Introduction to long tongue fly systems
Orchids such as the famous Madagascar Star Orchid (Angraecum sesquipedale) have extraordinarily long spur lengths, which exceed 30 cm - a phenomenon that interested Darwin (1862) intensely. He proposed that a hawk moth pollinator existed with an equally long proboscis and that the flower and insect had coevolved in a type of “arms race.” This hawk moth was found only 40 years later!
Figure 1 . A modified version of Darwin 's hypothesis on the evolution of deep flowers in plants. Long-tongued pollinator individuals are not compelled to contact the floral sexual organs in short-tubed plant individuals, which therefore receive fewer pollinations than long-tubed individuals that pollinate freely with each other via pollinators. From Nilsson (1988).
His idea was that plants with relatively shallow flowers would be less fit than plants with longer tubed flowers because pollinators with very long tongues would not make physical contact with the reproductive parts of the plant in order to obtain nectar. As a result there would be a strong selective pressure for plants to evolve deep throated or long spurred flowers. This in turn would drive pollinators to evolve longer tongues so that they could get nectar in the depths of the flowers. In a classic experiment (Fig 1), Nilsson (1988) provided the first empirical evidence in support of Darwin 's hypothesis and found that by constricting the spurs of orchids (and thereby artificially shortening them), these short spurred individuals were less fit than long spurred individuals (they set less seed and exported less pollen).
These long spurred orchids and their pollinators have become one of the best known examples of how natural selection may work and it is used frequently as a text book example of co-evolution. But in South Africa we have a unique and equally bazaar system which is similar to Darwin 's orchid system in many ways.
A plethora of plant taxa (in excess of 200 species) have exceedingly long corolla tubes or spurs and none of them are pollinated by moths (Goldblatt and Manning 2000)! Instead they are pollinated by a polyphyletic group of flies (Goldblatt and Manning 2000) which are commonly referred to as “long tongued flies” where the champion fly has a proboscis of up to 80mm long (several times its own body length). The plants that are pollinated by these flies belong to several families and genera and it is clear that this pollination system has evolved many times in South Africa . These flies are likely to have been responsible for the radiation of several plant taxa in South Africa and one of the best examples of this is the relatively recent speciation within the Disa draconis complex which has recently been revised (Johnson and Linder 1995).
Disa draconis is a long spurred orchid with a wide, but disjunct range in the Cape . The plants are pollinated by different species of long tongue fly, which have very different tongue lengths (Johnson and Steiner 1997). Consequently the orchids have different spur lengths, which correspond to the length of the fly tongues in each part of the range (Johnson and Steiner 1997). Several other floral characteristics are also thought to have evolved in response to the different pollinators and the Disa draconis complex now consists of three different species (Johnson and Linder 1995).
Darwin C. 1862. On the Various contrivances by which British and Foreign Orchids are Fertilised by Insects . Murray , London .
Goldblatt P and Manning JC. 2000. The long-proboscid fly pollination system in Southern Africa . Ann. Missouri Bot. Gard . 87: 146-170.
Johnson SD and Linder HP. 1995. Systematics and evolution of the Disa-draconis complex (Orchidaceae) Botanical Journal of the Linnean Society 118: 289-307.
Johnson SD and Steiner KE. 1997. Long-tongued fly pollination and evolution of floral spur length in the Disa draconis complex (Orchidaceae). Evolution 51(1): 45-53.
Nilsson LA. 1988. The evolution of flowers with long corolla tubes. Nature 334: 147-149.