Bibionidae

St.Mark's Flies

DIPTERA > NEMATOCERA | Lower Diptera

BIBIONOMORPHA > Bibionidea > Bibionidae (18 species)

(subfamilies: none)

Although this small family has the collective common name of St.Mark's Flies, only the largest species, Bibio marci, is actually referred to by that name as it usually emerges around St.Mark's Day on the 25th April.

The family is represented in Britain by two genera, Bibio and Dilophus, the latter containing just four small species known as fever flies. Of the 14 Bibio species, some are quite small and will most likely be overlooked, but that still leaves a number of species that may be encountered during the spring months of April and May particularly. Whilst the adults may only be on the wing for a few weeks, they can often be abundant on spring blossom. Habitat varies by species with some favouring heath or grassland, whilst others are more likely to be seen in woodland.

Bibio species are sexually dimorphic with the males being large-eyed, black-bodied, relatively hairy and usually with a pale wing membrane, whilst the females by contrast, have tiny heads with much smaller eyes, inconspicuous pilosity, bodies and legs that can be extensively red in some species, and wings that are darkened in places.

Family characteristics are given as tiny to large (2-15mm), compact, robustly built flies; body colour pale brown to black, in some cases partially red or yellow; ocelli conspicuous; male eyes are holoptic, female eyes dichoptic and simple; antenna inserted low on head below the compound eyes, with 6-12 segments; wings wide with distinct anal lobe; wings clear or darkened; vein R4+5 thickened and reaching the margin well before the wing tip; basal cells long, the upper basal cell extending to the middle of the wing; lower basal cell sometimes reaching beyond the upper; strong spurs on legs; femur on the foreleg usually swollen.

There are a number of very similar species where identification comes down to fine details; too many to list here, but the following brief notes may be useful. Bibio marci is the most common species; males have milky wings; a long basal section to vein R4+5, which is a useful feature to separate them from the similar, but smaller, Bibio leucopterus that has the base of R4+5 of roughly equal length of r-m. B.leucopterus also has a unique character in that the front tibial spur and outer spine are about the same size; the female of the species has dark wings. Bibio lanigerus males can be separated from Bibio varipes by the relative length of the hind basitarsus, which is noticeably longer in B.varipes, somewhere in the region of 4x the thickness, rather than 3x as in B.lanigerus. Other useful features for Bibio varipes are the pale brown wing veins, more conspicuous in the males; a short basal section of R4+5 as B.johannis and B.reticulatus; a dark stigma, but not as defined as B.johannis; females usually have legs that are extensively reddish-brown, and pale brown wing membranes lacking the darker veins of the male; the males are more variable, some with black hairs on the thorax, others with yellow, and legs that can be entirely blackish in some individuals, although usually with the hind tibia reddish-brown. Bibio johannis is best recognised and separated from B.varipes by the more noticeably dark and well-defined wing stigma; the female's legs are almost entirely reddish. Bibio reticulatus is an interesting one as it's by far the least recorded of any of these species, so my assessment of the mating pair below may be incorrect, but they can be distinguished from the others in their group by the dark wing veins and stigma; males with paler hairs on the thorax and blackish legs; and the fact that this particular species is most frequently encountered in ancient woodlands.

Dilophus febrilis is apparently one the most frequently observed bibionid species in many areas, although I have to admit that, at present, I've only seen the one individual featured below. It could be that I've just passed them by as the males visually resemble the Bibio species until you look closer. The females stand out as they have darkened wings with white tips, which unfortunately are blown out in my photo due to the use of flash. If this had been a better detailed image it would have shown several blunt spines both on, and around the end, of the front tibia, and two rows of spines on the pronotum behind the head. The Dilophus species also have larger fore coxae than found in Bibio and a costa that extends beyond R4+5. Dilophus febrilis is the largest of the four species in the genus with a wing length of up to 7mm. They can be found in various habitats from spring through to early autumn, peaking in May and early June, and again later in the year with a second generation in July and August. The adults may congregate on various flowers dependent on the season, such as Cow Parsley in the spring and Wild Parsnip in late summer.