Crab spider (Mecaphesa sp.) on Yarrow (Achillea millefolium)
eating a fly (Tachinidae? Exoristini sp.?). (1)
South of Big Lake, White Mountains, AZ. June 19, 2020, WJ Beaver
A few months ago I was sitting on my patio when I noticed that the tiniest yellow crab spider was dangling from my arm on a bit of web. Crab spiders are called such because they are shaped and walk like a crab. The females hide in flowers and feed on visiting pollinators. There are all yellow species that only feed on yellow flowers and there are white ones that mainly feed on white flowers but can change from white to yellow and can also change the intensity and thickness of the reddish stripes on their abdomen and can feed on other flowers besides white. While walking in a semi-burned forest in the White Mountains of Arizona I encountered a small stand of Yarrow covered with insect life. While taking pictures I realized there was a rather large crab spider on a flower head eating an equally large fly. I took some pictures then we walked on. On the way back I looked for it again and it was gone, either hiding among the clusters or underneath.
Crab spiders don't build webs but use their silk to weave “kites” that allow them to fly through the air. Young spiders are born in the thousands and need to escape the nest fast to avoid cannibalism so they take to the air. This is seen even in larger adult spiders. In fluid mechanics, the Reynolds number is used to predict how fluid flows around an object. There are two main types of flow, viscous or smooth flow, and inertial or turbulent flow. A ball moving through a jar of honey is an example of viscous flow with a small Reynolds number while any odd object in an air stream that creates turbulence behind it is an example of a large Reynolds number. Air is not viscous but as objects become very small in at least one dimension, inertial properties disappear and the air becomes viscous. Studies of large spiders have shown that the spider waves a leg in the air to test the wind currents then anchors itself with a bit of web and stands on its “toes” with its abdomen pointed up and spins long strands of silk, two thicker pieces and dozens of thin ones. Objects with at least one dimension in a range of fewer than 800 nanometers have special properties as quantum effects start to take over. The larger silk strands are around 800 – 500 nanometers and 1 to 8 meters long. The thinner strands are around 200 nanometers. Aerodynamically, this drastically reduces the Reynolds number of the kite, reducing all inertia and producing a purely viscous flow. This dampens turbulence and allows the silk to lift the body of the spider above the boundary layer and into the sky at low wind speeds. (2)
Take Off (2)
Flowers signal their availability to pollinators with color, pattern, and odor. Pollinators have visual and olfactory systems attuned to flower signals. The insect color spectrum is very different than ours and includes ultraviolet wavelengths and polarized light. Crab spiders disappear on a flower due to their color. If they can look like the flower they are on they can wait until their prey comes to them. Some species can change from white to yellow and back again and can also change the intensity and number of their stripes. It is known that different species react to uv light differently but I could not find out whether they could change uv pattern also. I would suspect that they can. (3,4)
I had a couple of chance encounters with a common creature but just a bit of research can turn up amazing and complex behaviors.
“Genus Mecaphesa - BugGuide.Net.” Accessed July 5, 2020. https://bugguide.net/node/view/4999.
Cho, Moonsung, Peter Neubauer, Christoph Fahrenson, and Ingo Rechenberg. “An Observational Study of Ballooning in Large Spiders: Nanoscale Multifibers Enable Large Spiders’ Soaring Flight.” PLOS Biology 16, no. 6 (June 14, 2018): e2004405. https://doi.org/10.1371/journal.pbio.2004405.
Heiling, Astrid M., Lars Chittka, Ken Cheng, and Marie E. Herberstein. “Colouration in Crab Spiders: Substrate Choice and Prey Attraction.” Journal of Experimental Biology 208, no. 10 (May 15, 2005): 1785–92. https://doi.org/10.1242/jeb.01585.
Anderson, Alissa G., and Gary N. Dodson. “Colour Change Ability and Its Effect on Prey Capture Success in Female Misumenoides Formosipes Crab Spiders.” Ecological Entomology 40, no. 2 (2015): 106–13. https://doi.org/10.1111/een.12167.
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