Key to the known genera and species of Bibionidae from Uruguay based on adult males (except Dilophus collaris and D. aff. tetracanthus that only females are known).

1. Fore tibiae without spines; Rs furcated; Pleciinae; Plecia Wiedemann. .......................................... 2

- Fore tibiae with strong apical spine or apical circlet of spines; Rs not furcated; Bibioninae ........................... 5

2. Mesonotum matte..................................................................................... 3

- Mesonotum shining................................................................................... 4

3. Mesonotum orange; mouthparts about as long as head; male terminalia as in fig. 2A, male tergite 9 with a broad cleft on the posterior margin, the bottom of the cleft medially with a small point often only visible in posterior view; in ventral view, male gonocoxites with lateral lobes extending posteriorly over half the length of the gonostylus, posterior margin of gonocoxites with a truncate median lobe and a pair of rounded submedian lobes; gonostylus simple, stout, apex tapered to a point; male terminalia illustrated by Hardy (1945, figs. 143d and 143e)............................................ Plecia gibbosa Hardy

- Mesonotum blackish; otherwise as above for P. gibbosa ...................... Plecia aff. gibbosa (see Comments below)

4. Mesonotum mostly black, sometimes with orange coloration just anterior to scutellum; male terminalia as in fig. 2D, male tergite 9 with a deep v-shaped cleft, inner (ventral) surface of tergite 9 developed into a pair of slender bifurcate processes; in ventral view, gonocoxites without lateral or median lobes gonostylus scoop-shaped, in ventral view convex, apically rounded; male and female terminalia and female head illustrated by Fitzgerald (1998, figs. 71–76)........ Plecia ramosa Fitzgerald

- Mesonotum mostly orange with anterior portion black; male terminalia as in fig. 2C, male tergite 9 with broad cleft on posterior margin and cleft with a small, rounded, median lobe which has a slight median indentation; gonocoxites with posteroventral margin with a pair of median lobes; gonostylus broad, irregular, appears bifurcate in lateral view with the upper lobe beak-like; male and female terminalia illustrated by Hardy (1945, figs. 135 a–e)....................... Plecia collaris (Fabricius)

5. Fore tibia with apical circlet of spines and one or two sets of medial spines; Dilophus Meigen. ....................... 6

- Fore tibia with one strong apical spine in addition to an apical spur; male thorax black; antennae with 7 flagellomeres; femora yellow with dark brown to black apex and base, remainder of legs reddish brown to blackish; inner spur of fore tibia very short; hind basitarsus not swollen; tergite 9 deeply and narrowly cleft................... Bibio dacunhai Hardy ( B. dacunhai is the only species currently reported from Uruguay, but four other species have been reported from Argentina (see Hardy 1953))

6. Fore tibia with one set of medial spines with 4–8 spines............................. Dilophus pectoralis Wiedemann

- Fore tibia with two sets of medial spines (upper and lower set)................................................. 7

7. Rostrum elongated (fig. 4A)................................................ Dilophus collaris Guérin-Méneville

- Rostrum short (figs 4C, D).............................................................................. 8

8. Upper set with one spine.................................................................... Dilophus sp. 1

- Upper set with two spines.................................................. Dilophus aff. tetracanthus Edwards