1923 ] 
Studies in Asilidce (Diptero) 
215 
Dioctria sackeni, new form rivalis. 
Male. — Length 7 mm. Basal half of wings smoky hyaline, 
distal half merging into blackish; legs black, the anterior knees 
and the basal half of hind tibiae reddish yellow. Face entirely 
silvery, mystax, hairs of front, of antennae and of upper occiput 
black, vertex with scant fulvous coating and not golden. Coating 
of mesonotum fulvous and not heavy, hairs black; the golden 
patch in front of wing connected with the more silvery patch 
on upper sternopleura. The appressed hairs on posterior half 
of abdomen black, not golden as in form Sackeni] dorsal plate 
of hypopygium continued posteriorly as two long narrow clavate 
processes, each tipped without by a closed cluster of black 
bristles and bearing on inner side of knob a pronounced parallel- 
sided prong, long black hairs on ventral lip and on stout lateral 
valves. In Sackeni hairs fringing the ventral lip are brown. 
Female. — Length 8 mm. Face deep golden. Wings uni- 
formly blackish. Legs black. 
Morphotypes. — Priest Lake, Idaho, Aug. 1920; Coeur d’Alene, 
Moscow Mt., Avon, Id.; Big Fork, Mont.; Friday Harbor, 
Quilcene, Wash.; Nelson, B. C. (Melander) : Stuart Island, 
Wash. (H. S. Davis); Wolf Fork of Touchet River, Wash. 
(V. Argo). Thirty-two specimens. 
Late one afternoon while collecting insects at Priest Lake, 
Idaho, I noticed many specimens of a Dioctria running over 
the foliage of some alder bushes growing near the water’s edge. 
On mounting the captured specimens there were found to be 
seven males of D. Sackeni, four males of the present black form 
and nine black females. The only interpretation is that Sackeni 
is dimorphic in the male sex. I have also taken the lighter 
colored male of Sackeni together with the dark female at Nelson, 
B. C. 
Light-colored males, similar to D. Sackeni, have been found 
associated with the dark albius in several places in the East. 
Dr. Back ventured the opinion that Sackeni, therefore, might 
prove to be the same as alhius, but the recent note by Banks 
that the Eastern reddish males have genitalia of the alhius 
structure suggests that male dimorphism in Dioctria is probably 
