This post is an attempt to detail my current thinking with regards the fluorescence of immature stages and how this phenomenon relates to their ecology.
I’ve made the mistake previously of assuming that if larvae/pupae are fluorescing under UV light then this will make them MORE visible to predators that see well in the UV part of the spectrum. Clearly it’s hard to make this scenario make sense, and I’ve struggled to imagine some kind of benefit that would outweigh this apparent cost. This has led me to consider all kinds of possibilities, some better than others but unfortunately all stemming from that fundamental misunderstanding.
When we go out at night and flood our subjects with UV light, the phenomenon we witness is UV induced visible fluorescence.
Although an extremely useful method for surveying for certain species, the real interest for me is that this practice gives us an insight into what they might be doing with the UV light they receive during the day. A process that is normally hidden from our eyes in daylight is dramatically exposed when UV is isolated from the rest of the spectrum at night.
Although we don’t know how much of the UV is being reflected (I guess we must be able to measure this with some specialist equipment) some of it is certainly being absorbed, and after some molecular excitation (that obviously I don’t understand) it is reflected back as longer wavelengths in the visible part of the spectrum.
It seems pretty well accepted that birds see well in the UV part of the spectrum and quite possibly better in that range than in the visible (to us) part of the spectrum:
https://www.nwf.org/Magazines/National- ... ird-Vision
So although I don’t know much about this subject at all, I like the idea that we might be witnessing a solution to the difficult problem of what to do with UV. UV light can apparently be very harmful to insects, and as a result many have evolved a high degree of UV reflectance. Although this potentially keeps them safe from the harmful effects of UV, it enables predators with good UV vision to home in on them very effectively. By absorbing it in order to then reflect back light outside of the UV range perhaps we are seeing a solution where the harmful effects of absorbing too much UV are avoided, while simultaneously remaining hidden from predators by reducing their UV reflectance.
If that is what’s going on then it’s very neat and makes the UV induced fluorescence we see at night make a lot of sense. In daylight these fluorescing organisms will be melting into their surroundings by reducing the UV signal that would give them away to avian predators.
Unfortunately, however, I can’t really find this spelt out in the literature, which makes me think I must have got something upside down or backwards. The only study I can find that focuses directly on this is one by Czarnecki et al. (2022), and although this produced some interesting results with painted dummy caterpillars, it’s hard to say how well the study mimicked natural conditions. So as usual I’m left trying to make things work without understanding the subject and of course that’s a dangerous game. There seems to be lots of question marks in this area though, so I wouldn’t be too surprised if the conclusion is correct even if all my working out is completely wrong.
Just one final point relating to the literature. The suggestion very often is that the UV induced fluorescence is acting as a warning signal. Just one example from Sourakov (2019):
“Finally, in Asbolis capucinus (Hesperiidae) (Fig. 1D), which is, to my knowledge, not known to be chemically defended, the fluorescence is localized to the last abdominal segment. While in the first three species the fluorescence seems to enhance the aposematic pattern, in A. capucinus the caterpillar inhabits a tube-shaped leaf shelter in a palm leaf, so that the last segment is the one that ‘plugs’ the entrance. In this case, the last segment may therefore have a possible warning pattern that is visible to predators, in the same way that shelter-feeding skipper caterpillars and their pupae frequently have eyespots and other warning patterns, on both ends of caterpillars (Janzen et al., 2010).”
Perhaps this is right, but if the caterpillar in question isn’t chemically defended then i’m not sure how effective this ‘warning’ would be? Surely once the deception is exposed a glowing bum plug (sorry) just becomes an invitation to dine (interesting sentence!). I guess if it’s mimicking a genuinely toxic species then it could work but I don’t think that’s what is being suggested.
It would make more sense to me that rather than acting as a warning the fluorescing final segment that remains on view is actually undergoing some kind of process along the lines that I’m suggesting, and the result is that it reduces its visibility to predators.
Anyway - however much I’ve misunderstood this subject, I’m much more comfortable travelling in this direction. Emperor pupae and hairstreak caterpillars are masters of disguise and the idea that they’ve taken this ability a step further in some way sits much better with me than any interpretation that requires an explanation of the opposite.