Most of the
genera of plants within the subtribe Stapeliinae are to varying degrees
stem succulents, often referred to as "stapeliads". Many of these resemble
cacti, though are not closely related, as an example of
convergent evolution. The stems are often angular, mostly four-angled in cross-section, but in some species there are six or more, with some species of
Hoodia having more than thirty angles. In size they vary from less than 2.5 cm/1" in length to over 2 m/6" tall. The leaves are in most species reduced to rudiments, sometimes hardened and thornlike, arranged on bumps or tubercles on the angles. Some species, however, still have recognisable leaves, most notably the Indian species
Boucerosia frerei (syn.
Frerea indica), and some members of
Tridentea. Stapeliads are most abundant in warm, dry climates. In Africa, there are two separate regions where stapeliads have most diversified: northeast
Africa, and
Southern Africa. Several species are endemic to the small island of
Socotra off the
Horn of Africa. The
Arabian Peninsula, and most specifically the country of
Yemen, contain another concentration of species. Several more are found in the drier parts of
Pakistan,
Afghanistan,
India,
Nepal, and
Myanmar. A single species,
Apteranthes europaea (syn.
Caralluma europaea) is found in
Europe, in the very southern part of the
Iberian peninsula and in the island of Lampedusa. Stapeliads are often regarded as a climax group within the family because of their often structurally complex
flowers. Certain aspects of these reproductive parts mirror the
pollination systems in the
orchid family and represent a case of parallel
evolution though both groups are quite unrelated and have developed similar, though not identical means to achieve the ultimate goal of pollination and therefore reproduction. Most stapeliads use flies as pollinators, that are attracted to odours resembling dung or rotting meat, emanating from the flowers. Many of the flowers also bear some physical resemblance to rotting animal carcasses, leading to their popular name of "carrion flowers". However, not all stapeliads smell bad, or attract flies. Some species use
beetles,
bees,
wasps,
butterflies or
moths as pollinators. Stapeliad flowers range in size from mere millimetres in species of
Echidnopsis and
Pseudolithos to those of
Stapelia gigantea that can reach 40 cm/16" in diameter, and are some the largest of flowers to be found on any species of succulent. Flowers are actinomorphic (radially symmetrical) with 5 free or fused sepals and petals. Some petals are formed in a shape of a star while some have a tubular shaped corolla. On some species, a raised ring or annulus is present in the center of the corolla. Instead of stamens and pistils, stapeliads contain a pollinarium. ==Taxonomy==