Stenobasipteron Lichtwardt, 1910

Barraclough, David A., 2005, A review of the type material of the Southern African genus Stenobasipteron Lichtwardt, 1910 (Diptera: Nemestrinidae), with transfer of two species to Prosoeca Schiner, 1867, Zootaxa 1094 (1), pp. 41-51 : 43-46

Stenobasipteron Lichtwardt, 1910

Stenobasipteron Lichtwardt, 1910: 614 . Type species: Stenobasipteron wiedemanni Lichtwardt, 1910 , by designation of Bernardi (1973: 259).

Stenobasipteron is a genus which for almost 80 years has been considered to include five species ( Bowden 1980). Bequaert (1925a) was the last worker to revise the genus; in the same paper he described two new species from Zimbabwe. In a subsequent paper ( Bequaert 1925b) he provided a revised species key, this necessitated by misidentifications in his earlier work. Despite this taxonomic treatment, Bequaert left the genus in a taxonomically confused state, and without a workable identification key to species. His publications were flawed by lack of recourse to the original type material.

Even though Stenobasipteron and Prosoeca appear to be readily identifiable generic concepts in the literature, in reality it can be extremely difficult to distinguish them based on previously used characters. Further study, particularly of the male and female postabomen, may reveal other characters, but I believe that this is unlikely (preliminary dissections of several species have already been undertaken by me). Bernardi (1973) was the most recent worker to discuss the distinctions between the two genera; key data provided by earlier workers such as Bezzi (1924) are either misleading, incorrect or redundant.

The characters Bernardi used were: 1) whether the wings are infuscated or not, 2) wing shape, 3) development of the costa, and 4) development of the alula at the wing base. None of these characters can meaningfully separate the genera, even when selectively combined; in fact Bernardi (1973: 259) conceded that the genera were very similar. Bernardi considered that the wing is hyaline in Prosoeca and strongly infuscated in Stenobasipteron . However, many Prosoeca species have variably patterned wings (in some species the markings are striking) and not all Stenobasipteron ­like species have a uniformly infuscated wing. Similarly, wing shape and costal development are of no real taxonomic significance; in Stenobasipteron the costal margin has a prominent anterior flexure in one species (male only), but in other species there is either no flexure at all or the flexure is only moderately developed. In several species of Prosoeca there is also an anterior costal flexure. The development of the alula—and thus the apparent width of the wing base—is probably the most widely used character used to separate the genera, but even this cannot reliably separate the generic concepts. There appears to be a continuum of variation between the two extremes, with a greatly reduced alula and narrowed wing base in typical Stenobasipteron ( Figs 1–3 ), and a relatively well developed alula and thus an apparently broader wing base in typical Prosoeca ( Fig. 4 ). The distinction between the two character states is further complicated by the furling of the alula in some specimens, which sometimes makes a wing base appear to be narrower than it otherwise would be.

Despite the difficulty in distinguishing the two genera, I consider it to be taxonomically pragmatic not to sink Stenobasipteron as a junior synonym of Prosoeca . The validity of Stenobasipteron is upheld for three main reasons. Firstly, at the very least, species of Stenobasipteron appear to represent a reasonably coherent grouping, although not yet demonstratably monophyletic. Secondly, Prosoeca is currently under review and finality on generic limits cannot be reached prior to a comprehensive revision of Prosoeca . Finally, the Stenobasipteron generic concept is currently widely used in the pollination biology literature, and I prefer to uphold it to preserve nomenclatural stability.

Provisional diagnosis

As stated above Stenobasipteron does appear to represent a group of related species, even though it is difficult to characterise. This group is here defined by the following combination of character states:

Striking body markings absent. Head with frons and ocellar protuberance not prominent in profile; proboscis 1.0 to 2.0 times body length. Wing never completely hyaline or with prominent dark markings, usually appearing mostly brown infuscate, although infuscation sometimes paler towards posterior margin. Wing base markedly narrowed, this resulting from a marked reduction in alula size ( Figs 1–3 ). Abdominal terga with posterior half of T2 and the entire surfaces of T3 and T4 with short recumbent vestiture; long, erect hairing almost always absent (a few apicomedial hairs rarely present on T2 and longer hairs occasionally present on lateral margins of all three terga).

Figures

FIGURES 1–4. Wings of Stenobasipteron and Prosoeca. 1–3. Stenobasipteron. 1. S. arnoldi, female holotype. 2. S. gracile, male syntype. 3. S. wiedemanni, male syntype (bulla at base of first longitudinal vein arrowed). 4. Prosoeca umbrosa, male syntype.