1a-b: Horismenus propodeum (left), and scutellum and propodeum of another species (right)
2a: Horismenus mesosomal pleuron, with episternal projection indicated
Biology: Primary larval or pupal parasitoids of Coleoptera (including Psephenidae), Diptera, and Lepidoptera or secondary parasitoids mainly of Braconids and Chalcidoids (some reared from spider egg sacs, probably as secondary parasitoids).
Comments: Many common species. Certainly in a clade with Edovum and Alachua, and probably with Pediobius and Paracrias as well. Differentiation from these genera is possible in the cases I have seen, but Horismenus body form and superficial features approach them in some cases, and misidentification is likely if gestalt is relied upon instead of careful assessment of the characters.
Paracrias: Scutellum without longitudinal groove; scutellar-axillar border without dorsal pit (though rarely with a small pit at the corner of the axillula, on lateral face of scutellum). Occipital margin sharp or carinate, very near ocellar triangle. Pronotal collar not carinate, usually rounded anteriorly.
Edovum: Petiole longer than broad, with longitudinal ribs. Median longitudinal groove of scutellum extending no more than half scutellar length (usually longer in Horismenus); epicnemial carina present, extending from posterior edge of mesepisternal extension (more extensive and differently shaped than in Horismenus).
Pediobius: Scutellum without longitudinal groove (but often with broad smooth area). Vertex carinate or sharply margined posteriorly. Petiole usually with dorsal flange embracing propodeal nucha, and with ventrally-projecting tooth. Propodeum in most species with 1 median carina or strip splitting posteriorly or with 2 submedian carinae diverging posteriorly. Surprisingly difficult to distinguish from Horismenus in some cases. The posteriorly split median carina often forms part of a raised area, especially apparent in Pediobius alcaeus (Walker) and related species, but these are distinguishable, though sometimes with some difficulty, by the carinate or sharply margined vertex. The scutellum in Pediobius is usually without a median sulcus, but most species have a broad smooth strip. I regard the ventrally-projecting petiolar tooth to be an unrewarding character to rely upon, as it is often difficult to assess when the petiole and nucha are short.
Alachua: Upper face and vertex densely setose; posterior margin of eye bordered by carinate rim. Face and mesosomal dorsum smooth. Propodeum nearly featureless except for long setae and anterior tooth extending towards metanotum. Easily distinguishable, but similar in a few important ways.
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Burks, B.D. 1971. The Nearctic species of Horismenus Walker. Proceedings of the Entomological Society of Washington. 73: 68-83.
Schauff, M.E. 1987. Taxonomy and identification of the egg parasites (Hymenoptera: Platygastridae, Trichogrammatidae, Mymaridae and Eulophidae) of citrus weevils (Coleoptera: Curculionidae). Proceedings of the Entomological Society of Washington. 89(1): 31-42.
Schauff, M.E. 1989. A new species of Horismenus (Hymenoptera: Eulophidae) parasitic on the lesser cornstalk borer, Elasmopalpus lignosellus (Lepidoptera: Pyralidae). Proceedings of the Entomological Society of Washington. 91(4): 534-537.
Schauff, M.E. 1991. The Holarctic genera of Entedoninae (Hymenoptera: Eulophidae). Contributions of the American Entomological Institute 26.
Image credits: 1a, 2a: Schauff (1991). 1b: Schauff (1987).