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I stumbled upon the following paragraph on the Wiki page about Argiope bruennichi:

When a prey item is first caught in the web, Argiope bruennichi will quickly immobilise its prey by wrapping it in silk. The prey is then bitten and then injected with a paralysing venom and a protein-dissolving enzyme.

On hindsight, it seems a bit redundant to do both those things. If prey is caught in the web, which is rather sticky and therefore restricts its movement, isn't it possible for the spider to go right ahead and inject venom?

Or on the other hand, if it wraps and immobilises the prey in silk, wouldn't it be more logical to then leave it to die, from some sort of evolutionary standpoint of wanting to saving energy and resources?

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Nice question!

Based on some previous observations on other web spiders, I belive that these "redundant" behaviors are selected to avoid prey scapes, and are not trully redundant if we remenber that some preys are able to evade even if wraped. Some beetles can find a way out from the web, moving their strong legs with projections, eventually some silk lines are cut. Also beetles are more protected against bites then are other bugs, since many are under their natural armor.

Other possible explanation** :some bees, ants and wasps produces chemical signals when in trouble, attracting their sisters. From the spider point of view, this could be a way to get more food. But too many of these insects could possibly destroy the web, only because their are numerous. Obviously these are speculations.

Also, some wasps prey on web spiders. If the wasp eventually touches the web and entangles itself, now the wrap/bite strategy turns in defensive way, maybe with the "danger turns dinner" bonus.

 I did not search too much, but probably someone has alread tested some of these possibilities. Also, I read once that some spiders can treat distinct preys in distinct ways, even releasing some undesirable bugs from the web.

** This bee-wasp-ants-hypothesis is not based on observation, only on knowlodge about insects and spiders.

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