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Snares are anchored cable nooses set to catch wild animals. There are two types, active and passive. An active snare has the wire under tension and a trigger to cause it to snap closed, while a passive snare has a one way cinch so that as the prey struggles the noose tightens and strangles the prey. Based on admittedly messy and ill sourced numbers, it seems a realistically sized spider could build passive snares to catch and kill medium sized mammals. Also of note, spider silk undergoes Supercontraction when exposed to water so tensioning an active snare might also be possible. So my actual question is, are there any impediments to a spider evolving this ability, what interesting implications might it have for the overall ecosystem?

The admittedly messy and ill sourced numbers:

This gives 6 weeks, for a real sized spider. Which sounds like a lot, but these traps should be reusable. So the spiders would be capable of killing prey 20 - 100 times their own mass.

If someone finds better sources that lead to different numbers, that would also be great.

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    $\begingroup$ I assume the only thing that stops a spider from doing this would be they aren't smart enough to do it. These traps are complicated and spiders aren't really smart enough to design them just by themselves. You might be able to train one to do it but even then I don't know if they are smart enough for training for something this complicated. $\endgroup$
    – A. C. A. C.
    Commented Oct 31, 2017 at 21:56
  • $\begingroup$ The only impediment that I can see with this is possibly the lack of the incremental amelioration necessary for this to be an evolved behaviour. As @A.C.A.C. notes spiders lack the intelligence to design a trap like this on their own. So this would have to be something instinctual, an instinct which becomes slowly more complicated and intense over millions of years of being selected for. So the latter probably precludes the possiblity of an active snare trap: there doesn't seem to be a way for a partially completed active snare trap to be useful (no incremental fitness of this instinct). $\endgroup$
    – AngelPray
    Commented Oct 31, 2017 at 22:00
  • $\begingroup$ On the other hand even a badly created passive snare trap could be very useful. While it might not kill the prey, it could certainly maim it, rendering it easier to catch. $\endgroup$
    – AngelPray
    Commented Oct 31, 2017 at 22:03
  • $\begingroup$ Tarantulas do make silk, but I don't think they are as prolific as the web dependent species. It may be you could have more than one spider cooperate making the trap though. $\endgroup$
    – user25818
    Commented Oct 31, 2017 at 22:32
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    $\begingroup$ @user6760 that already exists. For a basic snare tying a knot would be about as necessary as it is for constructing a standard web. Spiders have shown themselves perfectly capable of binding two strands together. Admittedly a one-way cinch is more difficult. $\endgroup$
    – Lex
    Commented Nov 1, 2017 at 3:18

3 Answers 3

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Spider colony.

6 weeks for a single spider. 80 seconds for 50,000 spiders.

giant spiderweb http://texasento.net/Social_Spider.htm

http://www.bbc.com/earth/story/20160122-meet-the-spiders-that-have-formed-armies-50000-strong

Anelosimus eximius, the species I encountered in the rainforest, is not the only kind of social spider in the world, but it does construct the biggest webs. Some can reach more than 25ft (7.6m) feet long and 5ft (1.5m) wide. A web that size could contain as many as 50,000 individual spiders. That is a lot of legs, eyes and fangs.

There is no reason these spiders could not make a sticky web that, when an animal blundered into it, would fall on the animal and progressively bog it down.

The other thing some spiders have is venom. Like snakes, spiders use venom offensively and defensively. If you pin a rat in a web it is never going to quit chewing its way out. If you pin it and then inject a few hundred spiderworths of venom, it will settle down.

As regards not having enough enzymes to process large prey, I do not think that would be a problem.

*disclosure: web in image is from a different species than that discussed in pasted text.

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    $\begingroup$ I do like the colony idea as it balances out the somewhat low probability of a given trap capturing food on a given day, with the quantity of food in a single capture. On the other hand I was hoping to keep with the creep factor the goes with a single spider taking out a mammal two orders of magnitude larger. $\endgroup$
    – Lex
    Commented Nov 1, 2017 at 1:38
  • $\begingroup$ The point of an actual snare vs the large web you suggest is that the snare itself strangles the prey. The spider does not need to get anywhere near the thrashing animal, and it will be dead before it has a chance to chew through the webbing. $\endgroup$
    – Lex
    Commented Nov 1, 2017 at 1:41
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    $\begingroup$ @Lex the problem is spider webs aren't that strong (how many times have you unfortunately crashed through a web? ). Spider web fibers are actually incredibly strong, such that spider silk is reported to be able to catch bullets. So whats the difference? density. How a snare works is an animal gets caught in some obstruction and strangled to death. In this case an animal catches a ton of webs and gets strangled to death. The real question is if spider fiber is to elastic for strangulation. $\endgroup$
    – anon
    Commented Nov 1, 2017 at 13:32
  • $\begingroup$ @Lex fyi the knot in a snare is unnecessary in this instance because panic would probably drive the animal deeper into the web such that they would become entangled (creating their own knots). $\endgroup$
    – anon
    Commented Nov 1, 2017 at 13:36
  • $\begingroup$ I once saw a tiny spider take out a huge wasp - at least 50x as big. The wasp was in the web fair and square. The spider was very respectful and stayed clear. At one point it crept up and delivered a bite to a leg then backed off. The next day that tiny spider was feeding on the wasp. $\endgroup$
    – Willk
    Commented Nov 1, 2017 at 16:27
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Spiders don't seem to need snares, and they don't seem to need large prey.

First, spiders employ glue in the spiderweb, which is fairly strong. If you ever get caught in a fresh spiderweb, you know that it is easier to snap threads than scratch them off your clothes/skin/hair. So, if a spider can spun a web that can hold large prey, there should be enough glue in that web to hold that prey captive.

Second, spiders are feeding by injecting digestive enzymes into captured prey. If they capture a prey that is much larger than the spider, there is no way they have enough enzymes to process this prey, and most of it would just be rotting.

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  • $\begingroup$ This wouldn't be a full web, just a single looped strand forming a noose. It has the obvious advantage over sticky traps in that it will strangle and kill the prey so the spider can stay well away from it as it thrashes and struggles. $\endgroup$
    – Lex
    Commented Nov 1, 2017 at 1:48
  • $\begingroup$ Stickyness increases linearly with contact surface, which increases linearly with strand diameter, but strength increases quadratically with strand diameter. $\endgroup$
    – Lex
    Commented Nov 1, 2017 at 1:51
  • $\begingroup$ The waste is one of the issues I saw with this idea, the only obvious way to reduce waste being large enough colonies to consume the entire prey. But unless you have a reason the waste would be detrimental to the spider, wastefulness in itself is not a reason to stop something from evolving. $\endgroup$
    – Lex
    Commented Nov 1, 2017 at 2:02
  • $\begingroup$ @Lex the spider would need to be sure that the animal will get his neck, and not leg, into the snare. Also, he would have to weave multithreaded rope to make the snare strong enough - and this is not what spiders do. Waste would attract a lot of other animals, and spider needs to be strong enough to defend his kill. $\endgroup$
    – Alexander
    Commented Nov 1, 2017 at 4:56
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A single strand noose would not be very useful for a spider, but spiders have evolved lots of different hunting strategies which have analogues to things we are familiar with.

The common orb spider (which most people are familiar with) weaves the nets we are familiar with. While the net is not a noose, different species of spider weave different configurations of nets. Some are very tight (one reason is the spider sits not the web and feels for the vibrations, which alert it not only to the fact the prey has been trapped, but even the location on the net where the prey is.

Other webs are much looser, and could be comparable to a fisherman's net with fills and area where prey could be expected to fly into.

Large numbers of species of spiders actively hunt their prey, running on the ground and chasing them down so they can bite and inject their venom into the prey (these include dangerous species like black widows and brown recluse spiders, which can kill humans with their venom).

Other spiders ambush their prey, like trap door spiders, which hide un small holes dug into the ground. Their webs extend outwards from the camouflaged cover, and alert the spider when their prey is close enough to flip open the door and pounce on their prey.

Bolas and net casting spiders come closest to your idea of a snare, bolas spiders use a single scented strand to lure prey in then try to entangle the prey, while net casting spiders actually throw their web over the prey!

Some examples can be found in this article

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  • $\begingroup$ Can you elaborate on why a noose type snare would not be useful. $\endgroup$
    – Lex
    Commented Nov 1, 2017 at 4:30

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