I had the pleasure of coming across two large fly species in Spain this summer, both of which are thought to mimic the European hornet (Vespa crabro). They are both large and somewhat hornet-like in appearance and behaviour. In fact, in both cases, I initially thought they were hornets. Looks like a hornet, sounds like a hornet, but is in fact a fly!
The first one is called the hornet robberfly, Asilus crabroniformis (Asilidae)(above). A great big, hairy brute of a fly which probably feeds on anything it can get its tarsi on: beetles, grasshoppers, other flies, and so on. It appears to be quite common in Galicia in northern Spain, where these photographs were taken, but has sadly been declining in numbers in the UK (Holloway et al., 2003). It does not look much like a hornet to me (more on this later!), but with its yellow abdomen and feisty manner, it probably does fool quite a lot of would-be predators (into thinking it has a sting!). Before we go any further, let’s have a look at a proper hornet, the so-called model for these mimics (below).
Looking at the hornet robberfly again, from above (below), it is clear that – at least our human eyes – the two species are rather different. This is what biologists call ‘imperfect mimicry’. The appearance of the mimic is far from perfect. But is it good enough?
One might think that evolution would gradually craft perfect mimics? If the most accurate copies of the model survived better than the others and had the highest fitness in the population, natural selection should gradually hone perfection, in terms of looking more and more like the model (i.e. the hornet). The fact that this is patently not the case, suggests that something else is at play. One theory is that there is another evolutionary force, an opposing selection pressure, which is driving the resemblance in another direction. Perhaps a need to be dark (and hairy?) to absorb heat and keep warm for instance. Whatever it is, and we cannot be sure in most cases, the final product – the living organism – is a compromise. A balancing act of opposing selective forces; in other words, a trade-off between mimicry and some other desirable property, like thermoregulation perhaps (Taylor et al., 2016).
The other hornet-like fly which caught my attention was the so-called Yellow jacket hoverfly, Milesia crabroniformisis (Syrphidae)(above). It is quite common in southern Europe – there were lots of them about in Spain, but is not found as far north as the UK. Well not yet anyway! I think this one looks much more like a hornet, and it certainly does a good job of sounding like one. It really is quite large (over an inch in old money!) and it flies about looking very confident that it will not be attacked! I managed to grab a shot of one in flight (below). Look at those huge back legs!
We must remember that it is not us human that these flies are trying to fool. It is possible predators like birds, who will have learnt to avoid hornets on account of their nasty stings. Perhaps they do so instinctively? Anyway, an interesting study was carried out to see whether pigeons – a stand-in predator! – were fooled by hoverflies that look like wasps (wasp mimics). Well it turned out that pigeons did rank mimics the according to their general similarity to a wasp model (clever birds!), in an order which was broadly similar to our own ‘intuitive rankings’ (Dittrigh et al., 1993). But the pigeons do not appear to take any chances with these wasp-look flies, and still rank rather poor mimics as being wasp-like. In other words, predators just seem to avoid anything that looks vaguely like a wasp (or hornet in this case); perhaps it’s not worth taking a chance – of being stung. Better to move on and find something which is definitely edible and not questionably dangerous.
So imperfect mimicry appears to work, as long as there are plenty of models (the hornets) about, to educate the predators about what a dangerous insect looks like. The problem is, there is another kid on the block: the Asian hornet! This Asian species of hornet has invaded Europe and is very common in Galicia, in NW Spain. Indeed, although I saw many Asian hornets this summer in Galicia, I did not come across any native European hornets. Unlike, back in the UK, where there were very large numbers of European hornets about in August and September, 2018. We are in the UK, hopefully, still free from the invasive Asian hornet, Vespa velutina.
So how will the native mimics, like these flies, cope with the fact that there are now two potential models? The Asian and European hornets; and they are rather different in appearance. If the Asian hornet (below) becomes so abundant that it starts to displace the native hornet – I rather fear that it already may have in some regions – then the mimics will be under pressure to start looking more like the invasive hornet! How long would such a process take? Or at the end of the day, are the two hornets (European and Asian) sufficiently similar for imperfect mimics like this? I think they probably are. Thank goodness for imperfection! And perhaps that is one reason that Nature does not produce perfect copies? They soon go out of date if things change!
Boppré, M., Vane‐Wright, R. I., & Wickler, W. (2017). A hypothesis to explain accuracy of wasp resemblances. Ecology and evolution, 7(1), 73-81.
Dittrigh, W., Gilbert, F., Green, P., McGregor, P., & Grewcock, D. (1993). Imperfect mimicry: a pigeon’s perspective. Proc. R. Soc. Lond. B, 251(1332), 195-200.
Holloway, G. J., Dickson, J. D., Harris, P. W., & Smith, J. (2003). Dynamics and foraging behaviour of adult hornet robberflies, Asilus crabroniformis: implications for conservation management. Journal of Insect Conservation, 7(3), 127-135.
Taylor, C. H., Reader, T., & Gilbert, F. (2016). Hoverflies are imperfect mimics of wasp colouration. Evolutionary ecology, 30(3), 567-581.