Sunday, April 28, 2013

Be like a bee

I have written in the past about ant-mimicry (Why look like an ant?), and last week I was reminded how good their relatives, the bees and wasps, are as models for Batesian mimicry. It was just by chance that I noticed this yellow spot on a rock beside a stream running through the forest. 
It was evident that this was a moth, a hawk-moth for sure, confirmed by looking at the hooked tip to the antennae. The remarkable similarity in pattern and size to carpenter bees of the genus Xylocopa is readily evident. As kids we used to play a trick with Xylocopa males that would perch atop wooden posts. Toss a pebble in the direction of the male and it would set off and pursue the pebble. Handling a dead carpenter bee was a painful lesson on how spiny and hard they are. Females can sting but they rarely need to use it. There is probably no predator of adult Xylocopa bees. Little wonder then that several species in the day-flying hawk-moth genus Sataspes have been selected to appear like carpenter bees (other day-fliers like Cephonodes look like clear-winged bees). The genus Sataspes does not seem to be well known in southern India and this observation may be among the few reports from southern India.

Species in the genus appear to show considerable variation within populations in the distribution of blue and yellow on their body and wings. Several of these variants have been described in the past as species and the exact determination of species may be reliably established only by dissecting out their genitalia (which show species-specific lock-and-key patterns). In 1900, a year before he died of malaria, Lionel de Niceville described a species from Burma:
S. tagalica (=S. hauxwelli) and model
"I have named this handsome moth (which is unique) after Mr. T. A. Hauxwell, Deputy Conservator of Forests, who is an enthusiastic collector of birds and Lepidoptera. It is a beautiful mimic of the very common large blue carpenter bee, Xylocopa auripennis, Lepeletier, a male of which I have figured-for the first time-for comparison ..." JBNHS 13:173
Some people find the evolution of mimetic forms too implausible to occur by random processes and those that question evolution pose the question as to how the "exact" same pattern could arise. Interestingly enough there is considerable variation within the mimics and it is actually not an exact imitation. Here are a couple of Xylocopas (unidentified) from southern India just for a sampling.

A Xylocopa from the W. Ghats
A Xylocopa from Bangalore

Looking at the variations in Sataspes it is clear that a series of characters are being mixed in various forms and species.Yellow bands, thorax, tufts, bluish wing shine and so on.
Sataspes xylocoparis described by A G Butler in 1875
Writing about Xylocopa aureipennis in 1922, Cedric Dover notes incredulously:
In addition to the localities noticed by Bingham, the Indian Museum also possesses specimens from the Parjiling District, the Naga Hills and Sibsagar in Assam, South India and Nepal. The species is supposed to be mimicked by a Sphingid moth (Sataspes hauxwelli), which according to De Niceville (Journ. Bomb. Nat. Hist. Soc. XIII, p. 174) was "a beautiful mimic of the very common large blue carpenter-bee Xylocopa auripennis, Lepeletier." The wings of the moth are a deep indigo-blue with bronze markings, which scarcely resembles the wings of the bee, and in the cabinet the whole insect seems entirely different.
De Niceville does not say that the bee and the moth were taken together, and in the absence of definite field-observations, the moth has little claims to being a mimic of the Xylocopa.
My own impression was that the moth above was the right size, shape and colour to give the impression of a carpenter bee.

Although carpenter bees are free from predators, they are not free from parasites and their immature stages probably have predators. A peculiar feature of carpenter bee females is a cavity on the first abdominal segment, known as the acarinarium, which is home to mites of the genus Dinogamassus. Nobody quite knows what the role of these symbiotic(?) mites are. Dr B. Mallik, an acarologist (and one of my teachers) at the entomology department in the University of Agricultural Sciences (Bangalore) showed me some of these mites under a microscope a few years ago pointing out that they stack up in a toroidal pattern.
The fact that so few records of these amazing day-flying moths exist is either due to their rarity or the fact that we have so few people keeping an eye on life around them.

References
Postscript
1-May-2013: Ian J. Kitching of the Natural History Museum notes:
... all the evidence now points towards it being the first representative of infernalis from SW India. All other specimens I have seen from the Western Ghats are the relatively non-metallic forms as shown on SEP. None have the strong blue sheen on the wings of your specimen. ..., the moth does look a bit more like S. tagalica but this usually has a greenish metallic sheen and the females (which your moth is) never have a yellow thorax, only the males. That said, there is a definitive feature that could settle the matter BUT it is on the underside of the abdomen. This is a long shot but do you have a photo in which the colour of the underside of the terminal segments of the abdomen is visible, even slightly. These are yellow in infernalis/xylocoparis and black in tagalica.

All that said, no Sataspes in our collection of any species has the subdorsal rows of yellow spots; they either have more extensive yellow bands (see xylocoparis on SEP) or broad central patches (see tagalica on SEP). So I am minded to think that this may be a new species, or at least a distinctive subspecies of infernalis.

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