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Brisbane, Australia.

While no image is available, direct observations were made of flies doing something to the egg sac.

Spider nest, with flies ‘injecting’ into egg sacs

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  • $\begingroup$ Welcome to Biology.SE. What makes you think the flies are 'injecting' something into the structure? Also please give us any information you have relevant to species-identification — in particular it could be helpful to know the geographic location as well as the ids of the species involved (if known). $\endgroup$
    – tyersome
    Apr 12 at 4:20
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    $\begingroup$ Possibly cannot see in the photo, but I could see each fly had a part of themselves extended from their rear end going into the egg sacs. She is a ‘tent spider’ and that’s how they lay their eggs. Each ‘bump’ is a sac of eggs. $\endgroup$
    – Rose
    Apr 12 at 6:54
  • $\begingroup$ Cyrtophora moluccensis- tent spider. Injecting? I could see something from the rear end of the flies going into the sac. (Sorry, Not clear in photo). Location: Brisbane Australia $\endgroup$
    – Rose
    Apr 12 at 7:09

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(Converting from comment)

Based on internet searches, it appears likely that the fly is Sarcophaga arachnivora, which lives off of spider egg cases in its juvenile form; the fly is laying eggs.

If you follow that link you will see references to a couple of papers that describe the species in more detail, though I don't believe I have access to them.

The geographic location noted there of SE Australia agrees with your observation of the organism.

References about tent spiders also make note of parasitic "flesh flies" so the species interaction appears to be an established phenomenon.

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