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Why do Scorpions Glow Under UV light?

Many macro shooters have come to know that the scorpion's exoskeleton glows a bright cyan under Ultra Violet (UV) light. But few know why or how.

Why the need to glow?
After scouring around for answers, the most commonly accepted answer was... that the scorpion's cuticle or exoskeleton functions as a giant secondary eye, collecting light information and relaying it to the nervous system. It is believed to be used by the scorpions to detect shelter, as being under shelter would block the light signals on it's body. Douglas Gaffin of the University of Oklahoma "blindfolded" the scorpions and found that they could still detect light. See link here.

There is another theory that the nocturnal scorpions have properties in it's exoskeleton that repels UV light, protecting it from the sun. Scorpions normally hunt at night and are tuned for the light and temperature conditions. If it requires to move in the day for any reason, this might help to re-radiate solar energy from the sun.

How did it glow?
As for HOW the glow came about, further scouring found that the mature exoskeleton accumulates a chemical called beta-carboline, which glows under UV.

Update 20 Nov 2013: My UV photos were recently published on WIRED, with more details on this phenomenon: The Secret World of Fluorescent Arthropods.

Scorpion (Lychas scutilus?) - DSC_8288 #1 Had everyone practising UV shots with my UV torch, this shot used flash to fill the background. This Lychas scutilus is quite commonly found in Singapore.

Scorpion (Lychas scutilus?) - DSC_8291 #2 Same composition, but purely lit by UV light

Scorpions (Lychas scutilus?) - DSC_8314 #3 A pair of scorpions on the tree bark, again with fill flash

Scorpions (Lychas scutilus?) - DSC_8312 #4 The same pair lighted with only UV light.

Scorpion (Liocheles australasiae?) - DSC_6033 #5 Different scorpions glow as well! Possibly Hemiscorpiidae, Liocheles australasiae.

Black Forest Scorpion (Heterometrus sp.?) - DSC_6324 #6 Even the black forest scorpion glows. Without the UV light, it would appear completely black.

Scorpion with babies (Lychas scutilus?) - DSC_1132 #7 A mother with babies on her back. As mentioned earlier, the chemical causing the glow accumulates on mature exoskeletons. That's probably why the babies do not glow.

Scorpion (Lychas scutilus) - DSC_8982 #8 Another old picture of a scorpion with babies. The scene was lighted with a red light, showing again that the babies do not glow under UV light.

Some other simple finds at Nangka Trail...

Tarantula (Phlogiellus inermis) - DSC_8199 #9 James found this common male Tarantula (Phlogiellus sp.) on the leaf litter.

Tarantula (Phlogiellus inermis) - DSC_8203 #10 It ran off in hiding after this shot

Bird Dung Spider (Pasilobus sp.) - DSC_8208 #11 A very small female Bird Dung Spider (Pasilobus sp.)

Huntsman Spider (Sparassidae) - DSC_8217 #12 A beautiful Huntsman Spider (Olios sp.)

Huntsman Spider (Sparassidae) - DSC_8236 #13 Closer view of the face

Huntsman Spider (Sparassidae) - DSC_8253 #14 Wider view of the body

Tailless Whip Scorpion (Amblypygi) - DSC_8257 #15 A tiny Amblypygi, resident of Nangka Trail.

Tarantula (Phlogiellus inermis) - DSC_8258 #16 A baby Tarantula (Phlogiellus sp. running around on a rock.

Ground Spider (Mallinella sp.) - DSC_8264 #17 Ground Spider (Mallinella cinctipes) with supper

Wandering Spider (Ctenidae) - DSC_8270 #18 Appears to be a pisaurid, but check out the eye arrangement (below) and it should be a ctenid, likely to be from the genus Acantheis.

Wandering Spider (Ctenidae) - DSC_8276 #19 View from a lower angle

Wandering Spider (Ctenidae) - DSC_8285 #20 Close up on the face

Crab Spider (Thomisidae) - DSC_8305 #21 Male Crab Spider (Cebrenninus rugosa) ID kindly provided by David Court.

James blogged about this trip here.

The complete album can be viewed here.

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