I was very fortunate, I think, to come across this mating pair of bumblebee hoverflies (Volucella bombylans), which are well-known mimics of bumblebees. The interesting thing is that they were two different varieties, or morphs, locked together in copula. They look rather different from each other on account of mimicking different bumblebee models. The fly on the top, hanging on to the vegetation, is Volucella bombylans var. bombylans, and is the male, I think.
The one at the bottom, being carried in copula, is Volucella bombylans var. plumata, and is the female, I think. She is noticeably larger. These are the two main varieties of this species; but there is a third, much rarer variety (at least in the UK), called var. haemorrhoidalis (below). They have each evolved to mimic a specific kind of bumblebee, which I described in a previous blog on this species: Polymorphic mimics: flies that look like bumblebees.
Remarkably, the proportion of the different morphs reflects the frequency and abundance of the bumblebee that they most closely resemble. So, for example, the frequencies of V. bombylans plumata at any given site in England and Wales is positively related to the frequencies and abundances of the B. terrestris group of bumblebees. This positive association between the model (bumblebee) and its mimic (the hoverfly) was discovered by researchers Malcolm Edmunds and Tom Reader, over a 10-year study, and confirms that these Batesian mimics gain protection by their mimetic resemblances. Something long suspected but not easy to prove!
The varietal differences between the two individuals is most noticeable when the pair are in profile (below).
The fact that these different varieties interbreed, has been known for a long time. The entomologist George Henry Verrall (1848 – 1911) – well-known to those entomologists who attend the Verrall Supper (The Verrall Association of Entomologists) – reported a mating pair of Volucella bombylans var.bombylans and Volucella bombylans var. plumata in 1901! It would be quite surprising I think, if they did not interbreed, as the different varieties would soon become isolated and divergent, if they did not. Also, it is presumably necessary to generate different varieties which can match the changing frequency of the different bumblebee mimics, both over time and in different locations. The generation of these different morphs is determined by Mendelian Inheritance, so there will be individuals of both morphs in the offspring of this particular union. See Keeler (1926) ad Rupp (1989) for more details.
This mating of Volucella bombylans may be relatively rarely witnessed in the field? I am not sure, but a researcher in Germany (Rupp, 1889) mentions only observing one copulation of this species in the field, during several years observations. Amazingly, much of the biology of these flies was known about and described in a book by a French entomologist, Künckel d’Herculais, published in 1875! More recent observations were included by Fabre, in 1920!
These hoverflies lay their eggs in the nests bumblebees – which occur in the ground – where the hoverfly larvae are said to live as scavengers, feeding on debris. There are reports of them occasionally feeding on the host’s larvae, but this is largely discounted by Rupp (1989). At least the evidence is lacking. V. bombylans adult females enter the nests of bumblebees, and try to avoid contact with the bumblebees; only infiltrating the nest when the passageway is free of incoming and outgoing workers. They do not restrict themselves to those bumblebees which they resemble; any morph will enter a given bumblebee nest. Nevertheless, the hoverflies are usually attacked by the bumblebees, but according to David Alford, usually mange to lay clusters of their elongate eggs on the comb. They have a macabre trick of still being able to lay eggs via a sort of reflex oviposition, even if they are killed!
There is an awful lot more to find out about these flies and their relationship to their bumblebee hosts. According to Rupp (1989), although they require obligate development in bumblebee nests, they are probably commensals – just feeding on nest detritus and hardly noticed by the bumblebees – rather than being parasites.
More, beautiful pictures of these bumblebee hoverflies can be seen on Steven Falk’s website.
P.S. these photographs of the mating pair were taken at Felmersham Gravel Pits, Bedfordshire (UK), on 28th June 2019.
Alford, D. V. (1975). Bumblebees. Davis-Poynter..
Edmunds, M. & Reader, T. (2014). Evidence for Batesian mimicry in a polymorphic hoverfly. Evolution, 68(3), 827-839.
Fabre, J. H. (1920). The Life of the Fly: With which are Interspersed Some Chapters of Autobiography. Dodd, Mead,.
Keeler, C. E. (1926). Recent Work by Gabritschevsky on the Inheritance of Color Varieties in Volucella Bombylans. Psyche: A Journal of Entomology, 33(1), 22-27.
Künckel d’Herculais, J. (1875). L’organisation et développement des Volucelles. [The organisation and development of Volucella (Diptera, Syrphidae)]. Paris: G.
Monfared, A., Azhari, S., & Gilasian, E. (2013). Volucella bombylans (Syrphidae, Diptera) recorded from a colony of Bombus mesomelas (Apidae, Hymenoptera) in Iran. Linzer biol. Beitr, 45(1), 829-836.
Rupp, L. (1989). The central European species of the genus Volucella (Diptera, Syrphidae) as commensals and parasitoids in the nests of bees and social wasps: studies on host-finding, larval biology and mimicry. Albert-Ludwigs University, Freiburg-im-Breisgau, Germany. Inaugural Dissertation.
Verrall, British Flies, Vo. 8, p. 485. London, 1901.