Angraecum cadetii Bosser
The orchid genus Angraecum holds a special place in the history of pollination biology and evolutionary biology. Charles Darwin famously received some specimens of Angraecum sesquipedale from Madagascar, the flower of which has a nectar tube nearly 30 cm long. After careful study, Darwin predicted in his 1862 treatise on the reproductive biology of orchids that only a giant hawkmoth with a "wonderfully long proboscis" would be able to pollinate this spectacular orchid. Forty-one years later, the predicted moth pollinator, Xanthopan morganii var. praedicta, was found in the primary forests of Madagascar (although effective pollination of the orchid by this insect was only demonstrated 135 years after Darwin’s prediction). (Micheneau et al. 2009 and references therein)
Many of the approximately 200 Angraecum orchids show numerous clear adaptations for pollination by large moths, but this is not the case for all members of the genus. Angraecum cadetii represents one such exception. Recent work indicates that this species, found only on the islands of Reunion and Mauritius in the Indian Ocean, is pollinated by a not yet described gryllacridid, or "raspy cricket", a cricket-like orthopteran insect. This represents the first clearly supported case of orthopteran-mediated pollination in a flowering plant. (Micheneau et al. 2010)
Angraecum cadetii is one of three species in section Hadrangis of Angraecum. Species in both section Hadrangis (of Reunion, Mauritius, and Rodriguez) and section Humblotiangraecum (of Madagascar) have a racemose inflorescence with several white or cream-colored fleshy flowers and flower spurs with a wide opening at the entrance. However, in contrast to section Humblotiangraecum species, which have flowers that are strongly scented and spurs that are filiform and elongated, the flowers of section Hadrangis species are unscented and spurs are wide and short. (Garay 1973; Micheneau et al. 2006, 2008)
In their analysis of A. cadetii flowers, Micheneau et al. (2010) found that the conical spurs averaged 6.3 mm in length, 5.2 mm in height, and 8.4 mm in width at the opening.
Angraecum cadetii is an epiphyte occurring at elevations ranging from 400-1325 meters (Jacquemyn et al. 2007). In Reunion, A. cadetii is relatively common in primary lowland wet forests (mainly from 0 to 1000 meters elevation) (Micheneau et al. 2010).
Angraecum cadetii is endemic to the islands of Mauritius and Reunion (Mascarene Islands, Indian Ocean) (Micheneau et al. 2009, 2010). In Reunion, A. cadetii is relatively common in primary lowland wet forests (mainly from 0 to 1000 meters elevation), but the species is rare in Mauritius. (Micheneau et al. 2010)
Angraecum cadetii is one of three species in Angraecum section Hadrangis, which is endemic to the Mascarene islands (Mauritius, Reunion, and Rodriguez in the Indian Ocean) (Micheneau et al. 2009).
Angraecum cadetii plants usually produce one to four erect racemes of one to five fleshy white-cream flowers, with flowering ocurring during the austral summer, from January to March depending on the locality (Micheneau et al. 2010).
In their study of A. cadetti pollination biology, Micheneau et al. (2010) found that nectar volume averaged 14.5 ml per flower, with a concentration of 12.3% sugar in sucrose equivalents. The fragrance of A. cadetii, emission of which was clearly greater at night, was largely dominated by monoterpenes, of which (E)-b-ocimene (52.6%) and geraniol (18.3%) were the two dominant compounds. These major volatiles were accompanied by a lower content of aldehydes, esters and other mono- and sesquiterpenes. The fragrance is barely perceptible to the human nose. The closely related--but bird-pollinated--A. striatum and A. bracteosum produced no detected emissions (Micheneau et al. 2010).
In their 3-year study of pollination and reproductive success in natural populations of A. cadetii, Micheneau et al. (2010) found that the rate of pollinarium removal averaged 46.5%, pollen deposition reached about 27.5%, and fruit set averaged 21.9%. Fruit set ranged between 11.9% and 43.3% at additional sites sampled in 2008 (Micheneau et al. 2010).
Angraecum cadetii is self-compatible, but requires a pollinator to achieve fruit set. In their studies of this orchid, which included videotaping flowers at night, Micheneau et al. (2010), observed just one pollinator species, an undescribed species of raspy cricket (Gryllacrididae, Orthoptera). These insects, which are nocturnal foragers, reached flowers by climbing up leaves of the orchid or jumping across from neighboring plants and probed the most ‘fresh-looking’ flowers on each plant. Visits to flowers were relatively long (if compared with the behavior of birds or hawkmoths), averaging 16.5 seconds, with a maximum of 41.0 seconds. At the study La Plaine des Palmistes (Pandanus forest) study site, 46.5% of flowers had pollen removed and 27.5% had pollinia deposited on stigmas. The proportion of flowers that set fruit was relatively high, ranging from 11.9% to 43.4%, depending on the sites sampled. (Micheneau et al. 2010)
The other two Angraecum in section Hadrangis, A. bracteosum and A. striatum, are pollinated by small songbirds (Zosterops spp.) on Reunion (Micheneau et al. 2006, 2009).
Although further study is needed, observations and experiments to date indicate that (a) at least some orthopterans regularly visit orchid flowers, presumably for the nectar they contain (food reward), and (b) the role of these insects in the pollination of A. cadetii does not appear to be merely occasional or incidental. (Micheneau et al. 2010)
Evolution and Systematics
Systematics and Taxonomy
Angraecum cadetii belongs to section Hadrangis of Angraecum. Hadrangis is well supported as a monophyletic group in molecular phylogenetic analyses (Micheneau et al. 2008). Hadrangis is endemic to the Mascarene islands (Mauritius, Reunion, and Rodriguez in the Indian Ocean). It includes just three known species: A. bracteosum, A. striatum, and A. cadetii (Micheneau et al. 2006, 2008).
The Malagasay (Madagascar) section Humblotiangraecum is sister to the Mascarene Hadrangis (Micheneau et al. 2008).