Helophilus pendulus hoverflies

I am an amateur naturalist trying to learn something about everything living in my garden.

Photo 1, taken back in September, shows two mating hoverflies resting on a leaf of my garden Buddleja bush. The characteristic brown stripes on the thorax quickly let me identify the species as Helophilus pendulus in my copy Hoverflies (Francis Gilbert, Richmond Publishing), an excellent monograph I've talked about before.

I assume the pair in photo 1 is male and female. Apparently it's normally possible to tell the two sexes apart in hoverflies by the eyes: The eyes of males touch at the top of the head, whereas those of females don't. Unfortunately this isn't true of Helophilus species. Why, generally, hoverfly sexes should differ in this regard, and specifically why they don't in Helophilus species, I can't imagine. Can anyone comment?

As I've mentioned previously, it's possible to distinguish hoverflies ('Syrphinae') from other flies by the veinature of the wings. In photo 2 I've zoomed in and enhanced the shot with my camera's software. Hoverfly wings have a 'false vein' (running approximately vertical in photo 2) and a section of vein at the edge of the wing that other flies lack.

In The Encyclopedia of Land Invertebrate Behaviour, the authors, R&K Preston-Mafham, describe male H. pendulus hoverflies as searching near flowers for females but often pouncing on other species of fly by mistake.

My copy of The Colour Guide to Hoverfly Larvae (G. Rotheray, Dipterists Digest 9) explains that the larvae of H. Pendulus thrive in farmyard drains and wet manure. Nice! My web searches also turned up a paper by one E. Stanley in the Veterinary Record 1845, 1(4) that describes finding an H. Pendulus maggot infesting the spinal marrow of a horse. By appearance they are a dark brown maggot with a tail as long as the body that acts as a breathing tube.

An obvious feature of hoverflies is the resemblance of many species to wasps and bees. Indeed, as explained in the extensive and very readable paper The Evolution of Imperfect Mimicry in Hoverflies by Francis Gilbert, at least a quarter of European hoverflies are mimics. Clearly, the mimicry is simply a matter of hoverflies trying to fool birds into thinking they are a wasp that will sting them....right? Well no actually, or at least it can be said the situation is far more subtle. As the paper above highlights the subject of animal mimicry is a complex one, much studied by biologists, and a topic where numerous questions and controversies persist.

Firstly, one should not assume that the 'warning colours' of hoverflies are, in all cases, a response to the threat of bird predation. There are plenty of other animals such as dragon flies, wasps and spiders will also eat hoverflies. Spiders have been shown to have an ability to recognise the threat posed by wasps in their web, and to treat them with greater caution than they do with other insects. At the same time, some experienced birds such as Flycatchers have been shown to be rather skilled at 'seeing through' the disguise of hoverflies - readily distinguishing them from wasps.

Next there is the issue of whether what we, with our human eyes, see as a resemblance to a wasp, is the same as what a predator perceives with eyes that might have very different characteristics to our own (different colour sensitivity etc.) Experiments studying the willingness of pigeons to peck at images of various hoverflies and wasps have shown that pigeons do broadly rank resemblances of hoverflies to wasps in the same way that we do. But there are exceptions with pigeons regarding Syrphus ribesii as the most wasp-like hoverfly of all, a view not shared by humans.

Next, there is the question of whether it is a bee's or wasp's sting that a predator is avoiding and that a hoverfly is "pretending" to posses. In fact, it seems that birds hardly ever suffer wasp stings and actually it is the unpleasant taste of a wasp's internal venom sack that some birds avoid. With Bumblebees (which some hoverflies mimic) the situation is more subtle still. There is some evidence that the sheer effort (in terms of wasted time and energy) involved in removing all the various hairs and largely inedible, chitinous, 'body armour' of a bumblebee is enough to put off some birds. Contrary to proclaiming its sting, the colouration of bumblebees (and their hoverfly mimics) may be a way of saying "I'm not worth the time and calories you'll get from eating me!".

This brings us to the difference between so-called Batesian mimicry and Mullerian mimicry. Batesian mimincry is the type of mimicry most of us imagine at first, whereby a harmless insect species evolves to copy the warning colours of a harmful one. By contrast, Mullerian mimicry involves two or more species that each have defences of their own, but nevertheless carry a common warning colour or form. Lots of bees and wasps all have yellow stripes for example. One can understand how this might arise: Suppose a predator has a bad experience with a harmful species. If that predator comes to associate the bad experience with a warning marking for that one species (yellow stripes say), then there's clearly potential benefit in other poisonous species adopting similar markings. By adopting similar markings however, one wasp species is not really 'copying' another, rather, both species are gaining benefit by evolving in parallel to advertise their venomous natures through similar body markings (multiple species settling on a 'common format' for advertising their individual danger signals if you will) . Now, in saying that hoverflies 'copy' wasps we are making the tacit assumption that hoverflies are harmless Batesian mimics. This is probably mostly the case. But it need not be universal. For example, it has been suggested that some species of hoverfly concentrate unpleasant tasting chemicals in their bodies through eating aphids that have been feeding on noxious plants. Such hoverflies would have their own defences (their noxious taste) and any evolved resemblance to say wasps, might then be an example of Mullerian mimicry.

The list of fascinating questions surrounding mimicry goes on. There is the question of the drawbacks of mimicry. Naively, it might seem there could little detriment for a species in 'wearing the clothes' of another. But suppose the venomous species being copied becomes rare. If a non-venomous hoverfly were to become more common than a venomous bee (say) it might be copying, what then? Predators would rarely (or even never) meet with the bad experience of finding that the prey in their mouth was the truly venomous one. After a time predators might simply stop associating the mimic's bright colours with danger. Indeed, the bright colours of the hoverfly mimic might become a positive liability, gaudily advertising the presence of a tasty snack! In summary, by choosing to copy the colours of another species, a mimic pays the price of shackling its population size that of the target insect being copied. Exactly the nature of the constraint (i.e. the mathematical relationship between the sustainable population of the mimic vs. the copied species) is a rich topic in its own right.

I could go on still further into a discussion of polymorphism in mimics, whereby a single species of say, hoverfly, comes in a number of different (colour) forms called morphs, and the benefits this confers. I've written enough for now however and so will leave this topic for another day, or, if you cannot wait for then, refer you to the paper above by Gilbert.