Terrestrial herbs. Corm spherical, pale green to greenish purple, subtended by two or three sheathing bracts, green to greenish purple, upper bracts successively longer and enclosing the base of the leaf; smaller corms occasionally arising from the base of larger corms on stalks; corm decaying within several months after new corms produced. Leaf solitary, linear, deciduous, elongating significantly after anthesis. Inflorescence solitary (rarely two), rachis greenish with a single flower (rarely two), elongating after anthesis; floral bracts scale-like, lanceolate to broadly ovate. Sepals free, erect, white to lavender. Dorsal sepal oblanceolate, obtuse to acute, basally adnate to petals. Lateral sepals oblanceolate, falcate, acute, attached to the ovary at a steep angle, meeting basally at the front in a small crest. Petals of the same colour as the sepals, basally adnate to column, basally connate, linear-oblong to oblanceolate, falcate, positioned with labellum to form a loose tube around column. Labellum forming a narrow basal tube where attached to the ovary, shallowly trilobed, striped and spotted with pink to magenta, basal half erect with central, yellowish crests adaxially, lateral lobes rounded, midlobe greatly expanded, obtuse to truncate, spreading, crenate-denticulate, with a central disc of pink to magenta stripes alternating with yellowish crests of trichome-like appendages and papillae, vaguely resembling anthers, appendages often darker terminally. Column arcuate, winged, greatly expanded at apex, apex extending beyond anther, erose to denticulate at corners, truncate and smooth to denticulate in the middle; anther incumbent, pollinia four, soft; stigma incurved, elliptical. Ovary cylindrical, narrowed apically, erect, green to greenish purple. Capsule erect, sessile, ovoid to ellipsoid. (DG).
Arethusa is found in sunny Sphagnum bogs throughout most of its range and also in the shade of Rhododendron L. and Kalmia L. (both Ericaceae) in the southern Appalachians (Luer 1975), from low elevations and flowering in July and August in the north to around 1670 m and flowering in May and June to the south in North Carolina (Radford et al. 1968; Luer 1975; Correll 1978). Boland and Scott (1992) indicated that in Newfoundland Arethusa prefers similar microhabitats to Calopogon tuberosus but drier sites than Pogonia ophioglossoides, both of which occur sympatrically with Arethusa within most of its range. However, Arethusa occurs in slightly wetter microhabitats than Calopogon in Newfoundland and much of the rest of the range where they co-occur. Arethusa is thought to be short-lived in the wild (R. Yannetti, personal communication), flowering for only a year or two before rapidly declining. Such rapid decline in plants of this genus could indicate its strong dependency on frequent propagation by seed; the detrimental effects of the lack of successful pollination over even a few years could be substantial. (DG).