I recently noticed a fly got caught in a spider web in my house. The fly was very energetic and, even though caught, continued to flap it's one wing (which I think might have been partially free) with hyper-speed.

Initially I thought it was just trying to get free from the web but then I noticed that when the tiny spider (it was smaller than the fly) approached it to, presumably, try to inject its venom the fly started to again energetically flap it's wing and it scared the spider off.

I was wondering - why did the spider got scared? I know animals do not perceive threats quite like us and even a big lioness might give way to a tiny mongoose if the mongoose threatens with a fight (I'm guessing both because unnecessary fighting expends precious energy and any wound can be bad with no hospitals around) but still as far as I know the fly can't harm the spider in any way even if it is free and much less when it is bound and while it would be smarter for the spider to just sit and patiently wait for the fly to drop out of exhaustion he is also risking it getting free, eventually.

So why are spiders scared of flies caught in their web?


1 Answer 1


from https://www.researchgate.net/figure/234696071_fig5_Figure-1-Differences-in-attack-strategies-upon-safe-and-dangerous-prey-in-orb-web

enter image description here

The linked article is pretty cool. Bottom line: predators need to be careful. If they get hurt they cant hunt and they starve. Spiders have different strategies depending on perceived dangerousness of a prey item. The linked article, for example, categorizes dipterans (flies) as nondangerous to spiders but orthopterans (crickets) as dangerous. How are crickets dangerous?

For your spider I can think of 1 of 3 things.

1: Fly is dangerous to spider in a way not obvious to you. Maybe it is poisonous, or can bite, or has sharp spines on the leg.

2: Fly is perceived by spider as similar to dangerous prey. I have watched spiders address wasps in their webs and the spiders are careful. If there are flies that mimic the appearance of wasps / bees to get respect, why not one that behaves in the web like a wasp to get respect? A cool idea and testable too.

3: What you see is the danger. A big fly thrashing around might be enough to damage the spider. A spider not desperate for food might not want to risk getting hit. Spiders are not that durable. That makes sense to me.


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