Morphology

Orchids are renowned for their diverse and often elaborate pollination strategies (van der Pijl and Dodson, 1969; van der Cingel, 2001). Some reward pollinators with food (e.g., nectar, food-hairs and oils), floral fragrances and other compounds such as resin-like substances and wax (van der Pijl and Dodson, 1969; Dressler, 1990, 1993; van der Cingel, 2001) and these rewards, in turn, reinforce pollinator foraging behaviour (van der Pijl and Dodson, 1969; Proctor and Yeo, 1975; Dressler, 1990; Proctor, Yeo, and Lack, 1996; van der Cingel, 2001). Many, however, produce no rewards whatsoever, and here attraction by mimicry and deceit tend to predominate (Porsch, 1908; van der Pijl and Dodson, 1969; Ackerman, 1984; Neiland and Wilcock, 1998, 2000; van der Cingel; 2001). In fact, some one-third of orchid species attract potential pollinators solely by deceit (Ackerman, 1984) and it is thought that deceptive pollination evolved from reward-mediated pollination systems (Ackerman, 1986). The former may involve complex mimicry strategies such as food-fraud, pseudocopulation and pseudoantagonism (van der Pijl and Dodson, 1969; Dressler, 1990; van der Cingel, 2001) and once attracted to the flower by olfactory and visual cues, the precise configuration of the floral parts, the presence of honey guides and tactile stimuli provided by floral hairs and papillae ensure that orientation of the insect upon the flower is optimal for pollination.

Although the rewardless condition is common amongst orchids, a significant number of species, nonetheless, produce food rewards (van der Pijl and Dodson, 1969; Dressler, 1990, 1993; van der Cingel, 2001). Many angiosperm families reward pollinators with pollen (Proctor and Yeo, 1975; Proctor et al., 1996). However, that of epidendroid orchids is bound within pollinia and is thus inaccessible to foraging insects (van der Pijl and Dodson, 1969; Dressler, 1990, 1993; van der Cingel, 2001). Even so, floral, food rewards such as nectar, food-hairs and floral oils play an important role in the successful pollination of many orchids (van der Pijl and Dodson, 1969; Proctor and Yeo, 1975; Dressler, 1990; Proctor et al., 1996; van der Cingel, 2001) and their effectiveness in the attraction of pollinators has been convincingly demonstrated for a number of species (Dafni and Ivri, 1979; Inoue, 1986; Johnson and Bond, 1997; Johnson and Nilsson, 1999; Neiland and Wilcock, 1994, 1998, 2000; Smithson, 2002). Moreover, they have been shown to be potent even in small quantities (Ackerman, Rodriguez-Robles, and Meléndez, 1994) and Neiland and Wilcock (1998) have reported that species that offer rewards often double their chances of developing fruit and seed. However, reward production and the subsequent processes of fruit- and seed-maturation are costly both in terms of materials and energy expenditure and this may outweigh the benefits (Southwick, 1984; Pyke, 1991; Ackerman et al., 1994; Meléndez-Ackerman, Ackerman, and Rodriguez-Robles, 2000 and references therein). Despite the cost, floral rewards, nevertheless, generally confer evolutionary advantage.