Biology & Ecology

Biology
Egg. The egg is 0.4-0.8 mm long, ovoid milky white, and without reticulation. The number of eggs laid by each individual seems to vary from about twenty five to about a hundred. (Lyneborg 1992).

Larvae. As far as is known there are five larval stadia, the last of which either pupates or goes into diapause. Therevids are usually univoltine, but instances where two or more years are needed to complete the life cycle are known in European Thereva species (Lyneborg 1992). The larvae are smooth and vermiform, with a rather well developed head with an apically spatulate tentorial rod (Irwin & Lyneborg 1981a, Irwin & Lyneborg 1981b). The abdomen is secondarily divided into some 16 apparent segments, and terminates in a pair of tiny pseudopods (Woodley 1989). The snake-like larvae are very mobile and move with considerable speed through sand and loose soil (Lyneborg 1992). They are found mainly in sand or soil close to the surface (English 1950). Therevid larvae are voracious predators, feeding on a great variety of insect larvae and earthworms, but preferring Coleoptera larvae. At least one New Zealand species has developed a cleptoparasitoid way of life (Lyneborg 1992). In captivity, therevid larvae can be fed on larvae of flour beetles, but should be separated as they can be cannibalistic.

Pupae. In the prepupal stage the larva assumes a curved position in the soil, somewhat like the letter U, or almost in a circle (Lyneborg 1992). The pupal stage lasts for only a week or two. The pupa is especially vulnerable to desiccation and to attack by predators (Lyneborg 1992).

Adults. In contrast to much of the world's therevid fauna, many species of Australian Therevidae are brightly coloured, elongate flies with largely glabrous bodies. Bright colouration, long antennae, body shape, together with behavioural modification, often results in wasp mimics, particularly in genera such as Agapophytus and Ectinorhynchus (Power 1998, Winterton & Irwin 2001). Many genera have attractively patterned wings and males with distinctive silvery pruinescent markings on the body. Many species show marked sexual dimorphism. The antennae are sometimes very distinctive, such as the greatly elongate scape in Agapophytus and the thickened scape in Neodialineura.

Habitats
Adults frequent a wide variety of habitats, but have the greatest biodiversity in semiarid regions of the world (Irwin 1976, Winterton et al. 1999). A large proportion of Anabarhynchus species seem to be associated with coastal scrub and sandy beaches, habitats that are extensively exploited by therevids all over the world (Lyneborg, 1992). In Australia, many genera are found in semiarid areas and dry sclerophyll forests, but few are found in wet sclerophyll forests and rainforests.

Field work in Australia continues to regularly uncover specimens of undescribed species.

Habits
Adult therevids are seemingly all diurnal (Lyneborg 1992); however, there are many records of Australian therevids from many genera, including mating pairs, being collected at light during the night. Some genera only appear in the early hours of the morning (A. Zwick pers.com.).

Little is known of the food habits of Australian therevids, but adult therevids appear to feed on honeydew, flower nectar and pollen. Many taxa inhabiting subtropical regions with winter rain (i.e. parts of California, the Mediterranean area, southwestern Africa, and probably also south-western Australia) often have an elongated, projecting proboscis, and are recorded as visiting flowers, taking nourishment in the form of nectar (Lyneborg 1992). Several genera were collected in numbers from flowers of Baeckea and Nuytsia floribunda in spring in south-western Western Australia (Lambkin pers. obs). Therevids need to regularly drink water, and can often be found at drying pools and springs. Adult therevids often alight in sunny patches on trails and paths, or rest on sand, rock, leaves, stems, and tree trunks. They have a rapid flight of short duration. The flying period for some of the more common species is usually 3-4 months (Lyneborg 1992). In semi-arid and arid environments, therevids are stongly attracted to water. Pools of water in drying creek beds often attract a tremendous diversity of these flies. Many of the field photos included in this key were taken along the edges of such pools.

Lek formation on the trunks of large, smooth-barked trees has been observed in the males of some species of Agapophytus. At various sites in south-eastern Queensland, males were observed forming small oval-shaped leks of up to eight individuals on the shaded side of the tree approximately 1–2 metres above the ground. Periodically, most members of the lek take-off and fly in a tight loop perpendicular to the trunk and land again in approximately the same position within the lek. Mating was observed on two occasions and took place on the other side of the same tree or on a nearby tree. Males of second species were observed forming slightly smaller sized leks on the sunlit side of a tree approximately 0.5 metres above the ground (Winterton & Irwin 2001).
Males of some species form hovering swarms. The female enters a swarm and is caught by a male, and copulation takes place (Lyneborg 1992). Males of Ectinorhynchus can often be seen hovering in small swarms above and among shrubs in sheltered but sunny positions. Oviposition behaviour has been described by Irwin (1976). Many Australian therevids appear to belong to the group in which the female abdomen is embedded in the substrate by thrusting and contorting it until a major part is buried. As soon as the egg is laid, the abdomen is withdrawn.

Ecology
There have not been many extensive studies on the ecology of the family Therevidae. The larvae, like most underground dwellers, are rarely encountered unless these soils are actively sieved. They twist violently when exposed and are extremely quick and agile in their attempts to escape. Even pupae when disturbed will wiggle vigorously in an attempt to scare away intruders. Adults have been described as secretive, and in most circumstances are rarely collected by hand. Many are caught in well-placed Malaise traps along flight paths near moisture. The individuals captured this way provide no information about their ecology. Many times, adults may be found mating on the soil surface, and being thus encumbered and somewhat preoccupied, can be more easily observed and hand netted. Most published works include ecological observations that are more anecdotal than systematically gathered.

One of the most intensive quantitative surveys of Therevidae was conducted in Australia in 1997-1998. A long-term Malaise trapping study near Brisbane was completed by Narelle Power as part of her Honours thesis at the University of Queensland. Narelle trapped therevid specimens using eleven 6 metre Focks Malaise traps based in three diverse habitats: coastal heath on Bribie Island, sclerophyll forest in Brisbane Forest Park, and rainforest at Mt Glorious. The weekly collections revealed a very diverse fauna of Therevidae in southeast Queensland. Over 1,000 Therevidae belonging to 15 genera and 52 species were collected over a 54 week period (Power 1998).

Individual abundance varied across the three sites with 47.5% of specimens collected from sclerophyll forest, coastal heath accounted for 39.2% of the total therevids, and the rainforest site just 13.3%. Individual species appear to be spatially restricted, with site species diversity related to the heterogeneity of the environment (Power, 1998). The majority of therevid species were represented by few individuals, with ten represented by less than 3 individuals. The results of an analysis of seasonal abundance were significant, finding that most specimens and all species were taken in spring (September-November) (Power 1998). Cooler southern areas of Australia may reflect the seasonal abundance in New Zealand, where three species of adult therevids have been collected during September, 16 during October, 30 during November, 36 during December, 37 during January, 30 during February, 13 during March, and 3 during April (Lyneborg, 1992). A similar situation was found in the genus Agapophytus, with the majority of collection records in Australia and New Guinea between November and February (Winterton & Irwin 2001). An American study of the bioecology of therevids (Hartman et al. 1995) in Sand Ridge State Forest in Mason County, Illinois, showed no obvious correlation between any of the six therevid species collected and microhabitat. Females were much less abundantly collected and were present about a week after the males first appeared. Three species had single population peaks as would be expected of univoltine species and two species peaked twice. All species were present in the first population peak, which occurred in late May to early June. The two most numerous species then peaked again in late July and early August, suggesting a second generation. Therevids have typically been thought to produce a single generation per year but data such as these suggest that some species are bivoltine. Similarly, a study by Mike Irwin along the Kuiseb River, Gobabeb, Namibia suggested that adults of some therevid genera may have a second generation.

Therevids as Bioindicators
Xeric habitats are destroyed at an alarming rate through a number of threatening processes such as land clearing, salinity and invasive species. We lack tools to accurately evaluate the impacts of threatening processes on soil-dwelling invertebrates, underground productivity, and habitat heterogeneity. To gauge degree and speed of change on these lands, specific bioindicators that reflect alterations are needed. Generalist predators feeding on subterranean herbivores are potentially good indicators of diversity, productivity, and heterogeneity, and in desert communities, they are at the top of the food chain (Ayal 1998). Thousands of adult therevids have been collected in a couple of days in single Malaise traps placed across a flight path in dry gullies in semi-arid areas of Victoria and northern Australia, and in the forests of the Warrumbungle National Park in New South Wales. These numbers imply that in these areas there is a high biomass of therevids in the environment, both of adults and predatory larvae. Easily monitored above ground, adult numbers mirror below-ground productivity, while their diversity and richness reflects subterranean heterogeneity (Hartman et al. 1995). Because the larvae are non-specific predators in the soil, therevids have often been suggested as potential indicators of soil health. Furthermore, therevids may have potential for biological control because they suppress root-feeding pest larvae in sandy agroecosystems. Examination of the biocontrol and bioindicator possibilities is restricted by deficient taxonomy. To complement taxonomic and phylogenetic studies, David Carlisle, a student from Australian National University Canberra, is currently investigating spatial and temporal emergence patterns of therevid larvae.

Biogeography
Australasia has the world's richest therevid fauna and the genera show an extraordinary degree of endemism. Of the 24 described genera, 23 are found only in this region. Australia's aridity is geologically recent (beginning 25 mya) and many of its therevid species belong to morphologically similar 'species-swarms' that may have radiated in this period. Species-rich genera, such as Anabarhynchus, Parapsilocephala and Acraspisa exemplify these massive radiations. Clearly the Australasian fauna has evolved and radiated in isolation after the separation of Australia and New Zealand from Gondwana. As Australia, New Zealand and New Caledonia have drifted northwards towards Indo-Papua archipelago, only recently has the Australasian fauna moved into Papua New Guinea and Indonesia. Irwiniella Lyneborg, found in the Australasian region in Papua New Guinea and Indonesia is the only adventive originating from Laurasian stock; Irwiniella is diverse in Asia, and Africa. Reflecting their Gondwanan heritage, the Australasian therevid fauna evolved after the separation of Gondwanan continents and their closest relatives can be found in South America, such as Melanothereva Malloch, Pachyrrhiza Phillippi and Entisia Oldroyd. Anabarhynchus is the dominant genus in Australia and New Zealand, but is also known from Papua New Guinea, New Caledonia and Fiji, and has its closest relatives in South America (at present some are still incorrectly placed in Anabarhynchus). Ectinorhynchus is recorded from Australia and New Zealand, but not Papua New Guinea or New Caledonia; the only genus with this distribution pattern. Closely related to Ectinorhynchus, Squamopygia is represented by only one described and one undescribed species from Tropical North Queensland and Papua New Guinea. Agapophytine genera such as Agapophytus and Acraspisa are very species-rich in Australia and are also found in Papua New Guinea and Indonesia, respectively (Winterton et al. 2001, Winterton and Irwin 200).

References
Ayal Y. (1998). Arthropod communities in deserts are top-down controlled: An example from the central Negev, Israel. Israel Journal of Zoology 44, 68-92.

English K. (1950). Notes on the morphology and biology of Anabarrhynchus fasciatus Macq. and other Australian Therevidae. Proceedings of the Linnean Society of NSW 75, 345-359.

Hartman M., Irwin M.E. and Kampmeier G.E. (1995) Habitat partitioning by therevids at Sand Ridge State Forest. January Report. Illinois Natural History Survey, Champaign-Urbana.

Irwin M. (1976). Morphology of the terminalia and known ovipositing behaviour of female Therevidae (Diptera: Asiloidea), with an account of correlated adaptations and comments on phylogenetic relationships. Annals of the Natal Museum 22, 913-935.

Irwin M. and Lyneborg L. (1981a). The genera of Nearctic Therevidae. Bulletin of the Illinois Natural History Survey 32, 193-277.

Irwin M.E. and Lyneborg L. (1981b) Therevidae. Manual of Nearctic Diptera. (ed. by B.V.P. J.F. McAlpine, G.E. Shewell, H.J. Teskey, J.R. Vockeroth & D.M. Wood) 1, pp. 513-523. Research Branch, Agriculture Canada Monograph, Ottawa.

Lyneborg L. (1992) Therevidae (Insecta: Diptera). Fauna of New Zealand/ Ko te Aitanga Pepeke o Aotearoa 24, pp. 139. DSIR Plant Protection/ Te Wahanga Manaaki Tupu, Auckland.

Power N. (1998) Temporal and spatial diversity of Therevidae in South East Queensland (Diptera: Therevidae). Honours thesis, University of Queensland.

Winterton S.L. and Irwin M.E. (2001). Phylogenetic revision of Agapophytus Guérin (Diptera: Therevidae: Agapophytinae). Invertebrate Taxonomy 15, 467-526.

Winterton S.L., Irwin M.E. and Yeates D.K. (1999). Phylogenetic revision of the Taenogera Kröber genus-group (Diptera: Therevidae) with descriptions of two new genera. Australian Journal of Entomology 38, 274-290.

Woodley N. (1989) Phylogeny and classification of the "Orthorrhaphous" Brachycera. Manual of Nearctic Diptera. (ed. by J. McAlpine, Wood, DM) 3, pp. 1371-1395. Research Branch Agriculture Monograph No. 32. Canadian Government Publishing Centre, Hull.