Dasyomma basale Malloch

Woodley, Norman E., 2007, Notes on South American Dasyomma, with the description of a remarkable new species from Chile (Diptera: Athericidae), Zootaxa 1443, pp. 29-35 : 30-32

Dasyomma basale Malloch

( Figs. 1–4 )

Dasyomma basale Malloch, 1932: 226 .

Dasyomma atritarsis Malloch, 1932: 234 .

Diagnosis. Males of D. basale can be distinguished from other described species of Dasyomma by having the following combination of characters: head and thorax black and the abdomen with segments 2–4 yellow with isolated black median spots ( Fig. 1 ), a mostly hyaline wing with a dark cell r1 with slight infuscation around the base of the radial veins and discal cell ( Figs. 1, 2 ), and bicolored middle and hind femora ( Fig. 2 ). Males key out in the key of Coscarón & Coscarón (1995) except for difficulty at the first couplet (see remarks below).

Females of D. basale can be separated from other described Dasyomma species by having a shiny yellow frons with only indistinct tomentum medially, the scutum entirely yellow, and the wing mostly hyaline with a dark cell r1 with slight infuscation around the base of the radial veins and discal cell ( Figs. 3, 4 ) as found in males. Females key readily in the key of Coscarón & Coscarón to couplet 10, but differ from D. fulvum (Philippi) by having mostly black antennae and the two species that trace through the second half of the couplet because they lack dark medial markings on the scutum.

Description. Male. Overall body coloration black, with extensive yellowish areas on the legs and abdomen ( Figs. 1–2 ).

Head black and appendages black. Eyes holoptic with pale brownish pilosity about the length of antennal scape. Frons, face, and parafacials densely tomentose but without pilosity; frons depressed medially, the depression extending between antennae. Ocellar tubercle, gena and occiput with whitish hairs. Antenna black, base of flagellum with slightly paler color, scape and pedicel with a few dark and pale hairs intermixed. Basal part of antennal flagellum subequal in size to pedicel. Palp black but somewhat yellowish in some (teneral?) specimens, with pale pilosity, with a few dark hairs in apical part.

Thorax black, thinly, uniformly grayish tomentose. Postpronotal lobes and postalar calli sometimes yellowish or brownish. Scutum and scutellum with uniform long, erect, pale whitish pilosity. Erect, pale pilosity present on propleuron, most of anepisternum, upper part of katepisternum, anterior half of anepimeron and most of laterotergite. Wing mostly hyaline, but with infuscation present distally in cells c, sc, br, and bm and basally in cells r2+3, r5, d and m3. Cell r1 beyond proximal fork in vein R (pterostigma of Coscarón & Coscarón (1995)) brownish black. Veins brownish but pale at base of wing. Tegula dark. Veins R1, base of R4+5 to just beyond crossvein r-m, and CuA nearly to apex of cell bm with fine black setulae, but can be somewhat reduced and inconspicuous on CuA. Halter with stem yellowish, knob yellowish to dark. Legs black but with mid and hind femora dark yellow on basal two-thirds, front tibia vaguely yellowish on basal half or more, but this coloration not sharply delimited from darker apex, midtibia more conspicuously yellow on basal twothirds, all basitarsi yellowish on basal halves gradually darkening apically. Anterior face of coxae and femora, especially posteriorly, with fine, erect pale pilosity. Tibiae and tarsi with short black vestiture, but sparse, longer, slightly curled hairs present on ventral surface of front tarsus.

Abdomen pale yellow but with entire tergite 1, narrow central spot on tergites 2–4, approximately apical half of segment 5 but with a central, anterior extension medially on tergite, and entire abdomen beyond segment 5 black (tergite 4 more extensively black in apical half in one specimen). Pilosity of abdomen mostly whitish, but black hairs present on medial portions of tergites 2–6. Cercus yellowish brown.

Length 8.0– 8.7 mm.

Female. Figs. 3–4 . Differs from male as follows: Head completely yellow except for ocellar triangle, antennal pedicel and flagellum, and proboscis which are black; antennal scape brownish. Eyes dichoptic, frons 0.38 – 0.42 of head width. Eye pilosity shorter, less than length of pedicel. Frons convex, moderately shiny except for very narrow lateral margins that are whitish tomentose; pilosity mostly pale but with some scattered black hairs in upper medial part. Face with wider tomentose margins, medially slightly shiny, mostly bare but a few hairs present medially just below level of antennae. Upper occipital region with dark pilosity. Palp with pale pilosity but with some dark hairs on apical half.

Thorax orangish yellow, with only proepisternum black and some vague darkened areas around anterior and posterior spiracles. Scutum with some dark hairs intermixed with pale pilosity. Tegula with posterior half yellow. Forelegs mostly blackish, but indistinct yellow area on basal, outer region of coxa and basal half of basitarsus. Mid and hind legs with basal half of coxae yellowish, midtibia with some yellowish but not as distinct as in male, otherwise as in male. Some black hairs present on hind femur.

Abdomen dark yellow with segments 5 to apex black, but tergite 5 usually with basomedial area yellowish. Tergites uniformly clothed with dark hairs. Cercus dark yellowish.

Length 5.1 – 6.8 mm.

Specimens examined. 1♂, CHILE: Malleco Province, Galvarino, 28 November 1929 (USNM; holotype of D. basale Malloch ); 2♂, CHILE: Valparaiso Province, Perales, January 1925, A. Faz (USNM; holotype and paratype of D. atritarsis Malloch ); 2♂, 3Ψ, CHILE: Llanquihue Province, Cayutué, 41°14'S, 72°16'W, 175 meters, 12 February 1998, N. E. Woodley (USNM); 1Ψ, same data but 13 February 1998.

Remarks. Five of the specimens cited above that I collected were taken sweeping a tree branch overhanging a small stream. They were all taken at the same time so the males were clearly associated with the females.

All of the species for which both sexes were previously known have coloration that is similar in both sexes. This is the first report for significant sexual dimorphism in color in South American Dasyomma . Because the majority of species are known from only one sex, there is the distinct possibility that there will be additional synonymy when sexes are properly associated. However, this will require further material in which both sexes are taken at the same collecting event. From the material that I have examined of D. basale and other species, it appears that the wing infuscation and coloration of the stigma-like marking in cell r1 is uniform between the sexes and is useful for identification.

The specimens cited as doubtfully D. basale by Coscarón & Coscarón (1995: 91) from Chile: Malleco Province, near Los Gringos Camp, Nahuelbuta National Park clearly are not conspecific. As they noted, these specimens have completely dark legs, and the pilosity of the head, including the eyes, is longer and denser. Also, these specimens have the stigma-like mark in cell r1 narrower and more elongate, and it is much paler in color than in D. basale . They also lack the infuscation in the region of r-m that is present in D. basale . It is possible that these are males of a species described from yellowish females.

I think it is also possible that the male cited by Coscarón & Coscarón (1995: 93) as D. immaculatum Malloch (erroneously as D. inmaculatum ) from the Los Gingos Camp locality noted in the previous paragraph, and the series of females from the same locality cited as D. trivittatum Malloch ( Coscarón & Coscarón 1995: 81) are actually the same species. The male differs from D. immaculatum by having darker legs and a few setae present on vein CuA. The females differ from Malloch’s description of D. trivittatum because none has a trivittate scutum but have a single broad medial vitta that is somewhat variable in width between specimens (and is absent in one). The legs apparently have less yellow on the femora than Malloch noted. All the specimens, including the male, have the extreme base of the mid femur yellowish, up to about one-fifth of its length, and this coloration is present on the fore femur in some specimens. However, the stigma-like marking in cell r1 is definitely darker in the male.

The sexual dimorphism found in some species of Dasyomma makes Coscarón & Coscarón’s (1995: 77– 78) key problematic beyond couplet 10 which separates specimens with a yellow frons and scutum from those with a black frons and scutum. Since some species with mostly yellow females discussed in this paper have males with a black frons and scutum, the females cannot be identified using the key. Also, the pivotal first couplet of Coscarón & Coscarón’s key only works for females. All males I have examined for species that trace to couplet 6, including D. basale , have the small, triangular frons below the contiguous eyes densely gray tomentose. The second character used in the couplet, extent of pleural hairing, does not seem to differ as significantly as implied and I have found it very unreliable.

Clearly additional work is needed to resolve the taxonomy of several Dasyomma species. The rather small amount of available material does not allow a clear picture as to whether or not there is geographic variation in such features as leg coloration, which appears to be rather uniform at a single locality. It is unlikely that the species can be adequately understood without significant additional collecting.

Figures

FIGURES 1 – 4. Dasyomma basale Malloch. 1, male, dorsal view. 2, male, semilateral view. 3, female, dorsal view. 4, female, semilateral view.