2020 March 30 morning
Rosemary Jorna’s photographs of the empidid flies have generated a lot of interest. Here are two of the comments received, plus a further note from Rosemary:
From Dr Rob Cannings:
Definitely Empididae and maybe Empis, because it has the look of one. Other genera, especially the related Rhamphomyia, also have this appearance. However, I’m not really experienced with the family at the genus and species level and so am not sure. Empis has vein R4+5 forked near the end, whereas Rhamphomyia does not. I can’t see the venation well enough to tell here. In general, for empidids, I need a specimen in hand, a microscope and a good key!
The male is on the top. As you indicate, courtship in some genera of the family involves the male presenting the female with a “nuptial gift” as a mating stimulus. This is either a prey item previously captured and killed by the male or a prey item wrapped in a frothy or silken package. In some species there is no prey in the package. The gift probably also distracts the female from attacking the male. I think Rhamphomyia never wraps the gift, whereas some species of Empis do.
An observer usually can separate the empidid sexes if the male belongs to one of those species where the terminalia are bulky and complex (usually the case). Females usually have the terminal abdominal segments tapered and acute (used as an ovipositor). The behaviour noted above also is useful.
From Libby Avis:
He gives her the present first to keep her occupied while he gets on with it. I believe it’s a not uncommon strategy among insects. In some cases (e.g spiders) it’s a necessary ruse on the part of the male to avoid being eaten by the female. Given that she is going to produce and lay the eggs, it also gives her extra protein. Same general principle as chocolates on Valentine’s Day……..well, maybe not exactly……. but you get the general drift.
From Rosemary Jorna:
Looking through the file of photos I took it looks as if the one on top is hanging to its mate with the back pair of legs while the other two pairs grab on to the support. The abdomen of the lower one is consistently plumper than the top one so I too think female; she seems to be the one with the claspers, locking the male in place while she concentrates all her attention and legs on dinner.
One single fly landed with a small black & white moth as dinner. Most flying around seem to be single and carrying prey but they are small and fast until mating I have not observed a pair meeting yet. I wonder if they will be flying tomorrow when I am out and about.
And while on the subject of flies, here’s another one, from a quite different family, Bibionidae, from Mount Douglas Park, March 29, photographed by Mr E. Mr E suggests Bibio xanthopus. Although Bibio is a large genus, xanthopus is almost certainly correct. The large eyes show that it is a male. They are sometimes called “March Flies”, although March is usually a little bit early to see them. They should preferably be called “Saint Mark’s Flies”. A well-known European species is B. marci, where “marci” does not mean “of March”, but rather “of Mark”, so called because it is abundant near to Saint Mark’s Day, April 25 – which is also about right for our xanthopus.
Bibio xanthopus (Dip.: Bibionidae) Mr E