Gonioscelis sp.

Londt, Jason G. H., 2004, A review of the afrotropical genus Gonioscelis Schiner, 1866 (Diptera: Asilidae), with descriptions of new species, African Invertebrates 45, pp. 21-124 : 102-104

Gonioscelis sp.

An undescribed species from Somalia has been studied. Unfortunately there are no male specimens and so the species is left undescribed. Suffice it to note that this small species (wing length 6 mm) is a member of the large and widely distributed group of species possessing a poorly defined facial gibbosity, and represents the first record of Gonioscelis from Somalia, the most easterly record for the genus. Apart from noting the following details regarding the two females studied, and including the species in Tables 1 & 2, no further comment is made .

Material examined: SOMALIA: 1^, ‘ Somalia / Mogadishu [02°04'N 45°22'E] / iv-29- 86 / R. Lavigne’; 1^, ‘ Somalia / Mogadishu / v-10-86 / R. Lavigne’ .

DISCUSSION Taxonomy

Although the genus is easily recognised, the characterisation of species is far more difficult. Only a few species can be recognised immediately using superficial morphological characteristics, and most need to be identified using features of the male genitalia. This means that female specimens that are not correctly associated with males from the same locality may be impossible to identify with any certainty. Males can, however, usually be identified relatively easily by studying features of the post-abdomen. While the removal of the terminalia and their maceration greatly assists in exposing details of the genitalia, it is often possible to see the more important features using a lens or microscope without excising the terminalia. While there is no doubt that a number of useful characters may be found associated with the gonocoxite and aedeagus, normally not easily seen without dissection and maceration, identification can be made using the more externally visible epandrium and hypandrium.Although a few species groups can be detected within the genus, these are not defined in this study.

Biology and distribution

Nothing is known about the immature stages of any Gonioscelis species. Imagos usually frequent open habitats in a wide variety of biomes. They are capable of rapid and fairly sustained flight, but usually fly close to the ground and settle on the ground or on low vegetation. Mating takes place with male and female facing in opposite directions in a tail-to-tail position, and eggs are usually deposited into loose soil.

While detailed distributional information is provided for each species, some general comments are appropriate. Gonioscelis is widely distributed (Table 1, Fig. 143 ), being associated mainly with the very extensive Grassland, Savanna and Succulent Karoo biomes. Although label data attached to some specimens suggest a possible association with forests, it is highly unlikely that any species actually inhabit forests. This belief is supported by the fact that the large area occupied by central and west African forests has yielded very few specimens. It is likely that those specimens apparently linked with forests were actually collected in more open habitats adjacent to forest.

Of the 38 species, 30 (79%) are found in southern Africa, most of these being endemic to this subregion, perhaps suggesting that the genus has its evolutionary history centred in this part of Africa. It is particularly significant that of the 22 species occurring in South Africa (i.e. 58% of all species), 13 (59%) are restricted to the ‘west coast’ of the Northern Cape and Western Cape provinces. This region , dominated by Succulent Karoo and Fynbos biomes, is unusual in an afrotropical context in that it receives winter rainfall. Species inhabiting this interesting region are usually encountered in spring (mainly between August and October) when temperatures are relatively cool in comparison with the high temperatures experienced during summer, and are in most instances easily recognised as they tend to possess a combination of rather unique morphological characteristics. Most are fairly darkly pigmented and possess prominent facial gibbosities and black setation. From a thermoregulatory perspective, these features probably serve them well. Within this group of west coast species there is a sub-group (i.e. punctipennis and its allies) of small species that are usually found resting on dark rocks, where they are often difficult to see, rather than on sandy substrates. Four of these species possess spotted wings that may serve as part of their camouflage when they are at rest .

In comparison with these west coast species, those inhabiting summer-rainfall areas tend to be larger and paler, and have flatter facial gibbosities. These species tend to be associated with Grassland or Savanna biomes, where individuals are usually encountered resting on the ground or on low vegetation.

Predation

At the time of writing, there were 1976 prey records in the Natal Museum’s computerised asilid-prey database. Of these, 74 (3.8%) pertain to Gonioscelis . Species (16) for which prey has been recorded are as follows – bykanistes (6), haemorhous (2), iota (1), lacertosus (4), longulus (3), macquartii (1), maculiventris (25), mantis (2), pickeri (6), punctipennis (5), ramphis (1), scapularis (1), truncatus (5), ventralis (6), whittingtoni (1), zulu (1), undetermined species (4). Of the 74 records, 60 (81%) relate to females, while only 14 (19%) are for males. Such gender inequality is reasonably well documented, but not understood. Prey items belong to six orders of insects as follows – Coleoptera 51 (69%), Hymenoptera 11 (15%), Diptera 5 (7%), Hemiptera 4 (5%), Isoptera 2 (3%), Lepidoptera 1 (1%), graphically depicted in Fig. 154 . Most of the Coleoptera are Scarabaeidae (38 – 75%) and many of them are flower-visiting Rutelinae. While it is possible that the spurred prothoracic femora and ventrally setose fore tibiae ( Fig. 1 ) are adaptations for feeding on hard-bodied beetles, as suggested by Oldroyd (1974), many other asilids (particularly species of Promachus , Hoplistomerus , Microstylum and Neolophonotus ) that feed on scarabs do not appear to possess any special adaptations. It is significant that some 30 (79%) of the 38 scarab records were collected in the Western Cape province of South Africa and along the western coastal plain of the Northern Cape. Flower-visiting ruteline beetles can be exceptionally abundant during the short annual flowering period of many plants growing in these winter-rainfall areas of South Africa. Their dominance in the diet of Gonioscelis may, therefore, be purely coincidental.

Figures

Fig. 143. The distribution of Gonioscelis species.

Fig. 154. Recorded prey (insect orders) for Gonioscelis species expressed as percentages of total available records (74) for the genus.

Fig. 1. Gonioscelis ventralis Schiner, 1867. General habitus.