This family includes only two species native in Michigan. The others that have been collected or reported in the state are mostly occasional waifs. The frequent occurrence of cucurbits on shores probably results not so much from picnickers at those sites as upon the large seeds surviving inadequate sewage treatment and hence washing up, still viable, far from their origins. Many of these species doubtless occur more commonly than the sparse collections suggest.
Adequate herbarium specimens of garden species, with often very large leaves and bulky fruit, are sparse. Temporary waifs are probably more common than the maps indicate, and species additional to those documented and included here may be found. The flowers are unisexual, and in our species both sexes are on the same plant (monoecious) except in Thladiantha.
1. Leaves deeply pinnately lobed (usually at least halfway to the midrib).
1. Leaves palmately (if at all) lobed.
2. Corolla at least 5 cm long and 5 cm broad, yellow.
2. Corolla less than 4 cm long and broad, yellow or greenish white.
3. Flowers yellow, ca. 1.5–3 cm long; tendrils unbranched; rare weeds or garden escapes.
4. Leaf pubescence entirely of pointed hairs, hooked hairs absent; tap-rooted annuals.
4. Leaf pubescence (especially on the underside of the leaf and petioles, also on the young stem) of short hooked hairs among narrowly conical pointed hairs (30 ×); perennial from tubers.
3. Flowers greenish white to cream, ca. 0.6–1 (–1.5) cm long; tendrils branched; native annual vines (though sometimes occurring in disturbed areas).
5. Calyx and corolla 6-lobed; internodes glabrous; fruit solitary, ca. 3–5 cm long, 4-seeded, ellipsoid and broadly rounded at apex.
5. Calyx and corolla 5-lobed; internodes pubescent; fruits each ca. 1.1–1.7 cm long, 1-seeded, ovoid and tapered to acute apex, but crowded into a ± spherical cluster ca. 2.5–3.7 cm broad.