I was very pleased to come across this butterfly, the common birdwing, recently. Although it is fairly common, I had not had a chance to try and photograph it before. There are at least 25 subspecies of Troides helena, but because of the location, in northern Thailand, this is almost certainly, Troides helena cerberus (C. & R. Felder, 1865), a protected species. Although I did not succeed in getting quite as good photographs as I hoped (always the case!), the wonderful black and yellow colours got me thinking, and researching, this striking butterfly.
The golden birdwing, Troides aeacus, is another, very similar looking species, also found in Thailand, but a different subspecies from that shown below.The Borneo birdwing (Troides andromache) is also very similar in appearance. Most, but not all, of these Troides species (birdwing butterflies) have prominent yellow marking on both dorsal (upper) and ventral (under) sides of their hindwings; particularly so in the males, where the yellow patches are much larger, and I think, brighter, than those on the females. The male Troides spp. therefore, display a highly contrasting colour scheme: bright yellow iridescent (and fluorescent) hindwings together with ultra-black forewings. These bright, sexually dimorphic colours are also, probably important for mate recognition and sexual signalling (as why else would they vary between the sexes?). N.B. pictures of 21 different Troides spp. can be viewed on the website: Papilionidae of the World: a pictorial summary. There are also very nice photographs of both male and female Troides helena cerberus on an excellent blog site called: Butterflies of Vietnam.
These butterflies are poisonous, or at the very least, highly distasteful. The larvae feed on several species of Aristolochia; climbing vines containing highly toxic aristolochic acids, which can cause cancers and tumors! So these butterflies accumulate these toxins during their larval feeding stages, and retain the aristolochic acids in their body tissues, as adults. Their colour pattern is therefore a warning, an aposematic signal which screams out to any creature thinking of eating them: “Watch out, I taste vile! Try to eat me and you will be sorry!” In the same way that black and yellow wasps (yellow jackets) are using stripy black and yellow colours to say: “Watch out, I sting!”
In males and females of those species, such as Troides magellanus, which have been studied and investigated, the yellow colour of the hindwings has been shown to be produced by the absorption and scattering of incident light by a special pigment, called papiliochrome. A novel molecule which was only discovered in the 1980’s (Umebachi, 1985). This species, T. magellanus, also has an unusual, but not quite unique property, of producing a so-called ‘back-scattered’ flash of white light, when the plane of the butterfly wing is close to the observation axis. This so-called ‘near grazing’ incidence iridescence is visible only when its hindwings are both illuminated and viewed at a very oblique, i.e. near-grazing incidence (or close to the horizontal. see below). The white iridescence is only just beginning to appear in this photograph (below) from Wikipedia, but is described in papers by Lawrence et al. (2002), Vigneron et al. (2008), Van Hooijdonk et al. (2012); for anyone who might want to investigate it further.I assume that this property is unique in Troides species? But I am not certain. Dechkrong et al. (2011) studied the yellow scales in the hindwings of the golden birdwing ( T. aeacus, see above) and found that they did not produce any back-scattering diffraction. The reason why they do not produce the iridescent sheen, was due to the particular arrangements of microribs on the wing scales. When these microribs are slanting, they produce an iridescent effect, but when they are perpendicular to the scale plane, no effect occurs. So the evolution of this effect was down to a particular microscopic pattern on the cuticle on the surface of the wing scales. Perhaps it first appeared by chance, as an aberration, but was found to be a useful feature, soon put to work by the butterflies in communicating with each other! Their own secret signal!
I could see no sign of any white iridescence coming off the hindwings of the common birdwing, when near the horizontal, and presumably it does not exist, but then again, this phenomenon only appears under certain lighting conditions. All very mysterious really; why only one species in a group of very similar looking butterflies may have evolved this form of iridescent flashing, which presumably it uses to communicate, perhaps during courtship?
To be effective, aposematic colouration needs to be highly conspicuous, or highly apparent, to a would be predator. The colourful pattern also needs to stand out against the background, i.e. the plants and flowers on which the butterfly may be feeding. Patterns – in this case the characteristic yellow and black network on the dorsal hindwing surfaces (see below) – are thought to be more effective, in terms of predators learning to avoid the organism, than simply solid bright colours alone (Bowdish and Bultman, 1993). The fact that it is such a striking pattern, especially when viewed from behind the butterfly (it almost looks like a face! See below), seems to be saying, take note, remember me! I.e. enhancing the aposematic effect.
The flight of Troides species is, I think, fairly unique and and characteristic. The forewings are flapped rapidly to provide propulsion, whilst the hindwings are kept remarkably still, almost as though they are a sign that the butterfly wants everyone to see! The following shot shows the rapidly flapping forewings almost invisible, with the hindwings remaining virtually motionless; creating the same aposematic pattern shown above, and surely characteristic of these butterflies?
The black portions of the wings have been described as ‘natural solar collectors’. In other words, the black wing scales have microscopic (nano-scale) anti-reflection nano-structures which are highly effective at capturing, i.e. absorbing, the light (Zhao et al., 2011). They may also be useful for emitting infra-red radiation and maintaining an optimal body temperature (Herman et al., 2011). The pigment, melanin is also thought to be present in the black portions of the wing.
This is a very active and ‘flighty’ butterfly and although I was using a shutter speed of 1/2000th of a second, this was not quite fast enough in many cases. The butterfly was photographed on 28 Oct 2019, in the gardens of Malee’s Nature Lovers Bungalows, Chiang Dao, north of Chiang Mai, Thailand.