A net-casting spider

Net-casting spiders hang down near the ground, and their web has become, through evolution of the spider’s behavior, a “net” held in the four front legs. These sit-and-wait predators simply bide their time until a prey item walks underneath, and then cast their net like a fisherman.

These are in the family Deinopidae, and, of course, are adapted to seeing well. They have the eight eyes possessed by nearly all spiders (remember, “eight legs; eight eyes”), but two of the eyes have become enormously enlarged (see picture at bottom). Most species live south of the equator, and on all continents save Europe and Antarctica. Also, as Wikipedia notes:

Their excellent night-vision adapted posterior median eyes allow them to cast this net over potential prey items. These eyes are so large in comparison to the other six eyes that the spider seems to have only two eyes. . . Its eyes are able to gather available light more efficiently than the eyes of cats and owls, and are able to do this despite the lack of a tapetum lucidum; instead, each night a large area of light sensitive membrane is manufactured within the eyes, and since arachnid eyes do not have irises, it is rapidly destroyed again at dawn.

This video, by Attenborough of course, shows the amazing talents of this spider. It’s interesting to contemplate how this derived behavior might have evolved, step by step, from the web of an ancestral spider. The earliest spiders probably used silk only to line their burrows or wrap up prey, and later developed adaptations to spin a web that could catch prey. It is from these web-building spiders that net-casters probably evolved. (I know there are spider experts reading here, so correct me if I’m wrong, and by all means refer us to relevant literature or theories.) I can imagine step-by-step adaptive scenarios for that, but I’ll leave it to you.

Here’s another one catching a fly:

Look at these eyes! (The species is Deinopsis subrufa, and the photo is from Spiders of Australia). You can barely make out the six tiny eyes.

14 Comments

  1. Posted October 8, 2017 at 2:35 pm | Permalink

    “The earliest spiders probably used silk only to line their burrows or wrap up prey, and later developed adaptations to spin a web that could catch prey. It is from these web-building spiders that net-casters probably evolved.”

    Yes, net-casting spiders are araneomorphs, so this is true.

    • Posted October 8, 2017 at 5:43 pm | Permalink

      Whoops, I forgot to h/t Hardy, who brought this to my attention. Thanks!

  2. ThyroidPlanet
    Posted October 8, 2017 at 2:59 pm | Permalink

    Sub

  3. Randall Schenck
    Posted October 8, 2017 at 3:09 pm | Permalink

    I remember my parents telling me, you have a brain, why not use it. The spider has eight legs and knows how to use all of them.

  4. Jenny Haniver
    Posted October 8, 2017 at 3:14 pm | Permalink

    How to account for the roughly trapezoidal framework of the net? I just found this additional information and more photos of net-casting spiders https://ednieuw.home.xs4all.nl/australian/Deinopidae/Deinopidae.html. It states that “The web is made of non-sticky cribellate (woolly) silk. Insects gets entangled in the wooly structure.” Am I to understand that this sentence refers to the wavy silk within the frame, or what?

  5. W.Benson
    Posted October 8, 2017 at 4:01 pm | Permalink

    Now, tell me, how did THAT evolve?

    • rickflick
      Posted October 8, 2017 at 8:12 pm | Permalink

      Natural selection, naturally. 😎

      • W.Benson
        Posted October 8, 2017 at 10:29 pm | Permalink

        No rickflick, you must have a gradual sequence of intermediate forms, all of which are adaptive and with each intermediate showing an advantage over the immediately preceding one. You can’t just proclaim “natural selection” because an [apparent] adaptation exists.

        I suspect that ancestral spiders started out tending webs built among dead leaves on the ground. A web with (fewer) free corners could be manipulated by the spider to intercept or entangle prey better. Once instincts for handling the web evolved, ones for carrying the it around would only be a step away.

        It would be good have an evolutionary series, but I suspect what evolution has left are not necessarily evolutionary series. See Mark Sturtevant’s comment below.

  6. Posted October 8, 2017 at 6:37 pm | Permalink

    I don’t know much about the taxonomic relationships withing spiders, but these spiders are related to other species that also make fuzzy webs that entangle prey. One could use various species to imagine some of the evolutionary steps toward the kind of spider shown here. An early step may be represented by the feather legged spiders, which make a fuzzy orb web: http://bugguide.net/node/view/865318/bgimage . I have come across a couple of these. A next step could be represented by the triangle spiders: http://bugguide.net/node/view/78763/bgimage . These make a triangular web which they hold taut by standing off to one corner. They let go when an insect flies into the web, entangling it.

  7. loren russell
    Posted October 8, 2017 at 6:56 pm | Permalink

    Interesting to compare the unrelated bolas spider [genus Mastophora and relatives], specialized predators on large moths. They attract their prey by mimicking female pheromones, then snag them with a silk filament with a big gob of glue at the end. These spiders are Araneidae [orb-weavers], so descended from complex web-builders.

    A clue to how this can happen — lots of species, lots of time, and availability of silk, a very versatile material.

  8. Posted October 9, 2017 at 5:58 am | Permalink

    Do these spiders get dazzled by bright light? Not having the ability to blink, presumably there is some other protection against direct sunlight blinding them? Is it down to the type of compound eye?

    • joanfaiola
      Posted October 9, 2017 at 9:40 am | Permalink

      I asked a friend, Astri Leroy, whose husband John has photographed Deinopis, and she said:
      “Oh gosh no I don’t know

      I’ve only had experience with ONE big Deinopis cylindricus that John photographed. Lighting was a normal torch covered in a red sweetie wrapper so he could focus. She was OK till the camera flash went off then she simply bundled up her web and went to hide. It was a really dark night in the sand forest in northern KZN. (Kwa-Zulu-Natal).
      That’s all I know. Menneus camellus doesn’t seem to care nor did Menneus capensis, but then their median eyes are smaller.”
      So I guess Deinopis does react adversely to harsh and sudden light.

    • loren russell
      Posted October 9, 2017 at 11:22 am | Permalink

      FWIW, spiders don’t have compound eyes. rather, each eye is a separate camera, with lens elements focusing images on a retina.

  9. joanfaiola
    Posted October 9, 2017 at 6:33 am | Permalink

    Catherine L Craig, an American arachnologist, is an expert on spider silk. The evolution of spider silk is covered in her books, of which the most readable is Spider Silk (Yale UP) by Leslie Brunetta and Catherine Craig. I recommend this book to anyone who is interested.


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