Anthrax oedipus Fabricius 1805

Anthrax oedipus Group

The oedipus group is a well-defined aggregate of species in North and South America. They generally are characterized by numerous spots on the wings at the bases of the cells, on apices of veins, and often scattered along veins and expanding into the cells. Setae on the face are concentrated along the epistomal margin with a few along the eye margins (Plate 1n). Light and dark scales on the thorax and abdomen are formed in a complex pattern with some enlarged and truncate along the posterior and lateral margins of the abdominal terga. The gonocoxites of the male genitalia taper apically and are not lobed; the distal segment of the gonostylus is flat, quadrangular basally, and ends in a deflexed hook. The base of the third antennal segment is wider than the second segment, hemispherical, and more or less as long as the styliform part.

Group Description

Body generally black, blue gray or brown pruinose, gray or silvery pollinose along eye margins; femora, tibiae, male genitalia, and basal antennal segments sometimes dark red or orange. Front with black setae and, on lower half, lanceolate scales. Face retreating, epistomal margin abrupt; upper half of face bare except for some setae and a few scales along eye margins; lower half of face with coarse black setae and linear or oblanceolate white or yellowish scales extending along eye margins into oral cavity (Plate 1n). Occiput with short, semirecumbent black setae and linear and lanceolate scales; fringe of pile on posterior margin dark brown or black, sometimes with light tips. First antennal segment narrow basally, enlarged mesad apically, slightly shorter to slightly longer than apical width. Second segment lenticular with apical margin rounded (Figure 15; Plate 1k) or saucer shaped with apical margin produced as a sharp flange (Figures 14, 16; Plate 1l); base of third segment bulbous, slightly wider than second segment in male, distinctly wider in female; styliform part arising medially or toward lateral margin, slightly shorter to distinctly longer than basal part; style short, about one-fourth as long as styliform part, tuft of hairs about as long as style.

Disc of mesonotum with sparse, fine black setae and linear, mostly black scales; white scales in transverse line in front of bases of wings, in submedial longitudinal lines on anterior half, along anterior edge, and in a triangle extending forward from posterior margin; brown scales sometimes intermixed with white and occasionally predominating on disc. Lateral margins of mesonotum with black setae and bristles and linear or elongate, threadlike, white or brown and white scales. Scutellum with linear or lanceolate black scales on disc and lanceolate, oblanceolate or obovate white scales on anterior and posterior margins; brown scales often intermixed with white, white or brown scales sometimes in medial patch on disc, black scales occasionally restricted to submedial spots. Sternopleuron, lower half of mesopleuron, and anterior half of pteropleuron with black setae and linear or lanceolate, recumbent to erect scales; some coarse setae on pteropleuron. Upper half of mesopleuron with coarse black or gold setae, and curly scales; pile sometimes present. Pile on prosternum, propleuron, and anterior margin of mesonotum white, black or mixed, some brown hairs often present, black setae present on anterior margin of mesonotum. Postalar tuft of pile black, white or yellowish brown, occasionally mixed, usually a few white scales at base. Hypopleuron bare, metapleuron usually bare, sometimes with a few scales behind and below spiracle. Coxae with black setae and black or white lanceolate or oblanceolate scales, occasionally with some brown scales.

Wing with basal infuscation extending into basal cells and with numerous spots in apical part of wing at bases of cells and usually at angles and apices of veins, sometimes also at other points on veins; spots often more or less coalesced into medial and subapical bands and sometimes elongated into transverse striae. Cells C and Sc with alternating hyaline and pigmented areas or entirely hyaline, subhyaline, or infuscated. Spurs at bases of cells R2+3 and R4, and at medial angle of m crossvein, and often at medial angle of vein R4 and basal angle of m crossvein; spur at base of cell R4 sometimes connected to vein R2+3 to form a sectoral crossvein. Anal cell slightly open or closed in margin; r–m crossvein at basal third or two-fifths of discal cell; junction of cells 1M2 and Cu1 one to three times length of base of cell Cu1. Cell 2A one to 1.5 times as wide as cell 1A; alula well developed; calyptere unpigmented, fringe of hairs white (Plate 1a–j; Plate 2a–o). Stem of halter brown, base of knob dark brown, apex of knob white.

Femora with oblanceolate-cuneate or obovate-cuneate scales, usually black anteriorly and white posteriorly on fore and middle pairs, entirely black or white posteroventrally and black anterodorsally on hind pairs; black scales sometimes more or less replaced by brown. Scales on tibiae usually white posteriorly and black anteriorly on fore and middle pairs, entirely black on posterior pairs. Fore femora with anteroventral row of bristles strongest toward base; middle femora with posteroventral row of bristles and occasionally with one or more bristles postmedially on anterior surface; hind femora with anteroventral row of bristles in female and additional posteroventral row in males.

Lateral margin of first abdominal tergum with white pile and a few black setae, occasionally some black pile, sides of second tergum with black setae and linear, lanceolate, oblanceolate or obovate-cuneate scales of variable color, occasionally some black pile; sides of posterior terga with oblanceolate and obovate-cuneate scales of variable color, occasionally some black pile; sides of posterior terga with oblanceolate and obovate-cuneate scales and black setae. Posterior margins of first tergum with a few black setae and with light scales laterally and dark scales medially; discs of remaining terga with sparse setae and complex pattern of linear or threadlike to ovate-cuneate scales; usually with broad white scales in submedial or medial and sublateral patches on posterior margins, less distinct posteriorly; smaller scales cover remainder of disc, brown or white in basal and subapical, poorly defined bands on second segment and basally on posterior segments, black elsewhere. Sterna with sparse black setae and linear or threadlike to ovate scales; light scales usually predominate posterolaterally and dark scales anteromedially.

Apex of gonocoxites of male genitalia simple, not elongated into lobes. Basal segment of gonostylus platelike, rounded dorsally, not produced apically; distal segment laterally flattened, quadrangular basally, apex a deflexed hook. Apex of epiphallus small, curled upward at tip dorsally, slightly flared laterally, with a basally produced spine just after junction of dorsal bands; epiphallus sometimes atrophied. Apex of aedeagus acute with gonopore ventral, or flared with gonopore apical, sometimes trilobed. Base of aedeagus bulbous, narrowing abruptly to apical part. Epandrium simple or with dorsal margin apically produced and with cerci produced ventrad.

Tenth tergum of female with four to twelve spines on each side. Sclerite on each side of gonopore hatchet shaped; lateral lobe acute, clavate or broad and blunt; dorsal or dorsomedial lobe short, acute or obtusely rounded; ventral lobe narrow and parallel sided or broadened medially, curving inward at apex. Ducts of spermathecae uniting medially into common duct or sometimes emptying separately into genital chamber, with one to seven convolutions before expanding to bulbs; bulbs elongate, expanded medially, postmedially or sometimes apically, sometimes bent medially or basally.

Taxonomic Characters

The wings of the species of the oedipus subgroup show many differences between taxa, but the differences in pattern are subtle and difficult to describe (Plate 1a–j;). That difficulty apparently has led some authors to lump all the species together in despair, although other less readily observed characters will separate the species. Discrete differences in the pattern of the wings often are obscured by variation in intensity of the pattern in different localities. That is especially true of irroratus irroratus where the wings vary from lightly to heavily pigmented in different parts of North America (Plate 1a–c). In the pluto subgroup the wing pattern remains almost the same (except for cybele), while differences between the species are due to restriction or extension and coalescence of spots (Plate 2a–o). The sexes vary in this subgroup with females having darker wings. The spur at the base of cell R4 may be connected to vein R2+3 to form a complete sectoral crossvein in the oedipus group. The character is stable in some species; in others it varies.

In most species of the oedipus group the second antennal segment is saucer shaped with the apical margin produced as a sharp flange (Figures 14, 16; Plate 1l), a character shared with many Palaearctic species. Only two species in the group, irroratus and cybele, have the second segment lenticular (Figure 15; Plate 1k) as it is in all other groups of the genus in North and South America except the tigrinus group. This is another character used to separate the “genera” segregated in the Old World by Sack (1909) and Bezzi (1924), but which is not applicable to New World species.

The chaetotaxy of the legs seldom has been found useful to separate species in the Bombyliidae and that is generally true in the genus Anthrax. Anthrax oedipus and peruvianus, however, have a distinctive bristle on the middle femur postmedially on the anterior side, which has not been noted in other species in the group (Figure 30). That is one of the qualitative characters that lead to the conclusion that the oedipus subgroup has evolved longer in the New World than has the pluto subgroup.

The male genitalia are of little practical use to separate species in the oedipus group, contrary to the situation in the albofasciatus group. While some good internal differences exist, external aspects of the genitalia are quite similar for almost all of the species. The most striking differentiation is in cordillerensis and inordinatus of the oedipus subgroup in which the apex of the epiphallus is atrophied and the apex of the aedeagus trilobed. Females of the species appear to be adapted to the trilobed aedeagus in that they have three spermathecal ducts emptying separately into the genital chamber rather than uniting into a common duct. Those two species also have the epandrium elongated over the cerci so they project ventrad. Female genitalia have several good internal characters, the shape of the spermathecae, the number of convolutions in the spermathecal ducts, and the shape of the sclerites on each side of the apex of the spermathecal ducts, but a part of the abdomen has to be destroyed to examine them.

Variation in vesture of the body is extensive. In general, there appear to be four types of ectodermal processes in the genus—macrochaetae (or bristles), setae, pile (hairs), and scales—although the cellular origins of the different types have not been investigated. While both setae and macrochaetae are tapered, macrochaetae usually are larger, few in number, and found in well-defined locations, whereas setae are finer, more numerous, and more or less scattered where they occur. Pile is formed of erect, parallel-sided hairs with minutely bristly apices. No intermediate structures have been found between pile and scales in the genus Anthrax. Scales are the most variable component of the vesture, in color, shape, and size, and often present useful characters. They may be white, black or various shades of brown, and vary from short and linear or lanceolate to long, ovate-truncate. Some of the various shapes discriminated are illustrated in Figures 17–26. An accurate classification is impossible since all sizes and shapes intergrade. “Tomentum” has been used in other genera of Bombyliidae for dense, woolly, hairlike scales, but such have not been observed in the genus Anthrax.

Mexico (Baja California, Coahuila, Nayarit, Morelos, Sinaloa, Sonora); USA (Nevada, Texas).