I’ve known that scorpions have fluorescent cuticles — if you go out into the desert with a black light and shine it on the ground, the scorpions will often glow green and blue and be easy to spot. I had no idea that many spiders exhibit the same phenomenon, but there they are, glowing away. I may have to visit my local head shop (in Morris? Hah!) and get some black light bulbs to see what the fauna in my living room is up to.
Fluorescence is actually a fairly common property: all it requires is a molecule called a fluorophore that can absorb and capture transiently photons of a particular wavelength, or energy, and release them at a lower energy. What this means is that a fluorescent substance absorbs light at one range of wavelengths, and then re-emits those photons at a longer wavelength; there is a color shift. In the case of black light posters and spiders and scorpions, they are absorbing light at wavelengths our eyes can’t detect (wavelengths below about 400nm, or ultraviolet light) and shifting it to a wavelength we can see, for instance to a nice blue at 450nm, cyan at around 500nm, or green at about 550nm. So to test this, all you need is a dark room, a spider, and a light source that glows at the wavelength that is absorbed by the fluorophore, and a detector (like, say, your eyes) that can collect light at the emission wavelength.
Or you can use a fluorimeter, a device that captures and measures the intensity of light emitted at specific wavelengths. Here’s what’s measured when you take various spiders, shine a UV light at them, and record the intensity of the emission at various wavelengths. Those are some respectable Stokes shifts from the spiders.
I know, charts and graphs are so bloodless and boring. What would you see in this situation? The photos below are of two species of spider, photographed on the left with normal visible light illumination, and on the right with UV illumination. Oooh, they glow!
Notice also the taxonomic distribution: it’s all over the place. All of the spiders have fluorophores in their blood — if you do an ethanol extraction of the hemolymph, you can get nice glowing test tubes of fluorescent spider blood — but they vary in how much of the fluorophore (whatever it is; it’s unidentified here) they deposit into the cuticle and into more thinly shielded tissues, like joints and hairs, that are exposed to the light.
I do disagree with the authors on one thing, though. They try to argue that this variation implies evolutionary significance, and I don’t think it does. This kind of spotty distribution that doesn’t show much coherence in the lineages implies to me that it is simply random variation in a trait that doesn’t have much of a selective advantage or disadvantage. Fluorophores are ubiquitous, and I wouldn’t be at all surprised if the spider fluorophore was simply an incidental property of something like spider hemocyanin, and its deposition pattern is a neutral phenomenon.
Now maybe it could be important. Fluorophores can act to enhance color in visible wavelengths (detergent manufacturers add fluorescent chemicals to laundry soap, for instance, to make “whites whiter than white”) and if these spider species were using color cues for courtship or camouflage, then maybe a case could be made. The authors do point out that flowers reflect light in UV wavelengths and some insects recognize and target flowers on the basis of colors we can’t see, so this could be important for camouflage, either to blend in or perhaps, by absorbing and thereby muting light at wavelengths that prey are tuned in to recognize, they are hiding themselves. The paper really doesn’t do any experiments to test their evolutionary hypotheses, but does show that spiders have some interesting color properties to which we humans with our limited eyes may not pay much attention. Unless, of course, we start decorating our homes in the styles of the 1960s.
Andrews K, Reed SM, Masta SE (2007) Spiders fluoresce variably across many taxa. Biology Letters 3(3):265-267.