Thoron, HALIDAY, 1833

JOHNSON, NORMAN F. & MASNER, LUBOMIR, 2004, The Genus Thoron Haliday (Hymenoptera: Scelionidae), Egg-Parasitoids of Waterscorpions (Hemiptera: Nepidae), with Key to World Species, American Museum Novitates 3452, pp. 1-16 : 2-5

THORON HALIDAY

Thoron Haliday, 1833: 271 . Original description. Type: Thoron metallicus Haliday , by monotypy. For subsequent taxonomic literature see Johnson (1992).

Neothoron Masner, 1972: 837 . Original description. Type: Neothoron lautus Masner , by monotypy and original designation. NEW SYNONYMY.

Masner (1972) proposed Neothoron and distinguished it from Thoron principally on the basis of the longer postmarginal vein in N. lautus . Later, Masner and Huggert (1979) referred to additional differences in the structure of the propodeum, the armature of the dorsellum, and the position of the lateral ocelli. With a substantially greater number of both specimens and species presently before us, we find that none of the character states above serves as a distinguishing character between Neothoron and Thoron . The distance of the lateral ocellus from the inner orbit varies from less than one diameter to more than two diameters, and it principally distinguishes only T. metallicus Haliday. The length of the postmarginal vein intergrades from being nearly absent to being distinctly longer than the stigmal vein. Species with a long postmarginal vein may have the propodeum deeply excavate. Similarly, most species that would be classified in Neothoron have the dorsellum clearly armed. We conclude that these character states are not correlated and thus cannot justify the recognition of two genera.

We currently cannot effectively argue whether Neothoron represents a monophyletic group. The internal pattern of phylogenetic relationships in the family is poorly understood, and it is unclear which group of parasitic Hymenoptera should be considered the sister of Platygastroidea. The elongate postmarginal vein of Neothoron is commonly found among scelionids, but this cannot be construed as convincing evidence that Neothoron is paraphyletic. The best cladistic reason to synonymize the two names is that today, even with much greater sampling around the world, Thoron sensu stricto remains monobasic. Our conclusion is that Neothoron is a junior synonym of Thoron (new synonymy).

GEOGRAPHIC DISTRIBUTION: Thoron is a widely distributed, but not cosomopolitan genus (fig. 1). One species, T. metallicus , is found in the eastern United States and Canada; Europe, from Ireland east to central Russia; and in northern Tunisia. A single species, T. dayi , has been collected from Indonesia. Thoron longicornis is known only from the Huachuca Mountains of southeastern Arizona. Six species are found from Panama to southeastern Brazil, and three of these are sympatric in Bolivia.

BIOLOGY: The only species of Thoron for which the biology is relatively well known is T. metallicus . This Holarctic species has been reared from eggs of Nepa cinerea L. ( Ferrière, 1916; Henriksen, 1918) in Europe. The only species of Nepa in the United States and Canada is Nepa apiculata Uhler , known from southern Québec and Ontario, west to Manitoba and North Dakota, and south to Kansas and Georgia ( Sites and Polhemus, 1994). This roughly corresponds with the distribution of T. metallicus in the New World (fig. 1). It seems likely that N. apiculata is the host in the Nearctic, although no rearings have been made.

Thoron metallicus appears to be a univoltine species that is particularly long­lived in the adult stage. Eggs of its presumed host in the New World , Nepa apiculata , are present from mid­May to mid­June in southern Illinois ( McPherson and Packauskas, 1987). Male specimens have been collected from June to September in the USA and Canada and from July to September in Europe (one male specimen from Tunisia was collected in April). In contrast, females have been collected from April to October and even as early in the year as March in Sweden (59 ° 31 ̍ N). We interpret these data to mean that the adult wasps emerge from the host in midsummer and that the females overwinter and then attack newly laid waterscorpion eggs the following year .

Specimens of Thoron longicornis were collected in numbers in the Huachuca Mountains of Arizona in association with nymphs and adults of Curicta pronotata Kuitert. The genus Curicta Stål , recently revised by Keffer (1996), has 16 species in the New World. Nepa does not occur in the Neotropics. We speculate that Curicta is the probable host of Neotropical Thoron because of the similarity in size and biology with Nepa .

There appear to be no records of Thoron attacking the widespread and relatively common genus Ranatra Fabricius in the Nearctic. This may well be due to a difference in oviposition habits: Nepa and Curicta generally lay their eggs in sand or mud along the shoreline, with the egg’s respiratory horns protruding into the open air ( McPherson and Packauskas, 1987; Keffer et al., 1994). Ranatra , in contrast, places its eggs within the tissue of aquatic vegetation, again with the respiratory horns protruding into the air ( Menke, 1979).

We have only a single specimen of Thoron from the Old World outside of Europe, Russia, and the Caucasus, described here as T. dayi from Indonesia, yet the Nepidae are most diverse in the tropics of Africa and Asia. This leads us to predict that the Old World may yet harbor numerous undescribed species of Thoron or its close relatives.

Most of the specimens of Thoron rivalis were collected in the cerrado of central Brazil along a small permanent stream at the end of the dry season. The first rain that season fell on October 1 (1995), but specimens of both sexes of the wasp were collected in the days preceding that event.

RELATIONSHIPS: Kozlov (1970) proposed a new higher classification of the Scelionidae , dividing it into four subfamilies ( Scelioninae , Teleasinae, Telenominae, and Baeinae) and 17 tribes. Of particular relevance for this discussion, Kozlov described as new the tribe Psilanteridini (six genera, Scelioninae ) and the monobasic tribes Thoronini (Baeinae), Tiphodytini (Telenominae), and Pseudanteridini ( Scelioninae ). The monophyly of Psilanteridini was based on the explicit recognition of a single apomorphic character, the presence of a skaphion on the mesonotum. Masner (1972) correctly noted that a skaphion is present as well in the other tribes, and, further, that those three are all associated with aquatic habitats. He placed the three, together with the new genera Neothoron , Microthoron Masner , Tanaodytes Masner , and Thoronella Masner , in a single tribe, Thoronini, within the Scelioninae . Thoronidea Masner and Huggert was later described and add­ ed to the thoronine cast of characters ( Masner and Huggert, 1979).

We now view the proposed monophyly of Thoronini with some skepticism. The basis for the hypothesis was primarily the structure of the female clava; the long, stiff bristles on the mesosoma; and the smooth surface of the body. Austin and Field (1997) demonstrated that one of the genera, Thoronella , possesses a complex apomorphic character complex, the Scelio ­ type tubular ovipositor. This strongly suggests to us that it should be removed from the Thoronini. Carlow (1992) discovered that Thoronella adults are phoretic on dragonflies. It therefore seems plausible to us that the character states used to define Thoronini may well be convergent and somehow associated with underwater behavior, respiration, and locomotion of the adult wasps.

There are no known species of Thoron in subsaharan Africa, none in most of tropical Asia, and none in far eastern Asia (Russian Far East, Korea, or Japan). The genus Elgonia Risbec is highly reminiscent of Thoron in both structural details and overall habitus. Species of this genus have been collected in both tropical Africa and Asia in pan traps in aquatic environments, including habitats rich in nepids. Elgonia is distinguished from Thoron by its abrupt, six­segmented female antennal clava, with all clavomeres separated; male antennal segment 5 carinate and with a small basal, ventral excavation (the sex­segment); metasomatic T3 is densely punctate; and the lower portion of the metapleuron above the hindcoxa is setose. We are continuing to investigate the question of the relationships of Thoron as a component of another, broader study, now in progress.

DESCRIPTION: Body smooth, shining, with no matte microsculpture, with scattered, long, semierect bristles, usually with underlying micropilosity; body black or, rarely, dark brown.

Head transverse, lateral ocellus separated from inner orbit by 1–2 ocellar diameters; compound eye glabrous (fig. 10); occipital carina crenulate; malar sulcus present; cheek with distinct fanlike striae; mandibles tridentate; clypeus truncate; female antenna appearing 8­segmented, radicle distinctly elongate, about one­third length of remaining A1, clava subcompact, 5­segmented, sensillar formula 1­2­2­2­1; male antenna 12­segmented, filiform, A5 not sexually modified; palpal formula 4–2.

Mesosoma about as high as wide, only slightly arched dorsally; skaphion well developed, glabrous, smooth, shining (fig. 10); notaulus varying from nearly percurrent (fig. 16) to reduced to small rudiment in anterior third of mesoscutum (fig. 15) or small furrows near transscutal articulation (figs. 17, 18); humeral and suprahumeral sulci crenulate; posterior margin of axilla deeply crenulate; scutellar disk weakly transverse, convex; dorsellum usually produced medially into subtriangular process, rarely long and pointed; propodeum variable in length, rugulose; netrion crenulate anteriorly, punctate; mesopleural carina present; mesepisternum rugulose; metepisternum narrow, usually sparsely setose; metepimeron usually glabrous, rarely densely pilose; submarginal vein in forewing closely approximated to costal margin; marginal vein short, slightly to distinctly enlarged; postmarginal vein variable in length, from rudimentary to longer than stigmal vein; basal vein nebulous, Vshaped, upper portion forming distinct acute angle with submarginal vein; submarginal vein in hindwing extremely close to costal margin, costal cell largely absent, near hamuli with narrow, short wing membrane visible.

Metasoma pedunculate, with 7 visible tergites in female, 8 in male; laterotergites well developed, deeply incised into sternites to form submarginal ridge; T1 elongate, in female sometimes with hump, longitudinally costate; T2 with longitudinal costae, subequal to T 3 in length; T3 widest segment, always smooth, nearly glabrous; T 7 in female external, subtriangular, articulating with T6 basally, not extruded with ovipositor; ovipositor internal, nontubular.