Nixonia, MASNER, 1958

Johnson, Norman F. & Masner, Lubomír, 2006, Revision of World Species of the Genus Nixonia Masner (Hymenoptera: Platygastroidea, Scelionidae), American Museum Novitates 3518 (1), pp. 1-32 : 3-7

NIXONIA MASNER

Nixonia Masner, 1958: 101 . Original description. Type: Nixonia pretiosa Masner , by monotypy and original designation. For subsequent taxonomic literature see Johnson (1992) or The Genera of Scelionidae ( Hymenoptera : Platygastroidea) of the World (http://atbi.biosci.ohio -state.edu:210/ hymenoptera /eol_scelionidae.home).

DESCRIPTION: Medium to large, length 4.0– 9.0 mm, robust, cylindrical, moderately elongate (figs. 2, 3); predominantly black in color, sometimes with mesosoma ( N. pretiosa ) or metasoma ( N. corrugata , N. pecki ) reddish in part, or with yellow markings ( N. flavocincta ); body with coarse surface sculpture and matte microsculpture; macropterous; with relatively low degree of secondary sexual dimorphism.

Head subglobose, moderately transverse when viewed dorsally (fig. 3); vertex rounded, hyperoccipital carina absent; occipital carina variably developed; lateral ocellus close to or contiguous with inner orbit of compound eye; compound eye large, bare; frons convex, without frontal depression, median longitudinal carina absent; head without raised interantennal process between toruli; torulus opening anteriorly; submedian carina absent; orbital carina absent; lower frons without fanlike striae; interocular space variable in width (figs. 7, 12, 13); inner orbits weakly to strongly divergent ventrally; clypeus short, broadly transverse, flat to weakly depressed, postclypeus and anteclypeus differentiated in some species by change in sculpture: anteclypeus smooth; malar sulcus absent; gena broadly rounded, convex; labrum sometimes partly exposed; mandible usually broad, usually bidentate (figs. 10, 11, unidentate in N. bini ), teeth transversely oriented; maxillary palpus 5-segmented; labial palpus 2-segmented; antenna 14-merous in both sexes; radicle inserted apically into A1, more or less parallel to longitudinal axis of A1; without expanded apical clava in female; gustatory sensilla on female antenna arranged in transverse pairs on apical antennomeres; claval formula A5-A14/ 2-2-2-2-2-2-2-2-2-2; male antenna with tyloid on A4–A5, sometimes on A6–7, rarely with rudiment on A8.

Mesosoma cylindrical (figs. 2, 3), in dorsal view longer than wide, in lateral view deep, somewhat flattened dorsally; pronotum in dorsal view strongly developed laterally, anterolateral corners rounded; transverse pronotal carina absent; epomial carina absent; pronotal humeral carina absent; anterior face of pronotum with pair of deep transverse pilose grooves near anterior margin, these often accumulate white exudate (e.g., fig. 35); lateral face of pronotum largely convex, without scrobe for reception of foreleg; netrion very broad, as wide as tegula, fusiform, open ventrally; anterior margin of mesoscutum meeting pronotum dorsally; mesoscutum semioval in outline; parapsidal lines often present; notauli absent; skaphion absent; transscutal articulation well developed, simple, not crenulate; scutellum semicircular in outline to slightly wider than long, unarmed, convex, sometimes with weak medial depression; axilla well developed; metanotum narrow, dorsellum weakly differentiated, unarmed; dorsal surface of propodeum with dense, fine pilosity; median keels of propodeum raised and fused to form a single strong medial tooth, tooth varying in shape from pointed to ligulate or cordate, apex of tooth turned downward and pointing posteriorly, parallel to longitudinal axis of body; lateral plicae on propodeum strong; posterior face of propodeum smooth, glabrous; foramen of propodeum subcircular in cross-section, opening on and occupying ventral half of posterior surface of propodeum; mesopleuron very large, prominent; mesopleural depression well developed: broad, deep; mesopleural carina absent; sternaulus absent; mesopleural pit absent; anterior margin of ventral portion of mesepisternum straight, not protruding between forecoxae; mesepisternum and mesepimeron separated by line of well-developed foveae; episternal foveae absent; dorsal corner of mesepimeron produced into strong, acute tooth directed posteriorly; anteroventral portion of metapleuron rounded, not separated from lateral face by carina; metapleural pit absent; posterior margin of metapleuron not produced; metapleuron separated from propodeum dorsally and posteriorly by deep, sigmoid groove; posterolateral corners of propodeum not projecting posteriorly; legs relatively short, strong; posterior surface of hind coxa with fine transverse striae; femora not incrassate; trochantellus present on all legs; outer surface of tibia with fine to strong spines; tibial spur formula 1-2-2, outer spur shorter than inner on mid-, hindleg; hind tibia with two longitudinal fine keels; tarsal formula 5-5-5; tarsomeres tapering in width apically; hind tarsus laterally compressed, higher than wide; hind basitarsus keeled ventrally and laterally, diamond-shaped in cross-section; pretarsal claws simple; forewing not extending to apex of T6, moderately to strongly infuscate, marginal cilia very short; R straight, extending through basal 0.4 of length of forewing, clearly forked apically, without large bristles; R 1 short, not reaching costal margin, therefore without true marginal vein, costal margin of forewing usually folded longitudinally to give impression that R 1 reaches wing margin; r-rs (stigmal vein) longer than R 1, slightly turned downward apically; no other tracheate veins in forewing; apical portion of forewing with six pigmented, convex longitudinal folds in position of R, M, Cu; hindwing with R tracheate at least in its basal half, never reaching hamuli and costal margin; no bristles on R; three hamuli present; cilia on posterior margin of hindwing longer than marginal cilia of forewing.

Metasoma (figs. 2, 3) cylindrical, slightly to distinctly longer than head and mesosoma combined; all segments, except apical segment in male, subequal in length, first segment slightly the longest; female with six terga and sterna visible externally, male with seven terga and sterna visible; male T7 much shorter than preceding segment, apically rounded, reflexed to almost vertical orientation; submarginal ridge well developed, defined by narrow laterotergites, laterosternites absent; no spiracles visible; anterolateral corners of terga with finely pilose ‘‘collecting area’’; female T6 apically emarginate, without medi- an raised field of microsetae or secretion; S1 flat, not laterally compressed, not extending anteriorly between hind coxae, anterolateral corner with finely pilose ‘‘collecting area’’; anterior margin of S2 straight; felt fields on sterna lacking.

DIAGNOSIS: Nixonia is superficially similar to many genera in the tribe Scelionini , particularly Scelio Latreille , and to the large, elongate species of the genera Macroteleia Westwood and Triteleia Kieffer. From all of these it is easily distinguished by the 14- merous, nonclavate antennae; the strong medial propodeal tooth; and the presence of two tibial spurs on both the mid- and hind tibiae.

GEOGRAPHIC DISTRIBUTION: The greatest concentration of species of Nixonia is found in southern Africa, but within that continent the genus ranges north to Egypt, Kenya, Somalia, Rwanda, and the Democratic Republic of Congo (former Zaire). A single species, N. krombeini , is found in in Sri Lanka, northern India, and Southeast Asia ( Thailand, Laos, and Vietnam).

BIOLOGY: Only one species has been reared: Nixonia watshami , n.sp. parasitizes the eggs of Acanthoplus discoidalis ( Orthoptera : Tettigoniidae , Hetrodinae ). The subfamily Hetrodinae is endemic to Africa and adjacent areas of the Arabian Peninsula (Otte et al., 2005). Hence, the host of at least N. krombeini in tropical Asia must be some other large orthopteran.

RELATIONSHIPS: The discussions of the relationships of the genus Nixonia that appear in the published literature are fairly superficial. The primary concern of authors has been to differentiate Nixonia from other genera, rather than to analyze its position within the family. Diagnosis is a task easily accomplished as it is the only living platygastroid with 14-merous antennae and is one of the few with a tibial spur formula of 1-2-2. These are, however, plesiomorphic characters. The monophyly of the genus is strongly supported by the arrangement of multiporous basiconic sensilla on the female antenna. These occur in transverse pairs on the apical antennomeres (figs. 29, 44, 72), rather than the typical longitudinal orientation in all other platygastroids. Additional characters supporting monophyly are the presence of a median propodeal tooth, the absence of a malar carina, and the longitudinal tibial carinae.

Previous authors have generally considered Nixonia as the most plesiomorphic of the living scelionids ( Masner, 1958; Kozlov, 1970; Masner, 1976; Kozlov and Kononova, 1990), implicitly placing it as the sister group of all other extant members of the family. The Eocene fossil genus Archaeoscelio lacks laterotergites on the metasoma, a characteristic of all other described platygastroids, including Nixonia . Archaeoscelio is also significant because of its 14-merous antennae. Thus, some basal rearrangement of the intuitive scelionid phylogeny soon may be warranted. Austin and Field (1997) depicted Nixoniini as one branch in the basal polytomy of 19 clades for the Scelionidae . Although their cladogram is only one possible resolution—it is not a consensus tree of all cladograms discovered—it suggests that Scelionidae is not a monophyletic family: two genera of the tribe Sparasionini cluster with the family Platygastridae . Thus, the hypothesis that Nixonia is the sister group to all other scelionids is open to question.

Determination of the closest relative of Nixonia is impossible at this time. The Australian endemic genus Neoscelio Dodd also has the upper posterior corner of the mesepimeron produced backward, but the value of this character has not been tested in a comprehensive analysis. The truncated R in the hindwing is shared with the tribe Scelionini . All of the species of this tribe, as far as is presently known, parasitize the eggs of Acrididae deposited in the soil. Similarly, the one known host record for Nixonia is also an orthopteran that deposits egg masses in the soil.

NOTES: Masner (1976) stated that specimens of Nixonia are the largest known scelionids. With further collecting, we must correct this. The largest Nixonia , N. gigas , is 9.0 mm in length, but this is exceeded by some undescribed species of Triteleia .