Ocypodomyia, Pont, 2006

Pont, Adrian C., 2006, Ocypodomyia gen. n. from the seashores of the Afrotropical region (Diptera: Muscidae: Limnophorini), African Invertebrates 47, pp. 321-334 : 321-325

Ocypodomyia gen. n.

Etymology: From Ocypode , a genus of ghost and sand crabs, and -myia (Greek, a fly). The name reflects the association between these flies and Ocypode crabs.

Type-species: Ocypodomyia stuckenbergi sp. n., by present designation.

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Systematic position: A genus belonging to the tribe Limnophorini of the subfamily Coenosiinae : frontal vitta without setae; anepimeron bare; lower calypter of the Phaonia - type, i.e. narrow and tongue-like; prealar seta absent; lower proepimeral setulae directed upwards; katepisternal setae not arranged at the points of an equilateral triangle; male hypandrium cup-like, not elongated and tube-like; female ovipositor with the cerci adpressed against the terminal membrane, not free-lying and projecting beyond epiproct and hypoproct.

Diagnosis: Ocypodomyia may be immediately recognised by a number of autapomorphies: the structure of the frons, which in both sexes is broad, entirely silvery and without apparent division into fronto-orbital plates, frontal vitta and ocellar triangle, and with all the setae greatly reduced, more like setulae than setae ( Figs 1, 2 ); the male genitalia which have two striking features, namely the broad and spade-like praegonite, and the pair of large rounded bulbous ventral prolongations on the hypandrium ( Figs 3, 6 ); the female ovipositor in which sternite 7 is atrophied, reduced to a pair of hindmarginal bristle-dots ( Fig. 9 ).

Description: Entire head except for eyes and antennae silvery-white pruinose. Frons short and very broad in both sexes, profrons high ( Figs 10–12 , 17 , 20 ). Frons uniform in appearance, with a differentiation into fronto-orbital plates, frontal vitta and ocellar triangle almost undetectable ( Figs 2 , 13 , 21 ). Head setae reduced and setula-like ( Figs 1, 2 , 13 , 21 ). Lower facial margin narrow, the vibrissae unusually close together ( Fig. 11 ). Prementum of proboscis glossy, undusted. Prosternum with or without setulae. Proepisternal depression with or without setulae. Katepisternals 1+2. Meron with or without a few short setulae below spiracle. Mid femur with 0 anterior and 2 posterior preapical setae. Hind tibia with 1 dorsal but 0 anterodorsal preapical seta. Veins bare, but vein R 4+5 sometimes with a few setulae at base. Sternite 1 with setulae at sides. Male hypandrium with a pair of large rounded bulbous prolongations ventrally ( Figs 4, 5 ), phallapodeme and epiphallus well developed, praegonite large, spade-like, postgonite curved, without setulae, aedeagus long and slender, the tip membraneous, without trace of any armature ( Figs 3, 6 ). Female ovipositor ( Figs 7–9 ) with tergites 6–8 present as paired sclerites, complete; sternite 7 reduced to a pair of hind-marginal bristle-dots; epiproct, hypoproct and cerci without spines, only with setulae.

Discussion: These two species are remarkable in their general appearance, being small and almost wholly silvery-white in colour. The characters of O. stuckenbergi place it very close to the genus Spilogona , and at first I considered it to be a highly derived species of that genus. The characters of the second species ( O. africana ) could place it in the genus Limnophora Robineau-Desvoidy , and again, I originally considered it to belong to that genus. My original conclusion was that the resemblance in external characters was homoplaseous and the result of similar habits, namely living on the seashore in the burrows of Ocypode crabs. However, study of the male and female genitalia immediately showed that the resemblance between these two species was not homoplasy but did indeed indicate a very close relationship.

In the male genitalia of both species of Ocypodomyia , the pair of large rounded bulbous ventral prolongations of the hypandrium is a unique feature within the Limnophorini , as is the large and spade-like praegonite ( Figs 3–6 ).

Spilogona Schnabl is a large and probably paraphyletic genus at the base of the tribe Limnophorini . In my view, an elongated female ovipositor is a ground-plan character of the tribe and of Spilogona , and among many species of Spilogona and in the other limnophorine genera there is a progressive shortening of the ovipositor, with concomitant modification of the tergites and sternites. An ovipositor such as that of Spilogona nitidicauda Schnabl (Holarctic) is probably close to the ground-plan of the Limnophorini —tergites 6, 7 and 8 are represented by paired long plates, sternites 6 and 7 by long plates, and sternite 8 by a pair of small platelets. In Ocypodomyia ( Figs 7–9 ), tergite 7 is reduced to more rod-like structures and tergite 8 is reduced to two narrow transverse plates that extend down the side of the ovipositor, thus giving additional rigidity to the terminal part of this shortened organ. Sternite 7 is reduced to two hind marginal bristle-dots.

Two Palaearctic genera of the Limnophorini , Neolimnophora Schnabl & Dziedzicki and Villeneuvia Schnabl , together include three species that are found on the shores of the North Sea and the Mediterranean Sea. They are much larger and darker species, and share certain apomorphies with Ocypodomyia (e.g. lower facial margin narrow, and vibrissae close together) but the differences are much greater (meron and sternite 1 bare, 4 postsutural dorsocentral setae, head structure and setation very different).

Some of the morphological features (large head, reduced eyes, large antennae, reduced head setae) are adaptations by species to life in burrows and tunnels. For example, the reduction in eye size is a recognised feature of Muscidae and Anthomyiidae associated with the burrows of marmots ( Marmota marmota Linnaeus ) in the European Alps ( Pont & Ackland 1995). The strikingly pale body appearance and the broad silvery frons are typical features of many seashore Diptera (e.g. Asilidae , Therevidae , Ephydridae , Chamaemyiidae , Anthomyiidae , Muscidae ). The densely silvery pruinose frons of Ocypodomyia resembles macroscopically the frons of the seashore flies discussed by Bethmann and Haupt (2005), who showed that this consists of a dense layer of microtrichia which are of importance for enhanced light scattering and thermoregulation under the harsh conditions of heat and radiation in the seashore environment.

Dr Stuckenberg (pers. comm.) kindly supplied the following information on his captures of these flies and their association with Ocypode crabs:

Ocypode species live as scavengers on the upper beach levels, and have special morphological adaptations that enable them to extract water from damp sand for storage in their gill cavities, thereby allowing them to live permanently out of water. Each crab excavates a burrow downwards into the sand, which it maintains and defends; the burrows usually terminate at the water-table level in the sand, and may go to a depth as much as one metre. The Ocypodomyia flies were observed to rest on the shaded sides in the burrows; they flew out when closely approached, and were easily caught by quickly placing a net over a burrow entrance. The burrows provide uniformly stable, cool and humid microhabitats, suited to such small flies living in the demanding environment of open beaches. The considerable surface area presented by the burrow walls could provide not only sheltered domiciles, but possibly also breeding sites. The fly larvae may exploit the resource of interstitial meiofauna, such as the abundant minute nematode worms and harpacticoid copepods.

Dr C.E. Dyte (pers. comm.) described his experience of collecting Ocypodomyia in Gambia as follows:

They were not very common, and only seen when disturbed, e.g. when a little sand was thrown in the Ocypode burrow, they flew out. However they were rapid fliers and were easily missed with a net. I found the best way to capture them was to hold the net in one hand with the frame horizontally, and with the fingers of the other hand lift up the apex of the net-bag so as to make a ‘tent’. When this ‘tent’ was lowered over the burrow entrance with a sharp smack the flies, if present, flew out into the ‘tent’ and were secured.

The predacious habit of the adults can be inferred from the structure of the proboscis which is typical for predaceous adults of the Limnophorini ( Werner & Pont 2006) : fulcrum slightly longer than theca; theca when dissected broad, bare except for a few setulae at base and at tip, when dry almost forming a closed tube with the labrum; discal sclerite U-shaped, but discal tooth not developed; 9 pairs of pseudotracheae; 4 pairs of short, stout and strongly sclerotised prestomal teeth.

The larvae may be presumed to be obligate carnivores like those of all other known Limnophorini ( Skidmore 1985) . It is probable that they live in damp sand at the base of the crab burrows in which the adults were found, as suggested by Dr Stuckenberg. This is certainly suggested by two females ( Senegal and Gambia) which have the ptilinal sac still largely extruded and with several grains of sand stuck to it .

Figures

Figs 1, 2. Male frons, dorsal view: (1) Ocypodomyia stuckenbergi sp. n. (Madagascar, Sainte-Luce); (2) Ocypodomyia africana sp. n. (Angola, Cuanza R.). Scale bar = 1 mm.

Figs 3–6. Male genitalia of species of Ocypodomyia gen. n.: (3–5) O. stuckenbergi sp. n. (Madagascar, Ambohitantely): (3) aedeagus, lateral view, (4) hypandrium, anterior view, (5) aedeagus, ventral view; (6) O. africana sp. n. (Senegal, Plage Somone), aedeagus, lateral view. Abbreviations: a – praegonite, b – postgonite, c – phallapodeme, d – epiphallus, e – distiphallus, h – hypandrium, hp – paired hypandrial prolongations. Scale bar = 0.5 mm.

Figs 7–9. Ovipositors of species of Ocypodomyia gen. n.: (7, 8) O. stuckenbergi sp. n. (Madagascar, Sainte- Luce): (7) dorsal view, (8) ventral view, sternite 8; (9) O. africana sp. n. (Angola, Cuanza R.), ventral view, sternites 6, 7. Abbreviations: tg – tergite, st – sternite. Scale bar = 1 mm.

Figs 10, 11. Male head of Ocypodomyia stuckenbergi sp. n. (Madagascar, Sainte-Luce): (10) head, lateral view; (11) face, frontal view. Scale bar = 1 mm.

Figs 12, 13. Female head of Ocypodomyia stuckenbergi sp. n. (Madagascar, Sainte-Luce): (12) head, lateral view; (13) frons, dorsal view. Scale bar = 1 mm.

Figs 17–19. Ocypodomyia africana sp. n. (17) male head, lateral view (Angola, Cuenza R.); (18) SEM of male head, frontal view (Gambia, Tanji Bird Reserve); (19) male sternite 5, dorsal view (Senegal, Plage Somone). Scale bars = 0.5 mm.

Figs 20, 21. Female head of Ocypodomyia africana sp. n. (Angola, Cuanza R.): (20) head, lateral view; (21) frons, dorsal view. Scale bar = 0.5 mm.