Sobarocephala

Introduction

Sobarocephala Czerny is the largest clusiid genus with well over 200 recognized species.  Most of these species occur in the Neotropics, but 17 species are found in the Nearctic, two species are known from Japan (Sueyoshi 2006), and at least a dozen described and undescribed species occur in the Afrotropical, Australian and Oriental Regions. 

Little is known of the biology of Sobarocephala, although adults of most North American species are commonly found around fallen, bare tree trunks under partial shade in moist, predominantly deciduous woodland (Malloch 1918, Sabrosky & Steyskal 1974). In Ontario, S. atricornis Sabrosky & Steyskal has been collected around Impatiens L., sedges and ferns. Sobarocephala flava has been collected in association with or amongst Poa L., Agrostis L., Phleum L., Festuca L. and Juncus L., and S. latifacies has been collected by sweep-netting in Impatiens, Clematis L., Rubus L. and grasses. In Saskatchewan, S. lachnosternum Melander & Argo has been caught in grasses in birch/poplar woodlands. In the Neotropical Region, specimens have often been observed on dung, especially on logs and around damp, mossy areas near water. Aside from dung, adults have been known to feed on nectar, sap and decaying vegetation (Malloch 1918).

Characteristics

Sobarocephala is an incredibly diverse genus that can be very difficult to characterize, largely due to the fact that only a portion of species have retained all of the genus and subfamily level synapomorphies (Lonsdale & Marshall in manuscript). As a result, a combination of characters is usually necessary to diagnose the genus: externally, vein R₁ is bare, the anterior fronto-orbital bristle is inclinate, the presutural intra-alar bristle is (almost always) absent, cell bm is often open (i. e., confluent with cell dm; also found in Procerosoma), and the mid tibia always has a dorsal preapical bristle.

The only synapomorphies defining Sobarocephala are male genitalic: the distiphallus has an additional shield-like sclerite at its base, and the lateral lobe of the distiphallus has an accessory 'thumb-like' extension (Lonsdale & Marshall, 2006). The thumb-like structure is frequently reduced or absent, but the basal shield is almost always present.

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Male genitalia of Sobarocephala flava. © Owen Lonsdale

Sobarocephala somewhat resembles the sobarocephaline genera Chaetoclusia and Procerosoma, but vein R₁ of Chaetoclusia is always setulose, and Procerosoma is missing several cephalic bristles, some notal bristles and all dorsal preapical tibial bristles.  Heteromeringia is most often confused for Sobarocephala, but Heteromeringia has highly derived male and female genitalia, one minute lateral scutellar bristle, no dorsal preapical tibial bristles, and very few Sobarocephala are as darkly pigmented.

Discussion of Phylogenetic Relationships

The genera Sobarocephala and Procerosoma together form the sister group to the small neotropical genus Chaetoclusia. These three genera make up the subfamily Sobarocephalinae, which was described and revised by Lonsdale & Marshall (2006).

Sobarocephala can be divided into nine species groups (to be named the S. archisobarocephala, S. erwini, S. flava, S. flaviseta, S. interrupta, S. isla, S. latipennis, S. plumicornis and S. rubsaameni species groups) that are almost entirely defined by male genitalic characters (Lonsdale & Marshall in manuscript). Only one species group can be reliably identified using an external synapomorphy, the S. flaviseta group, which is defined (in part) on a frons that strongly narrows towards the back of the head. This is the largest of the nine groups, and almost every species is characterized by either a densely plumose arista (below, left) or ivory-white patches on the notum (below, right) (Lonsdale & Marshall in manuscript).

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Sobarocephala quadrimaculata showing a densely plumose arista (left), S. flaviseta showing ivory-white patches on the notum (right). © Owen Lonsdale

Relationships between the species groups of Sobarocephala are largely unknown, and molecular data will likely be necessary to resolve them.

References

Lonsdale, O. & Marshall, S.A. 2006. Redefinition of the Clusiinae and Clusiodinae, description of the new subfamily Sobarocephalinae, revision of the genus Chaetoclusia and a description of Procerosoma gen. n. (Diptera: Clusiidae). European Journal of Entomology, 103: 163-182.

Malloch, J.R. 1918. A revision of the dipterous family Clusiodidae (Heteroneuridae). Proc. Ent. Soc. Wash. 20(1): 2-8.

Sabrosky, C.W. & G.C. Steyskal. 1974. The genus Sobarocephala (Diptera: Clusiidae) in America North of Mexico. Ann. Ent. Soc. Amer. 67(3): 371-385.

Soos, A. 1987. Clusiidae. In Manual of the Nearctic Diptera, Vol. 2, coordinated by J.F. McAlpine et al., Research branch, Agriculture Canada, Ottawa, pp. 853-857.

Sueyoshi, M. 2006. Species diversity of Japanese Clusiidae (Diptera: Acalyptrata) with description of 12 new species. -Annales de la Société Entomologique de France 42(1): 1-26; Paris.