Today reader Robert Lang, who previously sent us photos from Costa Rica, weighs in with some arthropods:
I realize this is close on the heels of my last set of Costa Rica photos I could not resist passing on a few more photos taken much closer to home (in the East Bay area of California), which I similarly hope you and readers might enjoy.First, we have a dragonfly, a Flame Skimmer (Libellula saturata), which has been hanging around the goldfish pond (a.k.a. the raccoon feeder) for a week or so. Pretty sure this one is a female, because she repeatedly makes passes over the pond, dipping her abdomen in it. (Sadly for her, I expect that the goldfish make short work of the offspring.) What I love about this picture is that the sunlight casts an orange shadow through her wings, looking for all the world like a fossil impression in the rock.
The reason I was in this particular canyon was not for the beetles, though; it was for this next creature, the California Common Scorpion (Paruoctonus silvestrii), which crawls out of its daytime lairs (typically cracks in cliffs and dirt) after sunset. Like many scorpions, it is fluorescent under UV, and while most scorpions fluoresce in the blue-green, this one is a brilliant chartreuse.
They’re tiny (most that I saw were 2-3 cm in total length), but you can spot even the smallest from several feet away; they stand out like a beacon. This one is a bit washed out (and I’m sure the camera is a wee bit confused by the bimodal color distribution), but by turning down the exposure, quite a bit of detail becomes visible. I’ve tweaked the colors of this next one to approximate what it looks like to the human eye.
Why they fluoresce is a bit of a mystery, but a few years ago Animal Behavior published a paper in which the authors suggested that it served the function of aiding light sensitivity by transmuting the dim level of UV from the night sky into green light that their existing photoreceptors could detect.Since there are noticeable differences in the peak wavelength of fluorescent emission among species, it would be interesting to see if there were corresponding differences mirrored in the peak photoreceptor sensitivity.