Asilidae

Taxonomy

Order: Diptera
Family: Asilidae Latreille, 1802
Common names: assassin flies, robber flies

Background

These predatory flies are found worldwide. Assassin flies feed exclusively on other insects and other arthropods, both in their larval and adult stages. Because of their predatory habit of feeding on other insects and their voracious appetites, they contribute to the maintenance of the natural balance among insect populations. A few species are known to prey on honey bees and can seriously deplete the populations of apiaries. The adults are generally large, active flies and readily attract attention (Hull 1962).

How to separate from Asian giant hornet

  • Flies have one set of wings instead of two (the hind wings are reduced to small halteres)
  • Many common asilids have an enlarged thorax, and none of them have a "wasp waist"
  • Flies have sucking mouthparts while wasps and hornets have chewing mouthparts

Distribution

Although assassin flies are found worldwide, most species occur north of the Equator. They have also established on major islands and even many smaller island groups. Most species occur in arid and semiarid regions, particularly in dry stream beds, which attract the greatest number. In more temperate regions, a few species occur in wet swamp areas, in deep forests, and concentrated on the edges of woodlands, where shrubs occur and give away to grassland (Hull 1962, Bromley 1946).

Distribution map of Asilidae by GBIF: https://www.gbif.org/species/7275

Diagnostic characteristics

Flies can be separated from Hymenoptera (wasps, bees, and ants) because:

  • They have one pair of wings instead of two.
  • Their hindwings are reduced to little appendages with a bulb at the distal end, called halteres.
  • Flies lack the thin waist seen in most Hymenoptera.

Although assassin flies are very diverse, there are a few ways to distinguish them from other flies:

  • They are generally large flies, ranging from very small (3 mm) to very large (over 80 mm long); most are 9–15 mm long.
  • They can be long and slender or short, robust, and bee-like.
  • Three simple eyes (ocelli) occur in a depression on the top of their head between the two large compound eyes.
  • Most also have a dense mustache of stiff bristles on the face called a mystax.
  • They have short, strong, and piercing mouthparts.
  • They have stout spiny legs.
  • They are often seen “perching” on twigs or leaves while hunting prey.

Diversity

Worldwide there are over 7,500 species of Asilidae in 560 genera, with over 1,000 species in 100 genera in the United States (Wood 1981).

Host/prey associations

Both male and female assassin flies prey on arthropods both as adults and immatures. The great majority of prey are insects, but some species have been reported to feed on spiders. Some species and genera show a preference for one type of prey. For example, species in the genus Mallophorina attack Hymenoptera almost exclusively (Linsley 1960).

Nesting and general behavior

Assassin flies are usually found in open and sunny habitats. They fly primarily during the hottest part of the day. Most species are present as adults during the summer months. However, there are many species that fly for short periods in the spring or fall. Complete development from egg to adult ranges from one to three years. Adults perch in open sunlit areas where they can see passing insects and fly out to catch them. Some assassin flies perch on exposed branches, others on logs or stones, and still other species perch on the ground. Each species shows a preference for type of perch and height above the ground. Assassin fly larvae are predatory and hunt other insect larvae in whatever substrate they hatched in. Mimicry is common in this family, and there are many examples of assassin flies mimicking wasps and bees in their appearance (Hull 1962).

Known invasives

Members of this family have been introduced to Hawaii.

<p><em>Efferia aestuans</em>; photo by B. Schoenmakers, Wikimedia</p>
<p><em>Laphria grossa</em>; photo by gailhampshire from Cradley, U.K., Wikimedia</p>
<p><em>Laphria grossa</em>; photo by cotinis, Flickr</p>
<p><em>Efferia</em> sp.; photo by John Sullivan, iNaturalist</p>
<p><em>Laphria thoracica</em>; photo by Bryan Thompkins, iNaturalist</p>