6533b82dfe1ef96bd1291251

RESEARCH PRODUCT

A survey of the function of the lethal kettle traps of Arisaema (Araceae), with records of pollinating fungus gnats from Nepal

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Abstract Evidence from recent research combined with an evaluation of the literature indicates that Arisaema is adapted to pollination by fungus gnats. It apparently shares this peculiarity among aroids only with the distantly related genus Arisarum . In addition to previous records from Japan and North America, systematic collections from nine Arisaema species during several expeditions in the Himalayas in Nepal showed that, although other less efficient insect groups may participate, the nematoceran families Mycetophilidae and Sciaridae are the principal pollen vectors; they best fit the pollination apparatus of the mainly (para)dioecious kettle trap blossoms. A total of 16 fungus gnat genera (both Mycetophilidae and Sciaridae) comprising 47 identified species (among them one genus and 22 species new to science) were observed. Usually members of more than one taxon are attracted per Arisaema species, and both sexes of gnats are involved. Visitor sets differed to some degree, depending on host species, area, and altitude; they do not, however, represent the complete fungus gnat fauna of a region. Relevant traits of growth habit and inflorescence structure are surveyed, and a detailed description of the pollination process is given, based on observations made on specimens cultivated in Europe, where vicariant fungus gnats are the pollinators. Attraction is deceptive by mimicking olfactory, visual and tactile cues characteristic of fungi where the gnat sexes normally meet and females oviposit. Odours produced by osmophores (the spadix appendage or spathe tip) play the most important part. The identified volatile compounds are mainly short-chained aliphatic aldehydes and alcohols. Flagelliform osmophores of some species, up to several dm long, reach the substrate and serve as conducting paths. Gnats become imprisoned when they inadvertently slide down the inner wall of the spathe tube. Its surface including the spadix is covered with ablative wax particles that inactivate the insects» tarsal pulvilli. This gliding device, in some species reinforced by zones of imbricate papillae, is irreversible, and no movements of floral parts allowing escape via the spathe mouth occur. Anthesis lasts several weeks. In the male spathes, victims are forced to wade through the pollen masses that have accumulated at the spathe's bottom; there they find an exit hole formed by local gaping of the spathe rims. The female spathes lack such an aperture. After potentially depositing pollen on the stigmas during their efforts to escape, gnats are detained until death. Except for an autogamous subspecies of A. flavum Schott, the few monoecious taxa of Arisaema are protogynous and possibly self-sterile. In monoecious A. tortuosum (Wall.)Schott, exit forming is delayed until pollen release. Three Arisarum species that are suspected to deviate in their floral syndrome are discussed along with araceous genera possessing similar one-way pitfall mechanisms.