Rhopalomyia medusa and R. medusirrasa are anatomically indistinguishable as adults, larvae and pupae. However, their galls are sufficiently different to warrant recognition of these species. Although the galls are the same general type, that of R. medusa is naked, whereas that of R. medusirrasa is thickly covered with long trichomes.

The mature gall (Fig. 36) consists of a larval chamber (Fig. 37) enclosing in a globular mass of reddish, green, or reddish and green filaments that lack trichomes. The filaments are thickened basally and become thinner, curly and branched distally. Frequently, galls form on several adjacent stem and leaf nodes and encompass large areas of the stem (Fig. 38).

One hundred mature galls from A. t. tridentata, A. t. vaseyana and A. t. wyomingensis collected at Sites 1, 7, 21 and 24 had a diameter (the fibrous mass measured perpendicular to stem) ranging from 8-29 mm (avg. 13.3). Seventy-five of these galls contained 1-3 larval chambers (avg. 1.2). Larger galls were usually observed on more vigorous roadside plants on A. t. tridentata.

The first sign of galls was during early September 1968 on both A. t. tridentata and A. t. wyomingensis. The developing galls were observed on the other host subspecies later in the fall. The galls first appear as groups of tiny leaflike structures growing from the nodes (Fig. 39). Gall development continues during the fall, is arrested during the winter, and continues in the spring with maximum size of the galls reached about the time pupae are present. Within a month after adult emergence the galls become dry and brown. They persist on the host plants for 2-12 months or longer. Developing galls were often found on flower stems but did not reach maturity because these stems died out following bloom in fall.

Rhopalomyia medusa
Medusa-gall Midge

A subtle characteristic of these galls that separates them from galls of R. medusirassa (Plate 534) is the reddish-brown, curled leafy bracts that stand above underlying hairs covering the gall body.

30b Modified leaves lacking trichomes (160) . . . . . . . . Rhopalomyia medusa Gagne

Galls are green to red, or mixed green and red, 8-30 mm in diameter, and begin development in fall but grow mainly in spring. The cecidomyiid overwinters as early-instar larvae, and adults appear in spring. One generation occurs per year. Hosts: A. arbuscula, A. cana, A. nova, A. tridentata, A. tripartita. Distr.: Montana to Nevada.