Fruiting body 5-11 mm tall, arising from a basal, yellowish, hyphal swelling, i.e. a trophocyst, the latter elongating into a filamentous, hyaline stalk, yellowish at the apex when young, later swollen into an ovoid hyaline vesicle, at maturity forming a small black apical sporangium; odor not distinctive; taste not determined.
Spores asexual, 5.0-7.0 x 3.0-3.5 µm, ellipsoid-oblong, smooth, thin-walled, pale-grey mounted in KOH; zygospores not seen; subsporangial vesicle approximately 2/3 mm broad, 1.0 mm long; trophocyst yellowish, ovoid to turnip-shaped, approximately 1/3 mm long, typically buried in the substrate.
Gregarious, often in large groups on relatively fresh herbivore dung, e.g., cows, horses, pigs etc.; found throughout the year when moisture is available; common but inconspicuous.
A relative of the common bread mold, Rhizopus stolonifer (=R. nigricans), Pilobolus roridus is recognized by tiny, nearly hyaline, filamentous fruiting bodies tipped with pinhead-sized black sporangia. It is distinguished from other Pilobolus species by relatively small, thin-walled, oblong-ellipsoid spores, <7 microns long, and a flattened-convex columella, seen only after the sporangium has dehisced. The epithet "roridus" refers to jewel-like droplets often seen on the stalk of the fruiting body, a conspicuous character, but not unique to the species. Pilobolus has fascinated generations of amateur and professional mycologists by virtue of fruiting bodies that track the sun and launch sporangia like cannonballs, in some cases up to two meters, a remarkable feat for a tiny fungus. This "long distance dispersal" addresses a critical phase in the Pilobolus life cycle which requires ingestion by a herbivore. Because horses and cows tend to avoid feeding near dung piles, sporangia are more likely to be consumed if landing on fresh grass a distance from where they originated. Ingested sporangia pass through the animal's gut unharmed and rapidly colonize freshly deposited dung. Within as little as a week, flushes of Pilobolus can appear. Interestingly, the Pilobolus-herbivore association sometimes involves a third organism, a parasitic nematode, species of the roundworm genus Dictyocaulus, a lung parasite of herbivores. Like the spores and sporangia of Pilobolus, one of the larval stages of the lung worm can pass through the herbivore's gut unscathed. Deposited in dung along with Pilobolus, their development coincides with that of the sporangiophores/fruiting bodies. As the sporangiophores nears maturity, some of the larvae climb past the gauntlet of stalk droplets to reach the sporangial cap where they and the sporangia are catapulted literally to "greener pastures."