Pretty Little Wallflowers

Found this little guy on a signpost at a local forest preserve today:

Tmarus angulatus, Van Patten Woods FP, Lake Co, IL 5/28/2014

What the heck are we looking at? It's a spider, specifically Tmarus angulatus, one of the Crab Spiders in the family Thomisidae. They're ambush hunters, waiting in one place for something to wander close enough to grab. Fortunately, they're all tiny enough to be completely harmless to us.

Here's an angle that may be a bit easier to process:

Tmarus angulatus, Van Patten Woods FP, Lake Co, IL 5/28/2014

Being an ambush hunter is well suited to wallflowers -- if you stand out from your environment, your prey isn't likely to come close enough. (You're also more likely to be eaten, of course.) Of course, being a wallflower on a flower can still be attractive enough:

Northern Crab Spider (Mecaphesa atratus), 

Illinois Beach SP, Lake Co, IL  6/2/2012

This is a Northern Crab Spider (Mecaphesa atratus). (At least I think it is -- it's certainly something close.) Like some of their relatives, these guys can slowly change color to match their surroundings, from white to yellow to pink depending on their favorite flowers.

From a researcher's standpoint, an ambush hunter that will spend several days hunting a single inflorescence is a godsend. Trying to examine predation behavior in wild wolf spiders, for example, would be pretty much impossible, given how much ground they cover. With crab spiders, though, you can watch (or even video these days) them for long periods of time, and get really good quantitative estimates of prey preferences, capture rates, and even net caloric intake with different prey types. Morse did this in Maine, looking for evidence of specialization on certain prey types based on optimal foraging theory, and not finding it. (1) I don't know whether he considered the issue of seasonal variation in prey type abundance. He also looked at the effect of flower type on prey capture rates, finding that roses were the least efficient places to hunt, with milkweed beating out goldenrod due to nocturnal captures of moths. (2)

Tmarus isn't a flower specialist -- it tends to wait on twigs and leaves instead. This sort of specialization makes sense, if the substrate makes a difference in capturing prey. Jimenez-Valverde & Lobo extrapolated that to a landscape scale, and found that species richness of crab spiders (and orb-weavers) was largely determined by the diversity of vegetation present in an area. (3) It's easy to see that specialist herbivores will be impacted by changes in plant distributions, but this shows that other species can be affected as well.

When Benoit Mandelbrot coined the word fractal, he was referring to very specific (but very hard to define) mathematical structures. Computer artists have taken advantage of the way that natural structures so closely mimic fractals. The more I delve into ecology, the more I think that ecologists should be doing the same thing.

(1) Morse, D. H. (1979). Prey capture by the crab spider Misumena calycina (Araneae: Thomisidae). Oecologia, 39(3), 309-319.

(2) Morse, D. H. (1981). Prey capture by the crab spider Misumena vatia (Clerck)(Thomisidae) on three common native flowers. American Midland Naturalist, 358-367.

(3) JIMÉNEZ‐VALVERDE, A. L. B. E. R. T. O., & Lobo, J. M. (2007). Determinants of local spider (Araneidae and Thomisidae) species richness on a regional scale: climate and altitude vs. habitat structure. Ecological Entomology, 32(1), 113-122.