Multiple mating Narcissus flies?

I took some shots of Narcissus flies, Merodon equestris, mating this sunny morning on a dandelion flower. Initially there seemed to be two males, but one of them left. Editing them tonight I realised that the male that left had been mating to start with, and then the other male had mated with the female as well. The male mounting the female beated his wings continuously as if in flight, and the female used her long rear legs to half-heartedly swat at the male.
This is the sequence of shots showing both males mating in succession with the female.

The male behind the female is mating, the one on top beats its wings

The first male is dislodged

The second male mating (also in the top shot)

I have been unable to find - or access - published research about Merodon mating behaviour or how frequent this multiple mating is. It is also possible that I missed a struggle between the males and the second managed to dislodge the first one.  After mating, the female will look for daffodil or bluebell bulbs to oviposit. The larvae will feed on the bulbs and these will die or bear no flowers next spring.

Flies on costume

A Bombus lapidarius flies by, and stops to sunbathe on a leaf...but, wait a minute!, bumblebees industriousness clashes a bit with this behaviour, and I have a second look. Doh! it's not a bumblebee, but a mimic: the large, colourful and furry hoverfly Merodon equestris. It is easy to be fooled. This individual is all black, with a bright red tail. The costume is so good, you can't stop watching: even the back femur is thicker like its mimicking the bee's pollen baskets! And it gets more fascinating still: Merodon equestris is highly polymorphic and different individuals colour patterns mimic different bumblebee species. Earlier in the month I came across one looking like Bombus pratorum and another like Bombus pascuorum. The different forms are actually just colour morphs that interbreed with one another, as shown in one of the photos below. Some of the colour patterns are sex-specific, that is, they are always male or female (the 'lapidarius' looking form is always a female). This fly can be found from April to September, but it is most abundant in May-June. The species is native from southern Europe and is thought to have been introduced in the UK from the continent - where it was first recorded in 1869 - in shipments of flower bulbs, on which the larvae feed, hence its common name, Large Narcissus Bulb Fly

Imitating Bombus pratorum

This male looks a bit like B. pascuorum

Two colour forms mating on the side of a pot.