Asilidae

Robber fly close-up

A member of the Asilidae feeding on a grasshopper. This asilid shows the mystax and ocular fringe typical of the Asilidae, with short, stout proboscis and spiny, powerful legs, adapted to the capture of prey in flight.

The Asilidae are a family in the order Diptera, the true flies. The common name for members of the family is the robber flies. The Asilidae are cosmopolitan, with over 7000 described species. Latreille was the authority for establishing the family in 1802.[3] The Asilidae, together with Bombyliidae and Therevidae, are the most representative families of the superfamily of Asiloidea and they form one of the most characteristic groups of the lower Brachycera.

Robber flies have stout, spiny legs and they have three simple eyes (ocelli) in a characteristic depression on the top of their head between their two large compound eyes.[4] They also have a usually dense moustache of stiff bristles on the face; this is called the mystax, a term derived from the Greek mystakos meaning "moustache" or "upper lip". The mystax has been suggested to afford some protection for the head and face when the flies deal with struggling prey; various Asilidae prey on formidable species including stinging Hymenoptera, powerful grasshoppers, dragonflies and even other Asilidae, in fact practically anything of a suitable size. Some Asilidae do, however, specialize in smaller prey, and this is reflected in their more gracile build.

In general the family attacks a very wide range of prey, including other flies, beetles, butterflies and moths, various bees, ants, dragon and damselflies, ichneumon wasps, grasshoppers, and some spiders. They do so apparently irrespective of any repugnatorial chemicals the prey may have at its disposal.[5] Many Asilidae when attacked in turn do not hesitate to defend themselves with their proboscides and may deliver intensely painful bites if handled incautiously.

Female Laphria flavicollis eating a boxelder bug

The antennae are short, have three segments, and sometimes bear a bristle-like structure called an arista.

Though they are a very characteristic group for such a large family, the Asilidae may easily be confused with the related and less widely known family Therevidae. Some points of contrast between the families include that the labium in the Therevidae is not a piercing, predatory organ, but ends in two fleshy labella adapted to the sucking of liquid foods. Again, the Therevidae commonly have fluffy setae above the mouthparts, unlike the stiff chaetae comprising the mystax of the Asilidae. Furthermore, in the Asilidae the depression on the vertex between the eyes tends to be more obvious than in the Therevidae.

The fly attacks its prey by stabbing it with its short, strong proboscis, injecting the victim with saliva containing neurotoxic and proteolytic enzymes which very rapidly paralyze the victim and soon digest the insides; the fly then sucks the liquefied material through the proboscis.

Many Asilidae have long, tapering abdomens, sometimes with a sword-like ovipositor. Others, for instance Laphria, are fat-bodied bumblebee mimics.

Female robber flies deposit whitish-colored eggs on low-lying plants and grasses, or in crevices within soil, bark, or wood. Egg-laying habits depend on the species and their specific habitat; most species lay their eggs in masses, which are then covered with a chalky protective coating.

After hatching, robber fly larvae generally seem to live in soil, rotting wood, leaf mold and similar materials, some being predatory and others detrivorous.[5] Larvae are also predacious, feeding on eggs, larvae, or other soft-bodied insects. Robber flies overwinter as larvae and pupate in the soil. Pupae migrate to the soil surface and emerge as adults, often leaving behind their pupal casing. Complete development ranges from one to three years, depending on species and environmental conditions.[6]

The Asilidae are predators, both in the juvenile stage and the adult stage, and feed on small arthropods, mainly insects. Although predatory forms in the adult stage are present in other taxonomic groups of Diptera, the Asilidae are the most representative for the number of species and for uniformity of feeding behavior. They are aggressive to the point of earning the common name, in English, of "robber flies". The combination of high biodiversity and high predatory activity leads to this family playing an important role in the ecological stability of entomofauna.

Blepharepium sonorensis, a robber fly that closely resembles Polistes paper wasp species such as P. apachus

The life cycle takes place in 1–3 years. The postembryonic development consists of four larval stages (instars) and one pupa. The larvae of the first instar differ from other stages in both ethology and trophic regime. The larvae of most known asilids live in the soil or in the case of some taxonomic groups, in rotting organic material, usually wood and the bark of dead trees. With regard to feeding behavior, most of the literature gives the larvae of the Asilidae as entomophagous, but there are doubts about the real nature of the trophic regime and its mechanisms. The entomophagy of some species had indeed been already hypothesized by some authors of the nineteenth century, based on the findings of larvae of asilids associated with larvae of other insects, but Melin (1923) asserted that, in reality, predation was occasional and secondary to the plant-based diet. More recent studies have confirmed the entomophagy of some asilids without extending this species's feeding behavior for the whole family. Less certain, however, is the mechanism of implementing entomophagy: in general, the behavior is cited as predation, but for some species may be ectoparasitoids. Musso (1983)[7] described the feeding behavior of the larvae of Machimus rusticus and its evolution during postembryonic development : in particular, the larvae of the first instar does not feed on insects, those of the second instar feed on secretions by larvae of beetles (and may cause death), while the larvae of the third and fourth instars actually behave like predators. In short, the feeding behavior of larval asilids can configure as intermediate between predation and ectoparasitism.

An asilidae fly with captured housefly.

Much better known and described in detail is the behavior of adults. In general, the activity of predation of adults is concentrated in the hottest hours in open, sunny spaces, while at night they take refuge in dense vegetation. The Asilidae are excellent flyers and, in most of the family, capture prey in flight. They are often seen stationed to ambush prey at strategic points. This behavior signifies that sight plays an essential role in the detection of prey and their capture. The prey is caught with the tarsi and immobilized as a result of the paralysis caused by the injection of saliva. The asilid pierces the integument of the prey with the prepharyx (hyopharynx) in preferential points of least resistance as the eyes, the membranous area of transition between the head and thorax (neck) or between thorax and abdomen, or between the last urotergiti. Puncture is followed by the injection of saliva, whose active components perform two functions: the neurotoxins cause paralysis of the victim, while proteolytic enzymes lead to the breakup and liquefaction of internal tissues; in a short time the predator is able to feed by sucking the internal fluids through the alimentary canal.

With regard to interspecific trophic relationships, there is a large number of reports on the prey captured by Asilidae. Lavigne (2003) has developed a database comprising over 13,000 reports.[8] The prey of Asilidi are predominantly represented by other insects, mostly winged, but several cases in which they have attacked spiders have also been reported. Within the insects, orders that include the most frequent prey of asilids include a wide range of families within Coleoptera, Hymenoptera, other Diptera, Hemiptera, and Lepidoptera; prey belonging to various other orders (Odonata, Neuroptera, Isoptera, Thysanoptera, Blattodea, etc.) is also mentioned.

With regard to the specificity of the trophic relationship, Wood (1981)[2] mentions the existence of some studies in the literature on the subject. Some genera have been found to be monophagic, but more generally the Asilidae manifest a more or less wide polyphagia with behaviors that vary from stenophagia[8] to euriphagia.[2] However, there are cases of stenohage asilids which if necessary focus their predatory activity against a species when this forms large populations.

Other studies conducted by Dennis, D. and Lavigne,(1975)[9] have shown that the ratio between the size of the prey and the asilid varies from 1.8:1 to 3.7:1, with an average of 2.6:1. The ratio tends to increase with decreasing size of the predator.

Egg-laying takes place, according to the species, with three different behaviors which relate to the structure and the morphology of the abdomen. Females with an undifferentiated ovipositor release eggs randomly and independently from the substrate. In other cases, however, the abdomen bears a differentiated, specialized ovipositor to lay eggs in the soil or sand, or lay them in cavities within plant tissues.

The Causse du Larzac is a limestone karst plateau, a habitat typical of those favoured by many species of Asilidae

Asilidae generally occur in habitats that are open, sunny, and dry, even arid. They favour open or scattered vegetation, and some species even frequent bare ground. Typical habitats include savannah, forest steppe, open steppe, semi-desert, maquis shrubland, and related shrubland types such as fynbos and chaparral. Their biodiversity is lower in forested ecosystems and, where Asilidae do occur in such environments, they tend to concentrate in the glades and margins. In those conditions the interrupted canopy leaves space for various species of shrubs and herbaceous plants suited to Asilid styles of predation.

In general the biology of the Asilidae is still poorly known. However, various authors have studied the population distribution in particular regions and ecosystems. They have classified the behavioral patterns in terms of microenvironments, ecological, and trophic factors, showing how different species of Asilidae favour particular habitats suited to particular patterns of reproduction and predation. Specific studies show correlations between the floristic composition and predatory behaviour. Particularly seen in parts of Arizona including Sedona and Phoenix.[10]

Robber flies (Asilidae) mating in a blade of lemon grass, in Western ghats India

Asilidae Robber fly from the Anaimalai hills, Western Ghats, India

Asilidae occur in all zoogeographical regions except Antarctica. In the Northern Hemisphere, some species are even adapted to tundra. Alpine species occur at altitudes exceeding 4000 meters. However, the highest levels of biodiversity are in warm climates; tropical or subtropical and arid or semi-arid regions tend to have the greatest variety of species, followed by areas where rainfall is highly seasonal.

Female Stenopogon martini feeding on a honeybee

Asilidae currently includes over 7500 described species in about 556 genera.[11] The taxonomy is still under study in the light of new specimens and cladistic analysis. There are 14 accepted subfamilies:[12]

• Asilinae
• Bathypogoninae
• Brachyrhopalinae
• Dasypogoninae
• Dioctriinae
• Laphriinae
• Leptogastrinae
• Ommatiinae
• Phellinae
• Stenopogoninae
• Stichopogoninae
• Tillobromatinae
• Trigonomiminae
• Willistonininae

Clade showing relationship of Asiloidea

Figures of some Asilidae in Europäischen Zweiflügeligen

Carl Linnaeus (Linné; 1758), in the tenth edition of Systema naturae, erected the genus Asilus, including 11 species, and added 4 others in the twelfth edition (1767). Asilus crabroniformis (1758) is the type of the genus. The rank of family is credited to Leach in Samouelle (1819). Johan Christian Fabricius in five publications dated from 1775 to 1805, erected the genus Damalis and described 76 exotic and European species. Wiedemann, in publications appearing between 1817 and 1830, described 235 species, many exotic. Johann Wilhelm Meigen in an early work of 1803 erected 4 genera, 3 of which now represent subfamilies. He also described many species in Systematische Beschreibung der bekannten europäischen zweiflügeligen Insekten (1800 to 1838). During the rest of the nineteenth century there were significant contributions by Hermann Loew in particular. Other prominent authors dealing with the Asilidae during the nineteenth century included Pierre-Justin-Marie Macquart, Francis Walker, Camillo Rondani and Jacques-Marie-Frangile Bigot.

• List of soldierflies and allies of Great Britain
• List of Asilidae species
• List of Asilidae genera

• Geller-Grimm F (2003): Photographic atlas and identification key to the robber flies of Germany (Diptera: Asilidae), CD-ROM, Amphx-Verlag Halle (Saale). ISBN 3-932795-18-0
• Hull, Frank M. (1962). "Robber Flies of the World: The Genera of the Family Asilidae". Bulletin of the United States National Museum. 224 (224): 1–907. doi:10.5479/si.03629236.224. hdl:10088/10126.
• Lavigne, Robert J. (2003). "Evolution of courtship behaviour among the Asilidae (Diptera), with a review of courtship and mating". Studia Dipterologica. 9 (2): 703–42.
• Musso, Joseph-Jean (1978). Recherches sur le développement, la nutrition et l'écologie des Asilidae (Diptera - Brachycera) [Development research, nutrition and ecology of Asilidae (Diptera - Brachycera)] (PhD thesis) (in French). OCLC 30534417.
• Oldroyd, Harold (1969). Tabanoidea and Asiloidea. Handbooks for the identification of British insects. Royal Entomological Society. OCLC 256410648.
• Papavero, Nelson (1973). "Studies of Asilidae (Diptera) systematics and evolution: I. A preliminary classification in subfamilies". Arquivos de Zoologia. 23 (3): 217–74. doi:10.11606/issn.2176-7793.v23i3p217-274.

Wikimedia Commons has media related to Asilidae.

Wikispecies has information related to Asilidae.

• Database service for Asilidae – Robber flies Comprehensive overview by Fritz Geller-Grimm, Torsten Dikow and Robert J. Lavigne
• Asiloid Flies: Deciphering their diversity and evolutionary history Smithsonian National Museum of Natural History.
• West Palaearctic species including Russia
• Australasian/Oceanian species
• Nearctic species
• Japan species
• World list species
• Robber flies of North America – reference photographs, descriptions, natural history
• Family Asilidae at Bugguide.net Images
• Family Asilidae at EOL Images
• Atlas of German Asilidae
• Key to the Asilidae of Central Europe
• Images (text in Japanese)
• Diptera.info
• Family description and images
• robber flies on the UF / IFAS Featured Creatures Web site
• bee killers, Mallophora spp. on the UF / IFAS Featured Creatures Web site
• Courting-dance of the robber fly (gallery)
• Robberflies of Germany Excellent. Includes structure.
• Wing venation
• Family Asilidae at EOL Images. Flowers visited by adults.