Overview
The Xylophagidae is an ancient family, with a fossil record dating back to the Cretaceous. The family contains over 100 extant species worldwide, distributed predominantly in the Northern Hemisphere. Only four species occur in Australia, and all belong to the genus
Exeretonevra
Macquart. The biological information of adults and all information on the immature stages presented below pertain to
E. angustifrons
.
Field sampling suggests that there is one generation per year, and the discovery of mature larvae in early spring indicates an overwintering larval stage. Adults are most common in summer, and are usually collected from December to February.
Adult
Exeretonevra
are less active than other Diptera, and often seem reluctant to fly. Males are more active than females, and often fly along or across open areas of vegetation. They also appear to bask, placing their pleural region perpendicular to sunlight. Mating has not been observed, however males repeatedly pursue and attempt to couple with other flying insects and other males. Females are weak fliers, and can be easily picked up by hand. They fly for short periods, often fluttering between nearby plants.
Females lay eggs in small sheltered crevices amongst leaf litter on the soil surface. When newly laid, eggs are covered in a creamy-white secretion, which becomes an identical colour to the soil after five minutes. This hardened mucus securely attaches eggs to the substrate. Immediately after oviposition the egg is completely opaque, and continues to be so until 2-4 days before hatching, at which time the developing larva can be seen within. In the laboratory first-instar larvae hatch after 2-3 weeks incubation time.
Upon hatching, first instar larvae immediately burrow down into soil. Mature larvae are found in the surface layer of soil, up to 5cm deep. Feeding has not been observed in larvae, but they are thought to be predacious on other soft-bodied invertebrates, based on mouthpart structure. The pupae are not known.
Description
Adult
Exeretonevra
possess a unique combination of morphological characters, which makes them relatively easy to distinguish from other flies, but has made them difficult to classify. The genus has only recently been placed in the Xylophagidae with certainty, based on larval morphology.
Adults vary from 13 to 20mm in length. One of the most obvious landmarks of the fly wing, the r-m cross-vein, is absent. The base of the radial sector, the area of confluence of M and Rs, M1, and M2, form a composite 'diagonal' vein that is superficially similar to that vein found in the Nemestrinidae. However, the diagonal vein of Nemestrinidae such as
Trichophthalma
Macquart includes a portion of M3, unlike the configuration in
Exeretonevra
which includes vein M2. Adult
Exeretonevra
possess one tibial spur on the foreleg, and two tibial spurs on each of the mid- and hind legs. The dorsal surface of abdominal tergite one bears two glandular areas, one on each posterolateral margin.
Eggs are elongate, 3.5-4.0mm long and 0.5mm wide. The egg's outer surface is ornately sculptured, with a surface of deep pits separated by elevated ridges.
First-instar larvae of
E. angustifrons
are elongate in shape, and are, on average, 4.2mm long. The head capsule is dark brown, conical, and strongly sclerotised. Mouthparts are inconspicuous and confined to the distal tip of the head. Abdominal segment 8 bears a strongly sclerotised, circular, terminal plate which ends in a pair of upwardly directed hook-like processes. This plate surrounds the spiracles. Visible beneath the cuticle on the prothorax are three patches of small, brown, circular spots. Spots are also present on the anterior dorsal surface of the caudal abdominal segment. The remaining body segments are creamy-white in colour. Creeping welts are present on abdominal segments 1-7 as convex areas, continuous around the circumference of the anterior margin of each segment.
Morphology of mature larvae agrees with that of the first instar, except for a few minor differences. The integument on the whole body is more heavily sclerotised. The prothorax is completely covered by small, circular, darkly sclerotised spots beneath the cuticle. The lateral margins of each of the posterior thoracic and first seven abdominal segments bears a small brown spot, representing nonfunctional vestiges of the spiracular system.
Distribution
All four described species are endemic to eastern Australia. They are distributed in eastern New South Wales, ACT, eastern Victoria and Tasmania. There is at least one undescribed species in south-west Western Australia. Xylophagids in Australia are found in a variety of habitats, such as alpine heath, warm temperate rainforest, and wet sclerophyll forest. They have been collected from a variety of altitudes up to 1700m. Larvae have been found in the same rainforest glades from which adults were collected.