Spiders: dimorphic, mimetic, and fluorescent

December 16, 2012 • 2:26 pm

I’ve discovered another superb photographer of arthropods: Melvyn Yeo, who has some amazing photos on his site. He calls himself a “just a hobbyist,” but, as you’ll see below, he’s selling himself way short. I’ll put up just three of his spider pictures to show you:

This one, Gasteracantha arcuata, has extreme sexual dimorphism: in this case females are large and ornamented, while males are drab and much smaller. Female above, male below (more information here):



This crab spider imitates mimics a bird dropping (for more pictures and information go here):

The araneid spider Pasilobus sp. builds its web at night close to bushes and small trees.
The araneid spider Pasilobus sp. builds its web at night close to bushes and small trees.

And here’s a harvestman, with Yeo’s photo passed on by Bug Girl, who said on Twitter:

It’s not just scorpions that fluoresce under black light! Harvest men (daddy-longlegs or Opiliones) also glow.

Remember that although harvestmen are arachids (that’s a class), they’re not spiders, for they’re in the order Opiliones, while “true” spiders are in a different order, Araneae.

Picture 2

28 thoughts on “Spiders: dimorphic, mimetic, and fluorescent

  1. I wish I had cones that could see ultraviolet like birds do, and presumably spiders too. And shrimps have even better eyesight. Amazing photos.

      1. Often sold as “fake note detectors” (because genuine bank notes often use cryptic printing which is UV-fluorescent).

    1. Actually, the phenomenon you’re seeing isn’t something that could see with UV vision. What’s happening is that the pigment is absorbing high-energy UV light and emitting lower-energy blue light. It’s the same as any other sort of fluorescence.

      I don’t know that these animals look like anything special in UV wavelengths…you’d need an actual UV-sensitive camera to tell. Just shining a light with a lot of UV wavelengths in it isn’t going to do the trick, any more than a heat lamp gives you night vision.



  2. Harvestmen that fluoresce in the ultraviolet–yowza! Can someone tell this slackjawed student in response to what environmental pressure that has evolved? I’m guessing it’s nothing to do with predator avoidance, as something like that would only make one easier to find (right?). Is it so that potential mates can find them? (And, do both males and females fluoresce?). If not, it’s probably something I can’t even think up. I’m sure I could google it, but it’s easier to ask the denizens of the web site. Thank you!

    1. Not everything is due to natural selection in nature! This might just be a side-effect of some protein that happens to fluoresce but that evolved for other reasons, or via genetic drift. On the other hand, there is some evidence that the fluorescence does have a function in scorpions.

  3. Usually it’s the other way around with sexual ornamentation is it? Why does this species have ornate females and drab males?

    1. Pure speculation, but it may have to do with a “once and done” deal for male spider mating (males can get eaten by the female after conjugation). The sexual selection pressure might then be on the female to advertise to the male: “I’m worth dying over!”

      1. Alternatively, it might have nothing at all to do with sex. The females are so much bigger than the males that they are surely subject to very different selective pressures. These might be defensive devices or something.

  4. Thank you for drawing our attention to these beautiful photos! Love ’em.

    In New England we have a butterfly larva that imitates a bird dropping during an early instar. The Viceroy. I found a great place two summer’s ago to take some photos of different stages. The fifth from the bottom is an example of the “bird dropping” stage. Keep in mind – my photos and write-up are totally amateur! Just wanted to share in case others were interested in another example of bird dropping imitation in the insect world. I’m sure there are more examples. Link:

    Diane G., I’m intrigued by the UV flashlight!

  5. Lovely “Newton’s Rings” on that harvestman. You could probably estimate the spectral quality of the UV lamp from that image.
    I get the same effect from raindrops on my spectacles when walking under “sodium” street lamps. It was a real intellectual thrill when I worked out what caused the effect when walking to school one morning.

    1. If you’re referring to the big cyan-colored features in the center on the beastie’s body, I don’t think those’re Newton’s Rings; I think it’s just that there happen to be concentric patterns viewed head-on.


      1. My reading of them is that there’s a droplet (air or water – I’m not sure of the medium) with a tapering edge, and it’s giving a “Newton’s Ring” like phenomenon. I’ve done a lot of polarized light microscopy over the years and we’re continually using such effects in, for example, the quartz wedge, the Becke line test, etc. And I’ve seen the same pattern so often on my specs that I’ve never figured any other way of producing the effect. That a “sodium lamp” would produce the effect on my specs where a “white lamp” wouldn’t produce anything noticeable, when viewing a particular raindrop’s meniscus, was a crucial observation in developing my interpretation. (It was so long ago that I had to dredge the memory for a while.) With care, a street lit by sodium lights and “white” lights on passing cars will allow you to select a raindrop and observe it’s meniscus against sodium lamp, white light, back to sodium lamp, back to white light … assuming, of course that you wear specs and that you get rainfall in AridZone? (Not a constraint in Britain!)
        Nope, I’m pretty convinced that it’s “Newton’s Rings”.

        1. This is a better description of how a quartz wedge works, and the comparison between interference fringes under white light versus monochromatic light.

  6. Serious skills and dedication.

    His video titled macro is visual overload. He has so many fascinating species he has captured. Half the time I was trying to figure out where predator ended and prey began or which was head or tail.

    Love his caption in the About Me section for “My Macro frantic friends”. Works for me.

  7. Gasteracantha is one of my favorite spider genera. I used to try and keep G. cancriformes as pets when I was a kid. It didn’t work out well but they lasted longer than the Nephila I tried to do the same thing to, the poor things.

    1. Somehow people manage to raise Nephila in commercial quantities, to make golden silk. A cloak or something made of pure Nephila silk was making the rounds in museums a few years ago.

  8. The 2nd spider is a Theridiid, not an Araneid. Macracantha arcuata (moved from sub-genus of Gasteracantha to it’s own genus almost 20 years ago) does exhibit sexual dimorphism but that spider is not the male.

    The 3rd spider is an Araneid, not a Thomisid (crab spider) as described.

    Other than scorpions and harvestmen, some millipedes fluoresce under UV light as well, particularly the legs. 🙂

Leave a Reply