Caterpillars by UV

[Padfield]

White-letter hairstreak caterpillars are notoriously well camouflaged. Even when you've got your eye in it can be easy to miss them. As I mentioned on the April 2020 page, I've been experimenting with a UV flashlight and a couple of nights ago found a caterpillar quite easily, which I then photographed with ordinary flash. Tonight I took a photo without flash to show how the caterpillar looks under UV alone:



This is a useless way of getting decent photos of the cats - I totally failed to get any good flash piccies tonight, even though I got a good one a couple of nights ago. But it might be a good way of finding and counting them. I wonder if other caterpillars are easier to locate like this - purple emperor cats, for example ...

Does anyone have experience of this kind of thing? I am using UVA, which is said to be harmless, not UVB or UVC (I use a handheld UVC sanitiser around the house to protect my dad from any nasty viruses ... Very good for things like newspapers and letters, that you can't wash in soap and water!).

Guy

[Padfield]

Tonight I confirmed this is a very effective way of finding w-album caterpillars. In about five minutes, in total darkness, I found three 3rd instar white-letter hairstreak caterpillars on two different trees.

The first was some distance above my head. In daylight, as revealed in a flash photo, it would have looked something like this (though not from so close up, as I used zoom for this photo):



Under UV it looked like this (hand-held, over half a second exposure - hence the shake!):



Clearly visible on a cursory glance.

The other two cats found tonight:





It might be better in future to use the iPhone for illumination when taking the pictures instead of flash. Stupidly, I didn't think of that tonight. The problem with flash is it's too bright, so you have to be sufficiently close for the caterpillar not to be illuminated directly.

I expect the same method can be used to locate other hairstreak larvae. Brown hairstreak are particularly difficult to find by day.

Guy

[bugboy]

I think these images show how small birds find it so easy to find and pick of green caterpillars from green leaves, being able to see into the UV wavelengths, fascinating stuff Guy

[Padfield]

There might well be something in what you say, Buggy, though of course all the light seen reflected in these piccies is in the visual part of the spectrum. The increased reflectivity in higher frequency light might indeed mean they are more visible to creatures that can see beyond visible light than they are to us. So why, then, has evolution selected for this in the caterpillars? I guess one answer might be that higher reflectivity is the caterpillar's way of protecting itself from the UV (A and B) present in sunlight. I read that direct sunlight has about 30 times as much UVA as a typical UVA torch (intensity, so Watts per square metre). UVA is less harmful than UVB but can still cause some damage over time, so it makes sense to have some protection from both, but I would guess it's probably principally a defence against UVB. All this is speculation.

As a caution to anyone who tries to copy this experiment: your own exposure to UVA should be limited, even though the rays are regarded as essentially safe. At night, the pupils are wide open and they don't constrict in response to UV, so the retina risks getting a big dose without you noticing. Moth trappers often use goggles.

Guy

[MikeOxon]

An interesting paper on 'Fluorescence in Insects' can be downloaded from: http://www.victorialwelch.com/download/ ... t%20al.pdf

In summary, the paper shows that fluorescent molecules are frequently found in many arthropods, including within their internal organs. In many cases, the fluorescence is apparently non-functional but, in some butterflies, there is evidence of it being involved in sexual attraction. For example, Philips notes that Glaucopshyche columbia is a butterfly in which the females have a red-purple fluorescence but the males have none, whereas Pieris melete agaope is a species in which females have a “Rich, bright red-purple burgundy” fluorescence from parts of their wings, whilst the males remain “dull”. The paper also notes that "it is conceivable that fluorescent proteins may afford insects bearing them some protection from ultra violet light."

I once had fluorescence in a scorpion demonstrated to me by Darren Mann (Head of Zoology at the Oxford University Museum of Natural History), as shown in my photo below:

OUMNH Oxford - 7th May 2013
Lumix TZ25 - 1/4s @ f/3.3 ISO 1600

Rather him than me.
Mike

[Padfield]

Thanks for that link, Mike - very interesting. The world is strangely different when viewed under UV. I'm not sure if the w-album cats really fluoresce or if they just reflect certain wavelengths much more than the leaves do - but they are easily visible even when the torch is some distance away and I think there might be an element of fluorescence. I took this shot tonight (with the torch quite close):



And this one by iPhone light, on a different tree. I think it is laid up for its moult into 4th instar:



Guy

[David M]

It's amazing how you are aware of this, Guy. What led you to develop this approach?

[Matsukaze]

I have heard of UV lights being used to search for certain moth larvae by night - Limacodidae and certain hawk-moths, where the countershading stripes that break up the outline of the caterpillar get illuminated by the UV.

[Padfield]

It's amazing how you are aware of this, Guy. What led you to develop this approach?

I was thinking in this kind of direction a couple of years ago, frustrated by the way purple emperor cats just vanished off the face of the planet at pupation time. Taking my inspiration from forensic scientists in crime dramas, I wondered if they or their silk might be more visible under different wavelenghts of light at night. But I never got round to doing anything about it until I was ordering a UVC sanitiser wand on Amazon to help me keep my father virus-free. For the next few weeks, Amazon kept offering me anything with UV in it and I saw these UVA penlamps (metal, not plastic) at very reasonable prices. So I got one!

I have heard of UV lights being used to search for certain moth larvae by night - Limacodidae and certain hawk-moths, where the countershading stripes that break up the outline of the caterpillar get illuminated by the UV.

I found references to this on the web too, Matsukaze, during my 'shall I, shan't I?' period of compulsory dithering before making the purchase I was always going to make. I couldn't find out exactly which caterpillars responded but went for it anyway - and no regrets.

Guy