Some years ago, Ole Heie published a paper discussing what he called aphid mysteries not yet solved (Heie, 2009). These included such gems as the shark’s fin on the giant willow aphid and why are aphids so fussy about their host plants? I could add a few of my own; of the three very common aphids that feed on sycamore, one, Periphyllus testudinaceus, is commonly attended by ants another P. acericola, is sometimes attended by ants and the third, Drepanosiphum platanoidis is attacked by ants. The question that one would ask is why this gradation when all three aphids live on the same tree and all produce lots of honeydew, the ant’s reward. People have pointed out that those ants that are obligately ant-attended have evolved a specific structure, the trophiobiotic organ (Heie, 1980) and that the siphunculi in non-ant attended aphids are longer than those of ant-attended species, which presumably enhances their defensive function (Way, 1963). These observations do not, however, answer the question as to why the association or lack of association arose, they just allow us to speculate about how the association has shaped the aphid, in essence an example of a circular argument. Although I have raised the question, and you might respond and say that the question has been asked by voicing it, in my opinion, the question remains unasked (untested) and the mystery unsolved.
There are plenty more aphid examples I could throw into the mix, but given the time of year when I started to write this, I thought I’d do a more topical unsolved mystery. Why are St Mark’s flies (Bibio marci) so easy to catch? A simplistic answer is they are so easy to catch because they fly very slowly and appear to make no effort to avoid being caught. I can literally grab them out of the air.
One that I caught earlier -)
The real question is how come they are so easy to catch? Why, unlike the ubiquitous housefly, Musca domestica, which is nigh on impossible to sneak up on, (well by me at any rate, but perhaps you are better at it than me), has evolution produced such a dozy animal?
Dipteran phylogeny (after Yeates et al., 2007).
Bibionids appeared earlier in the evolution of flies than the Muscids and if you look at the phylogeny above you can see that as new Orders of flies arose, they moved from being long-legged, relatively clumsy fliers, such as the crane flies and Bibionids to the much more agile species we see in the Empids (dagger flies), Asilids (robber flies) and blue bottles and house flies. Bibio marci is found across most of Europe while the housefly is found everywhere that humans are to be found, a truly global beast. Based on distribution alone we could argue that M. domestica is the more successful of the two.
Their structure apart, are there any differences in their life history traits that might explain why natural selection has shaped adult Bibionids into being such easily caught organisms when compared with houseflies? Given the big differences in their flight agility we might expect their respective predators to be markedly different. The eyes of a typical housefly have about 3400 ommatida (Sukontason et al., 2008) and process visual information around seven times more quickly than humans, enabling them to identify, and easily avoid attempts to catch or swat them, since they effectively see the human’s movements in slow motion. The eyes of bibionids on the other hand are divided into two halves, with one half pointing upwards, the other downwards. The upward pointing half is used to locate mates while the downward pointing half is used for positioning (Zeil, 1983), so they are good at hovering (Ennos,1989), but not very agile in comparison with other flies, including, in my experience anyway, crane flies, which although looking clumsy are surpassingly good at not being caught*. Despite these differences in flight ability, their predators are not very different. Adult houseflies have many predators, including birds, reptiles, amphibians, various insects, and spiders which is pretty similar to St Mark’s flies, the adults of which form a substantial proportion of the diet of birds, such as starlings and chaffinches and are eaten by spiders and attacked by Empids (dagger flies) (D’Arcy-Burt & Blackshaw, 1991).
What about their diets then? You might not realise it but we can describe adult houseflies as being mainly carnivorous; their primary food is animal matter, carrion, and faeces, but they also consume milk, sugary substances, and rotting fruit and vegetables. Although they don’t have jaws per se, they deal with solid foods by liquefying them with saliva before sucking it up. Given their food preferences they are great at moving bacteria around the environment, hence their bad reputation as public health pests. Adult bibionids on the other hand are nectar feeders (Lewis & Smith, 1969; Smith & Lewis, 1972), so can be classified as beneficals due to their pollinating ability (Lewis & Smith 1969). As larvae, B. marci feed on leaf litter, both coniferous (von Schremer, 1958) and deciduous (Pobozsny, 1982), so again have a very important role in humification and soil formation (Pobozsny, 1982). House fly larvae feed primarily on muck, dead and decaying material, animal faeces, pig manure being a particular favourite (Larrain & Salas, 2008; Pastor et al., 2011), which, like B. marci, makes them important components of the ecosystem. We are, however, concerned with the adults and their exposure to predators, and looking at their respective life styles it seems odd that B. marci is such a lethargic flyer as it would seem to be just as, or even more so, exposed to predators as the house fly.
That leaves us with the life cycle. Is there something about B. marci’s life history traits that enables it shrug off the possibility of predation? The adult has a short life cycle, one week, there is only one generation a year and the typical female lays 3330 eggs (Skartveit, 2002), so pretty prolific. The house fly has a longer adult life, and at 25oC lays just over 700 eggs (Fletcher et al, 1990), so although fecund, nowhere near as productive as B. marci. They do however, whip through the generations, in temperate regions of the world getting through 10-12 generations in a year, so their multiplication rate is massive compared with that of our bumbling bibionid.
Given all the evidence, I would have thought that B. marci would benefit greatly by being a faster flyer and less conspicuous and/or unpalatable. It is none of these things. It might be spatially aware, but its predator avoidance mechanisms seem to leave a lot to be desired and birds love to eat it. That said, it has been remarkably successful and was, in the past, regarded as an agricultural pest (Morris, 1921). There does, however, seem to be growing evidence, that B. marci is not as numerous as it once was, (Grabener et al., 2020), so given the close association that the house fly has with humans and the current direction of global heating, I would bet that the former will, over the next few years decline in numbers and the latter become an even bigger pest. Sadly, this seems to be the direction we are heading with regard to insect numbers, those we love are threatened, those we hate are doing well ☹
D’Arcy-Burt, S. & Blackshaw, R.P. (1991) Bibionids (Diptera: Bibionidae) in agricultural land: a review of damage, benefits, natural enemies and control. Annals of Applied Biology, 118, 695-708.
Ennos, A.R. (1989) The kinematics and aerodynamics of the free flight of some Diptera. Journal of Experimental Biology,142, 49-85.
Fletcher, M.G., Axtell, R.C., & Stinner, R.E. (1990) Longevity and fecundity of Musca domestica (Diptera: Muscidae) as a function of temperature. Journal of Medical Entomology, 27, 922–926.
Grabener, S., Oldeland, J., Shortall, C.R. & Harrington, R. (2020) Changes in phenology and abundance of suction-trapped Diptera from a farmland site in the UK over four decades. Ecological Entomology, 45, 1215-1219.
Healy K, McNally L, Ruxton GD, Cooper N, Jackson AL (2013). Metabolic rate and body size are linked with perception of temporal information. Animal Behaviour. 86, 685–696.
Heie, O. (1980) The Aphdioidea (Hemiptera) of Fennoscandia and Denmark. 1. Fauna Entomologica Scandinavica 9.Scandinavian Science Press, Klampenborg, Denmark.
Heie, O.E. (2009) Aphid mysteries not yet solved (Hemiptera:Aphidomorpha). Monograph Aphids and Other Hemipterous Insects, 15, 31-48.
Larrain, P.S. & Salas, C.F. (2008) House fly (Musca domestica L.) (Diptera: Muscidae) development in different types of manure. Chilean Journal of Agricultural Research, 68, 192-197.
Lewis, T. & Smith, B.D. (1969) The insect faunas of pear and apple orchards and the effect of windbreaks on their distribution. Annals of Applied Biology, 64, 11-20.
Morris, H.M. (1921) The larval and pupal stages of the Bibionidae. Bulletin of Entomological Research, 12, 221-232.
Skartveit, J. (2002) Variation in fecundity in relation to female size and altitude in Palaearctic Bibioninae (Diptera, Bibionidae). Studia Dipterologica, 9, 113-127.
Smith, B.D. & Lewis, T. (1972) The effects of windbreaks on the blossom-visiting fauna of apple orchards and on yield. Annals of Applied Biology, 72, 229-238.
Sukontason, K.L., Chaiwong, T., Piangjai, S. et al. (2008) Ommatidia of blow fly, house fly, and flesh fly: implication of their vision efficiency. Parasitology Research, 103, 123–131.
Pastor, B., Cickova, H., Kozanek, M., Martinez-Sanchez, A., Takac, P. & Rojo, S. (2011) Effect of the size of the pupae, adult diet, oviposition substrate and adult population density on egg production in Musca domestica (Diptera: Muscidae). European Journal of Entomology, 108, 587-596.
Pobozsny, M. (1982) The feeding biology of larval St Mark’s fly Bibio marci (Diptera: Bibionidae). Acta Zoologica Academie Scientiarum Hungariaicae, 28, 355-360.
von Schremer, F. (1958) Bibio larvae as utilisers of litter of dead needles. Anziger Schadlingskunde, 31, 151-153.
Way, M.J. (1963) Mutualism between ants and honeydew-producing Homoptera. Annual Review of Entomology, 3, 307-344.
Yeates, D.K., Wiegmann, B.M., Courtney, G.W., Meier, R., Lambkin, C., & Pape, T. (2007) Phylogeny and systematics of Diptera: Two decades of progress and prospects. Zootaxa, 1668, 565-590.
Zeil, J. (1983) Sexual dimorphism in the visual system of flies: the compound eyes and neural superposition in bibionidae (Diptera). Journal of Comparative Physiology, 150, 379-393.
*I speak from bitter experience; my second-year undergraduate insect collection was Tipulids, which turned out to be a lot more difficult to catch than I had anticipated.