The
thallus of
Heterodermia rubrotricha is
foliose and can form tufts up to 10 cm in diameter. It features whitish lobes that grow linearly, ascending or becoming , reaching lengths up to 4 cm and widths of 0.9–1.6 mm. The lobes taper at the tips, where they are narrower (approximately 0.4 mm wide) and recurved. Branching is , occurring every 0.5–2.0 cm. The upper surface of the lobes is smooth, shiny, and ranges from flat to convex, while the lower surface lacks both a and
rhizines, being instead (cobweb-like) and whitish with corticate margins. A distinguishing characteristic of this species is the presence of prominent, black marginal , which are or branched, dull, 0.1–0.2 mm thick, and can grow up to 8 mm long. These cilia are mostly covered with a containing dark red pigment, excluding the very tips and bases. The species does not produce rhizines, soredia, isidia, or
pseudocyphellae.
Apothecia (fruiting bodies) are common, measuring 4–7 mm in diameter and occurring (on the surface of the lobes). Their margins feature 6–15 tapering lobes similar in form to the primary lobes. The apothecial are brownish-grey, typically heavily covered with white pruina. The
hymenium (fertile tissue) is 100–180
μm tall, while the (supporting tissue below the hymenium) is pale brown and about 30 μm thick. are brown, contain one septum, have few , and measure 35–43 by 19–22 μm. Chemically, the cortex of
H. rubrotricha reacts yellow with potassium hydroxide (K+), the
medulla faintly yellow, and the red pigment on the cilia produces a purple reaction.
Thin-layer chromatography has identified
atranorin (in the cortex),
zeorin (in the medulla), and a red
anthraquinone pigment on the cilia. ==Habitat and distribution==