New light photo-emitting biological things...

New light photo-emitting biological things...
Cocoon Fluorescence

By Dominique Adès

If we were looking at our dear Actias reared cocoons under different light, what would we see????

In publications, descriptions of cocoons very often come after the breeding of caterpillars and butterflies (1). Such cocoons can present very different characteristics according to species on the one hand, and on the food-plant and the conditions of spinning (hypogaeous = "remaining under ground" or epigaeous = "close to the ground" on the other hand. With Actias species, the spinning is epigaeous, either on the ground or in plants, into soft or hard leaves etc...).

So, it is possible to point out in our breeding a very particular characteristic of the cocoons of most species of Actias: their fluorescence upon ultraviolet light irradiation.

Fluorescence is a luminescence [glow] that is mostly found as an optical phenomenon in cold bodies, in which the molecular absorption of a photon triggers the emission of another photon with a longer wavelength. The energy difference between the absorbed and emitted photons ends up as molecular vibrations or heat. Usually the absorbed photon is in the ultraviolet range, and the emitted light is in the visible range. Thus we can see it. Yes, many Actias cocoons give off a visible glow during exposure to ultraviolet light.

This fluorescence (cocoons that glow), well known on lepidopterist sciences for only some selected races of the mulberry silkworm Bombyx mori (2) and some mites, is a very usual property within the genus Actias. To my knowledge, it is unknown in our case (as well as in other genera) because of the difficulty of observation due to the fugacity in a wet atmosphere. (fugacity = a short lived phenomenon = transient = fleeting = ephemeral). Fluorescence is a very short lived emission that disappears almost immediatly (within 6 micro-seconds) after irradiation.

This led to the interest here of the rearing or spinning in low hygrometry (humidity).

The fluorescence of cocoons finds its source among molecules constituting the silk gum (sericin), the gelatinous liquid (a gum-like one) secreted by the caterpillar. This liquid impregnates (ratio of approximately 25 %) the space between silk threads thus giving a certain cohesion to the cocoon (and raw silk, of course).

This sericin has a rather high molecular weight (meaning a rather big one: MW around 10.000 to 300.000). The highly hydrophilic glycoprotein contains 46% amino-acids of which three are basic fluorescent molecules (Tryptophan, tyrosin and phenylalanin) but of very short wavelength fluorescent emission (<350nm). So, their spatial association (with other amino-acids) / chemical modification may give rise to higher wavelength emissions upon higher wavelength photo-excitations just as in the Green Fluorescent Protein found in the well known jellyfish Aequoria victoria (4).

But what is the colour of these Actias cocoons, most of the time, when they are freshly woven in low hygrometry? For sure you will be amazed: they often are silver white (Graellsia, dubernardi, felicis, luna, gnoma etc. and their hybrids), sometimes yellow (sinensis), sometimes but more rarely (with respect to species) directly dark brown with the colour acquired within the time of silk extrusion (in that case, the fluorescence won’t appear!).

Graellsia isabelae galliaegloria cocoon upon natural light (left)
Graellsia isabelae galliaegloria cocoon during irradiation (right)

Actias dubernardi cocoon upon natural light (left)
Actias dubernardi cocoon during irradiation (right)

When these clear cocoons are observed during irradiation with ultraviolet rays, you will be well surprised to see them behaving as good light photo-emitters: yellow-green particularly bright for sinensis, blue or white bluish for Graellsia, dubernardi and felicis, blue-green for luna, gnoma, and intermediate for their hybrids.

Actias sinensis cocoon upon natural light (left)
Actias sinensis cocoon during irradiation (right)

Actias luna cocoon upon natural light (left)
Actias luna cocoon during irradiation (right)

If these clear cocoons stay in a wet (more natural) atmosphere, their gum fluorescent components oxidise in one to no more than few hours, and they generally develop brown colours and produce almost no fluorescence after completion of the chemical oxidation processes.

Actias dubernardi cocoon (left); Actias gnoma cocoon (right)

In the above image of the Actias dubernardi and Actias gnoma cocoons, the small dark cutaway patch shows how the silk darkens after exposure to high hunmidity. Not all fluorescent molecules/associations have the same reactivity towards oxygen. The small samples were oxidized within 20 hours in a wet atmosphere (36h in the Graellsia case) and no longer presented a fluorescence able to be detected by the unaided eye.

Whether the proportion of the fluorescent molecules in the gum, yielding the fluorescent colour, and/or the immediate or delayed impregnation of the silk with dark compounds, masking/eliminating the fluorescing completely, is sexual dependent, seems probable. Such is the case with a few Bombyx mori selected races (2) and has to be verified with felicis, Graellsia and dubernardi). I will try to examine this next.

Actias felicis cocoons, +/- oxidised upon natural light

In the image above, the darkest Actias felicis cocoons correspond to males and were immediately impregnated by dark gum. They have never been fluorescent.

It’s a fact that in the rearing of Actias felicis, the silk threads of the first cocoons woven (probably males) were immediately dark brown impregnated.

Moreover this fluorescence is also the privilege of most of the caterpillars of these Actias.

Actias sinensis and Actias felicis larvae, irradiated

All pictures presented here are resulting from a 404 nanometers (nm) wavelength irradiation (“Xenopus electronix” Cree diode light torch; FWHM 19 nm) and were taken through a pass-band “420-3000 nm” filter to overcome important reflected UV artefacts arising from numerical photographic captor sensitivity as well as from blue residual LED light pollution.

Actias sinensis x Actias dubernardi larvae, irradiated, no filter

Saturation was then computer corrected with respect to real visual results. Irradiation at lower wavelengths would give rather different results both in colour and intensity.

All detailed results (other excitation wavelengths, spectra and discussions) have been accepted to be published in « Lambillionea » Belgian revue (5).

* The phenomenon of obtaining white cocoons has sometimes already been observed and is reported here on Bill Oehlke's site by Scott Smith with another genus in another tribe: Hyalophora euryalus (3). It is also easily observed with other species in Attacini tribe such as S. Ricini and its interspecific crossings (1).

** Note that the ultimate dark colour may be the result of two distinct processes: immediate or delayed impregnation of silk with a dark brown gum (which stops any possibility of emission of light either by absorption of exciting ray or by re-absorption of emitted light) and wet oxidation process of a clear gum impregnated silk which destroy fluorescent molecules at various speeds.

*** Please note also that the fugacity of these light emissions in wet conditions do limit drastically easy wild species capture and its possible environmental perturbations (by endangering species), and that’s the reason why this phenomenon can be presented here! For scorpions of which all species do permanently fluoresce between molts (maximum excitation curve at 392 nm for always high intensity blue-green emissions), the problem of excessive hunting is in fact much more real!

**** Actias hybrids origins are here either natural (gnoma x luna) or manual (sinensis x dubernardi) mid-2006 author’s pairing of respective parents.

***** On oxidation, cocoons of Actias sinensis are changing from gold-yellow to off white/pale beige as they become non-fluorescent. None of the numerous oxidised products of the gum get dark brown in this case. In fact, brown colour isn’t a specificity of oxidised fluorescent molecules but just a common overall result of oxidation on some unsaturated macromolecules/parts of them within sericin! Brown colours may also be partially removed by excess of water (rain in nature) especially when they derived from sufficiently small (so easily soluble!) oxidised molecules (some Actias luna cases for example) ...

Bibliography :

1- D. ADES, R. PLONTKE, R. VUATTOUX, this site, Hybrid Index / Saturniidae hybrids, 2006.
2- LI MUWANG, YAO QIN, HOU CHENG XIANG, LIN CHANG QI, CHEN KEPING, Sericologia ISSN 0250-3980 CODEN SERIDY, 2001, Vol 41, n° 4, pp 527-542 (18 ref.). Ed. Commission séricicole internationale, La Mulatière, France (1979).
3- SCOTT SMITH, this site, Members’articles, 2001.
4- http://userpages.umbc.edu/~jili/ench772/intro.html
5- D. ADES, Lambillionea, CVII, 1, mars 2007 (accepted).

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