Dioxys pacificus Cockerell

ROZEN, JEROME G. & ÖZBEK, HIKMET, 2003, Oocytes, Eggs, and Ovarioles of Some Long-Tongued Bees (Hymenoptera: Apoidea), American Museum Novitates 3393, pp. 1-36 : 8-9

Dioxys pacificus Cockerell

Because the mature oocyte of Dioxys cincta was so different from the egg of the North American D. pomonae pomonae Cockerell pictured and briefly described by Rozen and Favreau (1967: fig. 4), we examined the mature oocyte of another North American species, D. pacificus .

MATURE OOCYTE (fig. 7): Length 1.38– 1.80 mm; maximum diameter 0.40–0.53 mm (N = 3); egg index 0.78 (medium). Shape symmetrical around moderately curved long axis; maximum width near anterior end; anterior end rounded; oocyte tapering posteriorly from maximum width; posterior end narrowly rounded; micropyle not evident un­ der stereoscopic examination; under compound microscopic examination of inner surface of chorion, micropyle appearing as small circular area, presumably with numerous pores; area surrounding it with radiating lines extending outward in all directions 1.5– 2.0 diameters of plate before fading. Chorion uniformly very thin, clear, smooth, reflective, and colorless.

MATERIAL STUDIED: Two females, Arizona: Cochise Co., 5 mi SW Apache, V­15–1988 (J.G. Rozen); one female, Arizona: Cochise Co., 4 mi E Willcox, V­8–1986 (J.G. Rozen) ; one female, Arizona: Cochise Co., 2 mi E Apache, V­9–1986 (J.G. Rozen) .

REMARKS: Because of the exceedingly thin and fragile nature of the chorion, we were unable to remove the follicular tissue from the anterior end of the mature oocytes of this species. However, we were able to find evidence of the micropylar area by removing the chorion with follicular tissue attached from the front end and examining it with a compound microscope.

The egg index (0.66) of Dioxys p. pomonae , given above, is based on the average of the two egg lengths (1.65 mm) provided by Rozen and Favreau (1967) divided by the average intertegular distance (2.51 mm) of 13 female specimens collected in association with that study, now preserved in the American Museum of Natural History .

The great differences in size and morphology of the mature oocytes/eggs between Dioxys cincta on the one hand and D. p. pomonae and D. pacificus on the other are unknown among other cleptoparasitic congeners and even within tribes. In their study of D. p. pomonae, Rozen and Favreau (1967: 201) found ‘‘a small slit in the cell wall above the posterior end of the Dioxys egg apparently marking the spot through which the egg was inserted into the sealed cell.’’ The assumption here was that the female cleptoparasite entered the nest of the groundnesting host, which occupied short burrows, and penetrated the closed cell with her metasomal apex to lay an egg. The cell wall of the host bee was composed of masticated leaves. There is a tendency for cleptoparasitic bees that oviposit in sealed cells to have larger, relatively unmodified eggs (as do D. p. pomonae and D. pacificus ) than do cleptoparasites that oviposit in cells that have yet to be sealed by host bees (to be discussed in greater detail by Rozen, in press). The females of D. cincta were collected while searching a vertical bank containing nests of a number of genera of osmiines, one or more of which were the hosts, but cell linings with masticated leaves were undetected. All cell walls were composed of hard soil, suggesting that the eggs of this species have to be deposited in cells that are still open. Might then the dwarf size of its mature oocytes and modified chorion be ways to escape detection by a returning host female?

APIDAE : XYLOCOPINAE: XYLOCOPINI