Taken from:
R. M. Bohart and L. S. Kimsey. 1982. A Synopsis of the Chrysididae in America North of Mexico.
Omalus aeneus (Fabricius)
Chrysis aenea Fabricius 1787:284. Holotype female, Halae Saxonum, Germany (Copenhagen).
Omalus Laeviventris Cresson 1865:303. Lectotype female, Colorado Territory (ANSP).
Elampus cyanescens Provancher 1881:303. Lectotype female, Canada (Quebec).
Omalus diversus Aaron 1885:213. Holotype male, California (ANSP).
Discussion: The well developed notauli, distinct and usually acute notch of T-III, brown stained wings, and well punctured scutellum are distinctive in our members of the subgenus. Most specimens have the scutum extensively polished with only a faint scattering of punctures. However, this may vary toward a rather coarsely and closely punctate scutum. In any case the notauli are deeply impressed and gradually broadened from the middle to the posterior point. This feature will distinguish the species from those in Pseudomalus, particularly janus, which may have the scutum weakly punctate. Other differences are the less hairy abdomen, less angulate mesopleuron, and more weakly impressed scapal basin. The two subsidiary claw teeth are moderately stout (fig. 68). Male genitalia are shown in fig. 89. Modal body length is 4.0 mm but length varies from 3.0 mm to 5 mm.
Hosts: Passaloecus cuspidatus F. Smith, Pemphredon sp. (Krombein 1979:1222).
Material examined: 223 males including holotype of 596 females including holotype of aeneus and lectotypes of laeviventris and cyanescens.
Distribution: widespread in the U.S., recorded from nearly every part of the country except the Gulf states. In Canada it ranges from Nova Scotia to British Columbia and north to Labrador (Goose Bay), Northwest Territories (Nor man Wells), and Yukon Territory (Rampart House). In Alaska it has been collected at Matanuska and Anchorage.
According to Balthasar (1954), the species ranges over most of Europe, Turkestan and the Caucasus.
In America it is found from the Upper Sonoran and Upper Austral Life Zones to the Canadian where it is most abundant.
Subgenus Omalus Panzer
Omalus Panzer 1801:11. 85, no. 13. Generotype: Chrysis aenea Fabricius 1787, monotypic.
Homalus Saunders 1873:411. Emendation of Omalus Panzer. Ellampus (Dictenulus) Semenov 1932:6. Generotype: Ellampus specularis Semenov 1932, original designation.
The diffuse and often scanty scutal punctation along with the broadly rounded metanotum characterize the subgenus which contains four North American species as well as several in South America and Eurasia. Examples of male genitalia and claws are shown in figs. 68, 70, 89, 90.
Key to subgenus Omalus
1. T-III posterior margin with medial notch well developed and about 1.0 MOD deep or deeper, notauli entire (fig. 62)
2
T-III posterior margin with shallow medial dent less than 1.0 MOD deep or entire, notauli incomplete or indicated only by a very fine line (fig. 64)
.3
2. Scutellum with punctate area two thirds or more times impunctate area (fig. 62),
notauli deeply impressed and gradually enlarged from middle to posterior point,
wings brown stained
aeneus (Fabricius)
Scutellum with punctate area no more than half impunctate area (fig. 63), notauli a
fine line broadened at extreme posterior point, wings clear or nearly so
. butleri Bohart and Campos
3. Notauli indicated by a very fine but essentially complete line which ends posteriorly in a pit, T-III with a yellowish translucent rim about 1.0 MOD long
.iridescens (Norton)
Notauli obsolescent, represented only by posterior pits (fig. 64), T-III with a
narrow brown rim much less than 1.0 MOD long
.glomeratus (Buysson)
Genus Omalus Panzer
Diagnosis: forewing medial vein strongly arched (fig. 59), median and submedian cells practically asetose, RS stub at least as long as stigma and extending beyond it apicad; tarsal claws with two to five subsidiary teeth (figs. 68-72), forefemur partly to strongly carinate beneath but not angled; mesopleuron simple; T-III nearly always with an apicomedial notch.
Discussion: As presently constituted, Omalus in North America contains three subgenera and 17 species. This compares with more than 100 species worldwide. Nine of the U.S. species are found only west of the 100th meridian.
For the most part Omalus are medium to small wasps which as far as known parasitize relatively small aphid-storing pemphredonine wasps. Most of the latter are twig nesters and several have been reared from old bee burrows in rose canes. Transport of roses by ship in the 16th and 17th centuries may have been the route by which auratus and possibly aeneus reached our eastern shores.
Specimens of Omalus are fairly often taken by wasp collectors, and we have examined 4, 600. Bohart and Campos (1960) revised North American species of the genus and clarified the relationship to Elampus. Both genera have a practically asetose median cell and a strongly arched median vein. Also, two species of Omalus have the Elampus-like features of projecting metanotum and a membrane in the truncation of T-III. A critical difference in Elampus is the long genal fringe in males and short setiform fringe in females. Omalus of both sexes have the genal fringe rather short and sparse. Furthermore, the association of scutal punctures with the notauli in the Elampus-like forms points to Omalus (Diplorrhos).
Sex recognition in Omalus is difficult unless male genitalia or the female sting are visible. Other identifying features of males are the more evenly concave S-Ill toward the apex and the shorter more rotund T-III. Also, males tend to be darker than females but this character is not dependable.
Key to subgenera of Omalus
1. Scutal punctures clumped along notauli, leaving extensive lateral and medial impunctate areas (fig. 65); metanotum usually angulate in lateral view (fig. 77)
..Diplorrhos Aaron p. 42
Scutal punctures not particularly clumped along notauli, either scattered randomly or clumped posteriorly between notauli, occasionally almost impunctate
..2
2. Scutum with large shallow punctures clumped posteriorly between notauli (fig. 61); tarsal claws with three to six subsidiary teeth (fig. 69)
Pseudomalus Ashmead p.40
Scutum almost impunctate or with evenly scattered small punctures (fig. 62); tarsal
claws usually with two subsidiary teeth (figs. 68, 70)
Omalus s. s. Panzer p.37
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