New to nature: No 132 Empis vockerothi

This North American species of dance fly engages in courtship deception by presenting would-be mates with empty gifts
Empis vockerothi
Empis vockerothi with his deceptive balloon. Photograph: Allen B Coovert Allen B Coovert/PR

True, or two-winged, flies of the family Empididae are commonly known as dance flies, so named for bobbing flight movements on display in swarms formed to attract mates. Most of the 2,000 or so species of dance flies are found in the Holarctic region.

Males of the European species Hilara maura present females with a nuptial gift. The male captures a prey item, a small insect of some kind, wraps it in silk produced from glands on his front feet, then presents it to a female in the "hope" that she will be receptive to copulation. A similar ritual will be well known to readers and keeps florists, chocolatiers, and jewellers gainfully employed. Hilara is not alone in amazing mating practices among dance flies. In some species, males form swarms from which females select a mate, presumably the one bearing the most alluring gift. Less frequently, it is the females that swarm and the male who chooses. In such cases, females employ elaborate anatomical modifications to catch the male's eye including darkly coloured wings, feather-like leg scales, and inflatable bags between abdominal sclerites. Although the unusual sex lives of dance flies have attracted the attention of numerous scientists studying evolution, mating behaviour, and sexual selection, the majority of species remain poorly known, as does the phylogenetic history of the family.

A team from Ottawa, led by Dr Bradley J Sinclair of the Canadian National Collection of Insects, undertook a revision of dance flies of the subgenus Enoplempis (genus Empis) in North America, east of the Rocky Mountains. This involved reviewing a dozen previously known species and resulted in the discovery of seven new ones. One of the new species, E vockerothi, is found from Ontario south to Virginia, has a wing length of about 5mm, and may be distinguished from its relatives by several features, including such large eyes in the male that they appear to be one.

Among the more than 700 species of Empis in the northern hemisphere are males that present food gifts to mates in a manner reminiscent of Hilara, but achieved quite differently. Empis do not produce silk but instead encase offerings in a frothy "balloon" formed by the salivary glands. In others, this ritual is taken one step farther, forming and offering a balloon to the female, but one empty and devoid of food. The dipteran version of a date at a cheap curry house, instead of Veeraswamy. The chemistry of the balloons is unexplored, but some collapse within hours, while others are very persistent.

Males of E vockerothi have been observed to swarm for one to three hours, beginning shortly after midday. They swarmed 2-5 metres above the ground in the shade of a hemlock tree (Tsuga) with males grasping their balloons between their middle legs. Interestingly, the sister-species, E snoddyi, uses the same empty balloon strategy and has a complementary geography, occurring from Virginia south to the Great Smoky Mountains. Such deception is unique among eastern species of the subgenus, the others presenting prey with no balloons. In western species, males either make small balloons containing large prey items or large balloons with only small dried-up bits of prey.