnarets play any role, nor the shortened legs.

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    $\begingroup$ It seems like you've got it narrowed down nicely, thanks! $\endgroup$
    – uhoh
    Commented Aug 10, 2023 at 14:46
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    $\begingroup$ I do a lot of walking and hiking - I come from northern latitudes so the much higher species diversity of Taiwan is a never ending source of discovery. I've been here 15 years and still on every long hike I photograph a new flower or two and several new insects I'd never seen before. I've seen many of these in the same general area, and always wondered if they were "really well reinforced egg cases". But I suppose it's captured, digesting prey, and those short 3rd legs and long spinnerets are just the right lengths to make these? $\endgroup$
    – uhoh
    Commented Aug 10, 2023 at 21:59
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    $\begingroup$ @uhoh yo might well be right there on the 3rd pair being the right length to fit the spinnerets. Still I wonder what the evolutionary pressure is to have such long ones. $\endgroup$
    – bob1
    Commented Aug 10, 2023 at 22:40
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    $\begingroup$ This looks like the short 3rd set of legs are pulled under and forward and working with the mouth parts to hold prey youtu.be/u7VIHz9Dspc and this is just a nice, camouflaged specimen youtube.com/watch?v=OjnT-fzEV2E According to this they hunt by throwing silk, now I need to see it... $\endgroup$
    – uhoh
    Commented Aug 10, 2023 at 23:11
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    $\begingroup$ Interesting if they are web throwers - that would fit the extra long first and second pair of legs and the long spinnerets. Use the spinnerets to cast a web between all 4 pf the first 4 legs, then use these to spread and trap. The spinnerets wouldn't reach otherwise. The first two leg pairs might be too long for the mouthparts to reach and hold, so 3rd makes sense. The beauty of evolution in action! You'd need to see them at night to see the web casting, probably in a contrived set-up - that's how the doco crews do it. $\endgroup$
    – bob1
    Commented Aug 11, 2023 at 0:19

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