Pollination of the Orchid Microtis parviflora R. Br. by Flightless Worker Ants

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Abstract
Flightness worker ants of the species complex Iridomyrmex gracilis (Lowne) are effective pollinators of Microtis parviflora R.Br. in the Castlereagh Forest near Sydney. Field and laboratory observations and experiments showed that this orchid is self-compatible but not autogamous and ant pollination results in very high levels of seed set. This is the first study to show that wingless ants can be exclusive pollinators. Ants forage persistently, visiting individual flowers and inflorescences repeatedly for nectar. Foraging patterns most likely result in high levels of selfing, but leptokurtic distributions of pollinator travel distances suggest that some pollen transfers result in cross pollination. Microtis pollen carried by I. gracilis workers is unaffected by antibiotic secretions from the metapleural glands which, in other ant species, may reduce pollen viability drastically. Pollinia are separated from the ant integument by a short stalk and are always carried on the frons, remote from the metapleural glands. This study shows that worker ants, although wingless, can be efficient pollinators and suggests that the presence of metapleural glands is not necessarily inimical to the evolution of ant pollination, particularly if mechanisms, such as stalked pollinia, prevent direct contact between ants and pollen.

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