Morphology, Taxonomy, Distribution and Relationships of the Afrotropical Genus Isomerocera (Diptera: Stratiomyidae)

ABSTRACT The external morphology of Isomerocera Enderlein, an Afrotropical genus of Stratiomyidae, is discussed and illustrated with special regard to specific structures on the antenna, surface cover, and male and female terminalia. The species of Isomerocera are re-examined, based on 524 specimens (including four types) from 15 institutions, and two species were distinguished. The rare species I. heteraspis James, 1949 is characterised by some unusual autapomorphic characters. The common I. quadrilineata varies remarkably in colour characters, and the extent and density of hair patches. Isomerocera maculiventris (Macquart, 1850) is considered to be a junior synonym of I. quadrilineata (Fabricius, 1787) as is I. natalensis (Gerstaecker, 1857). Isomerocera quadrilineata var. melecta, as originally described by Speiser (1908), is identical with the nominal form. Generic characters are compared with the related genus Ptilocera from the Oriental and Australasian regions. Distribution data are summarised on the basis of the reliable published records and/or extensive material that we examined and collected in Angola, Burundi, Cameroon, Central African Republic, Democratic Republic of the Congo (Kinshasa), Equatorial Guinea, Gabon, Ghana, Ivory Coast, Kenya, Liberia, Malawi, Mozambique, Nigeria, Republic of the Congo (Brazzaville), Sierra Leone, South Africa, Tanzania, Togo, Uganda and Zimbabwe. I. heteraspis is known only from two localities in Burundi and Uganda. The known distribution of I. quadrilineata essentially covers the vast area of savannahs and the zone of lowland tropical moist broadleaf forests in Africa.


INTRODUCTION
The genus Isomerocera was originally erected by Enderlein (1914: 302) to include the single species Diphysa maculiventris Macquart, 1850. Many authors (e.g. Gerstaecker 1857Loew 1860;Grünberg 1915;Curran 1928) treated the African Stratiomys quadrilineata Fabricius, 1787 under the name Ptilocera Wiedemann, 1820, with the Oriental species P. quadridentata Fabricius, 1805 as the type species. Brauer (1882) in his "Identification key to the genera of Notacantha" noted under Ptilocera the distribution as "Ost-Indien, Afrika". Grünberg (1915) mentioned two forms of P. quadrilineata: one distributed in West Africa and based on the type originating from Sierra Leone and the second occurring in South Africa and described by Loew (1860) under the same name. Both forms were reported to differ by the colour of the last flagellomere. Kertész (1916) definitively proved that Ptilocera nataliensis Gerstaecker, 1857 is actually a mere synonym of Diphysa maculiventris Macquart, 1850. He compared distinguishing characters of Isomerocera and Ptilocera and came to the conclusion that they represent different genera (see also Mason & Rozkošný 2011). James (1949b) described the second Afrotropical species, I. heteraspis. In Lindner's series of taxonomic and faunistic studies from the Afrotropical Region , 1953, 1958a, 1958b, 1961, 1965a, 1966b, 1968, 1970, 1972Lachaise & Lindner 1973) he consistently identifies the examined Isomerocera specimens as I. quadrilineata; only in a paper from 1961, based on material from South Africa, did he use the name I. maculiventris with a misleading note that I. natalensis and I. quadrilineata are synonyms of it. Actually, Isomerocera maculiventris and I. natalensis are junior synonyms of I. quadrilineata. Lindner (1966b) asked L. Tsacas, then custodian of the dipteran collection in the Museum of Natural History in Paris, for a comparison of I. quadrilineata with the holotype of I. maculiventris. On the basis of Tsacas's expert opinion Lindner proposed the synonymy of both names. This was later accepted in "A World Catalog of the Stratiomyidae" .
Head of Isomerocera (Figs 2,30,37,44) holoptic in males and dichoptic in females, transverse in dorsal view and almost hemispherical, higher than long in lateral view, eyes bare in both sexes. Facets in upper part of male eye distinctly larger than in lower third, male ocellar triangle prominent in lateral view and frons divided by contiguous eyes into upper short part and lower, broader, subtriangular part, both covered with pale appressed hairs. Postocular area not visible in male but well developed in female. Female ocellar triangle is distinctly elevated above the level of eyes and this elevation visible also in front of the anterior ocellus as a preocellar tubercle (Figs 12 a, b). Female frons nearly parallel-sided, slightly narrower than scape is long. Antenna relatively long and slender, about 2-3× as long as head. Scape 3-4× longer than pedicel and pedicel cup-like, distinctly shorter than first flagellomere. Male flagellomeres 3-5 provided with paired finger-like projections, longer ventral and shorter dorsal ones. In females an additional, short ventral projection on flagellomere 2 distinct. Face slightly arched at middle below antennae. Proboscis (Fig. 25) unmodified and palpus two-segmented (Fig. 26).
Thorax without any apomorphic characters at generic level. Prealar prominence ( Isomerocera heteraspis James, 1949 Figs 13, 14, 17, 23, 24, 29-41 Isomerocera heteraspis James, 1949b: 106. Diagnosis: Both sexes of this species can be easily recognised by the scutellar spines located on the posterior margin of the scutellum in two markedly different planes (Figs 13,14). The male has paired projections on flagellomeres 3-5 (Fig. 31) and the female only on flagellomeres 3 and 4 (Fig. 38). lower frons almost equilateral, both covered with white tomentum but part of lower frons above antennae and its tip almost bare. Face brown, swollen below antenna, distinctly prominent as a rounded tubercle in lateral view. Antenna (Fig. 31): scape 3× as long as pedicel, brownish, darker than reddish pedicel. Flagellomeres 1 and 2 with sensory pits, the third only with some scattered pits hidden among short hairs. All flagellomeres dark brown, only last flagellomere snow white. Flagellomeres 3-5 asymmetrical, with paired projections, flagellomere 6 subconical and 7 subcylindrical. Apical flagellomere slender and elongate, twice as long as preceding flagellomere, completely snow white. Thorax: Black, finely punctate, with greyish scales on anterolateral part of scutum. Proepisternum with long silvery hairs, anterior part of anepisternum bare, posterior part covered with greyish shining scales. Katepisternum, meron and the area near the posterior spiracle with long silvery hairs. Proximal half of wing membrane infuscated, darker on distal part of br cell. Legs chiefly brown but coxae black, anteriorly with long silvery hairs. Distal third of mid and hind femur as well as all tibiae black, tarsi pale. abdomen: Black, finely punctate, with appressed silvery hair patches only on tergites 4 and 5 (Fig. 29), venter covered with fine appressed silvery hairs being longer on sternite 1. Male terminalia as in Figs 33a-c. Genital capsule (Fig. 32) with posterior medial process relatively deeply emarginate at middle, tripartite aedeagal complex (Figs 33a-c) slightly longer than the massive lateral aedeagal projections, parameral sheath bipartite as in I. quadrilineata but aedeagal apodeme distinctly dilated proximally.
Upper postocular area covered with three to five rows of silvery, appressed scales (Fig. 17). White tomentose patches on frontal band developed as two semicircular pilose spots on upper part of frons continuing with the brown short hairs around the ocellar triangle and as a pair of medially divided spots in middle of frons (Fig. 37). Antenna (Fig. 38) predominantly dark. Scape dark brown, pedicel and flagellomeres 1-2 pale brown, flagellomeres 3-7 dark brown, apical half of last flagellomere snow white. Sensory pits present on both sides of flagellomeres 1-5. Projections on flagellomeres 3-4 asymmetrical, shorter than in male. Face brown with low and rounded nose-like projection, bare on top, covered with silvery hairs along eye margin. Thorax: Black, finely punctate. Pile and scales on thorax as in male but generally with longer shining scales. Scutum with two metallic lines consisting of scales with purple reflections. Scutellum twice as wide as long with four spines (Figs 13, 14), lateral ones smaller and medial ones larger, brownish yellow with darkened tips. Proximal half of wing membrane darkened, blackish on br cell, as in males. Legs (Figs 39-41) orangeyellow and black. Coxae, mid femur except apex and almost entire tibiae darkened, all tarsi pale. abdomen: Black, finely punctate, with appressed silvery hair patches on margins of tergites 2-5 ( Fig. 36), venter with fine appressed short silvery hairs on sternites 2-5 and longer on sternite 1. Genital furca not examined. Condition: Very good, but the costal vein broken on both wings and the body with some rusty incrustation. The male genitalia dissected and attached on the same pin. Remarks: The unusual position of the scutellar spines (in two different planes) is also known in all three species of the Afrotropical pachygastrine genus Diplephippium (see James 1949a, Lindner 1966a). However, Diplephippium differs from Isomerocera by the absence of finger-like projections on flagellomeres, its scutum is provided with a long and pointed spine in front of the wing base and the wing pattern in both genera is different. Diagnosis: Scutellar spines located on the posterior margin of the scutellum in one plane (Figs 15, 16). The female with paired antennal projections on flagellomeres 3-5. male 15,(25)(26)(27)(42)(43)(44)(45)(46)(47)(48)(49)(50)(51)(52)(53)(54)(55)(56)(57)(58)(75)(76)(77)(78)(79)(80)(81)(82). Description: Length (mm): Body 5.3-10.2 (n = 24); wing: 4.8-9.9 (n = 24). Head: Nearly hemispherical in lateral view, about 1.6× higher than long, transverse in dorsal view (Fig. 42) and 1.6× as wide as long. Large eyes covering greater part of head, contiguous on frons for long distance, gena and postocular area not visible in lateral view. Ocellar triangle distinctly prominent in lateral view, no preocellar tubercle developed. Facets in lower third contrastingly smaller than upper ones. Upper frons elongate, triangular, barely as long as ocellar triangle, lower frons subtriangular and equilateral, both covered with snow-white tomentum. Tomentose area on lower frons with longitudinal black midline. Face somewhat swollen below antenna but this fairly flat tubercle barely visible in lateral view (Fig. 44). Lower face deeply concave. Face covered with short whitish hairs, these being longer below antennae and continuing as narrow whitish tomentum along eye margin. Antenna (Figs 45,46) slender and long, about twice as long as head in lateral view. Basal antennal segments and two basal flagellomeres reddish brown to orange, scape 4-5× as long as broad distally, pedicel cup-like, as long as broad. Flagellomeres 3-7 dark brown to black. Flagellomeres 3-5 with paired projections, upper projections short and lower projections twice as long as upper projections. Last flagellomere entirely white or blackish basally. Sensory pits small but distinct on basal two flagellomeres. Proboscis brown, two-segmented palpus slender and black. Thorax: Dark, black in ground colour, scutum covered with dark short appressed hairs and indistinct metallic iridescent scales. Two paler and narrow medial lines are more or less distinct in presutural area of scutum covered by greenish, purple or grey scales, usually widening toward scutellum. Posterior area of scutum beyond postalar callus with tuft of long whitish hairs. Scutellum with four yellow to brown spines, inner pair longer than outer, middle spines about as long as scutellum. Proepisternum, proepimeron and anterior part of anepisternum with long silvery hairs, narrow middle part of anepisternum bare and shining, but its posterior part covered by silvery and almost purple hairs and scales. Anepimeron, katepisternum, katatergite and meron silvery haired. Mediotergite bare, black and mostly matt. Anterior spiracle as in Fig. 20, posterior spiracle as in Fig.  19 and prealar prominence as in Fig. 21. Wing as in female (Figs 1, 4). Legs yellow or pale brown with extremely variable dark pattern. Coxae usually blackish, femora mostly darkened in apical third and tibiae predominantly darkened, tarsi pale or partly darkened (75)(76)(77)(78)(79)(80)(81)(82).
Variation: In addition to size (see above), both sexes may vary apparently in the pattern of scales on the scutum, and in the extent of the abdominal hair patches. The last flagellomere may be entirely white or blackish basally up to the middle. In some females the frontal velvety patches may vary in large extent. The semicircular lateral hair patches on the upper frons along the eye margin (Figs 62, 63) may be more or less developed or even completely absent even in females belonging to the same population. The narrow lateral frontal hair patch along the inner margin of the eyes may be present or absent. In a sample of 15 females from the same locality the pairs of silvery spots are present in the majority of specimens, but in five specimens these spots were absent. Kertész (1916) considered Isomerocera maculiventris and I. quadrilineata to be distinct species that may be distinguished by the basally darkened last flagellomere and the scutum having a dense, appressed, brassy-yellow pile in front of the scutellum in I. maculiventris, whereas in I. quadrilineata the last flagellomere is mostly wholly white and the scutum lacks pile as described above. Through our examination of a large amount of material we cannot confirm the species-specific character of these features. Both forms display an identical shape of the male and female terminalia. Also, the highly variable colour pattern of the legs does not show any relation to a specific distribution area.
Remarks: Lindner (1953) found larvae along with larvae of Ptecticus sp. (Sarginae) in an old carica papaya tree trunk in Tanzania. Later Lindner (1965b) found that the larvae develop in decaying plants and proposed to include this species among the hemerophilous forms ("Kulturfolger") accompanying plantations of different cultured plants. According to recent data from Togo (A. Ssymank pers. comm.), I. quadrilineata was regularly collected on and/or along plantations producing avocados, aubergines, bananas, coffee and cacao beans. Lachaise and Lindner (1973) found during an ecological survey of the tropical savannah of Lamto (Ivory Coast) that larvae of Isomerocera quadrilineata lived in gallery forests and were reared in a large numbers from a dead fallen trunk of a papaya tree, as do the larvae of Sternobrithes tumidus Loew (Pachygastrinae).

DISCUSSION
Isomerocera and Ptilocera represent apparently sister genera sharing two unique characters among Pachygastrinae: paired slender projections on medial flagellomeres and the well-developed parameral sheath on the male terminalia. The flagellar projections differ between the two genera and between both sexes within them. The male upper projections are partly reduced, shortened on flagellomeres 3-5 in Isomerocera (Figs 31, 45, 46) but slightly shorter only on flagellomere 3 in Ptilocera. An extreme situation was described in P. simplex Mason & Rozkošný, 2011, where flagellar projections are (probably secondarily) entirely reduced. The females of both genera may display an additional unpaired ventral projection on flagellomere 2 ( Fig. 65) but this is absent in I. heteraspis, in which also projections on flagellomere 5 are missing (Fig. 38). In both species of Isomerocera the female antennal projections are of the same, short shape as in the males, whereas in the females of Ptilocera all antennal projections are markedly elongate and relatively long, plumose compared with the males. The scape represents another distinguishing character. It is 3-4× as long as broad in Isomerocera spp. but only at most twice as long as broad in Ptilocera (with exception of both sexes of P. simplex where the scape is 3× as long as broad in males and 2.5× as long as broad in females). The preocellar tubercle as a part of the ocellar triangle elevation is present in females of both species of Isomerocera, but is not present in females of Ptilocera.
The prealar prominence is also known in Ptilocera (see Mason & Rozkošný 2011) and apparently is not too rare among other pachygastrine genera (e.g. Pachygaster, Zabrachia and the Afrotropical Platyna) as well as some genera of Clitellariinae. It is present as a subtriangular prominence in adoxomyia Kertész and as a strong spine in clitellaria Meigen and Nigritomyia Bigot.
The surface pile on the scutum and scutellum is partly transformed to a dense covering of scales (Figs 17,18) that are present in some other species from different genera. It is not precluded that the shape of the scales may be species-specific on some parts of the body but this presumption needs further documentation. The arrangement and shape of different projections and spines on the scutellum is characteristic for a number of different genera of Pachygastrinae but also for some genera of other subfamilies. The four strong marginal spines of I. quadrilineata (Figs 15, 16) is a widely distributed character state (see ) but I. heteraspis shows a more advanced state where four spines are arranged in two different planes (Figs 13, 14). The abdomen indicates a relation to the genera of Pachygastrinae with a rounded and a dorsally convex abdominal shape (Fig. 3) (see Kertész 1916).
The male terminalia found in Isomerocera and Ptilocera seem to be unique within the Stratiomyidae (see Mason & Rozkošný 2011). As we show here, the complex structure above the proper tripartite phallus is derived from the parameral sheath fixed to the inner dorsal side of the gonocoxites by an attachment structure (Figs 6-8) (see Woodley 1986Woodley , 1995Sinclair et al. 1994;Cumming & Wood 2009). The parameral sheath is apparently bipartite (though connected by a hyaline membrane). Another apomorphic structure (at least in the Pachygastrinae) is represented by the pointed lateral projections of the aedeagal complex. Kertész (1916) proposed the presence of paired lateral aedeagal projections as a suitable distinguishing character for Isomerocera and Ptilocera. According to our recent analysis, they are actually not present in Ptilocera spp. except P. simplex. This structure resembles similar projections illustrated in all the European species of Sarginae (Rozkošný 1982) and described by Woodley (1995) in the Neotropical beridine Neoberis brasiliana (Lindner, 1949).
In summary, in our opinion there are no reliable arguments for the synonymy of Isomerocera and Ptilocera, which has been discussed in the literature for more than one hundred years. Apparently Ptilocera simplex with its elongate scape and the welldeveloped lateral aedeagal projections seems to be more related to Isomerocera spp. than to the other species of Ptilocera. On the other hand, the male of this species differs from all other species of both genera by the absence of any flagellar projections, but the female shows no preocellar tubercle and has long, paired projections on flagellomeres 3-5 as the other species of Ptilocera. Moreover, P. simplex is known to occur far from the Afrotropical Region in a limited area (northern India and northern Thailand) near the centre of the Oriental Region. Perhaps some additional information (e.g. a potential comparison of immature stages or molecular analyses) will provide more information on the relationship of both genera under discussion.
Based on the extensive material from different collections examined, I. quadrilineata is reliably reported from 21 African countries (Figs. 89 & 90): Angola, Burundi, Came roon, Central African Republic, Democratic Republic of the Congo (Kinshasa), Equatorial Guinea, Gabon, Ghana, Ivory Coast, Kenya, Liberia, Malawi, Mozambique, Nigeria, Republic of the Congo (Brazzaville), Sierra Leone, South Africa, Tanzania, Togo, Uganda and Zimbabwe. Distribution in the Central African Republic is added on the basis of a reliable published record by Lindner (1970).