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

Abstract

Evidence from recent research combined with an evaluation of theliterature indicates that Arisaema is adapted to pollination by fungusgnats. It apparently shares this peculiarity among aroids only with thedistantly related genus Arisarum. In addition to previous records fromJapan and North America, systematic collections from nine Arisaemaspecies during several expeditions in the Himalayas in Nepal showedthat, although other less efficient insect groups may participate, thenematoceran families Mycetophilidae and Sciaridae are the principalpollen 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 Arisaemaspecies, and both sexes of gnats are involved. Visitor sets differed tosome degree, depending on host species, area, and altitude; they do not,however, represent the complete fungus gnat fauna of a region. Relevanttraits of growth habit and inflorescence structure are surveyed, and adetailed description of the pollination process is given, based onobservations made on specimens cultivated in Europe, where vicariantfungus gnats are the pollinators. Attraction is deceptive by mimickingolfactory, visual and tactile cues characteristic of fungi where thegnat sexes normally meet and females oviposit. Odours produced byosmophores (the spadix appendage or spathe rip) play the most importantpart. The identified volatile compounds are mainly short-chainedaliphatic aldehydes and alcohols. Flagelliform osmophores of somespecies, up to several dm long, reach the substrate and serve asconducting paths. Gnats become imprisoned when the; inadvertently slidedown the inner wall of the spathe tube. Its surface including the spadixis covered with ablative wax particles that inactivate the insects'tarsal pulvilli. This gliding device, in some species reinforced byzones of imbricate papillae, is irreversible, and no movements of floralparts allowing escape via the spathe mouth occur. Anthesis lasts severalweeks. In the male spathes, victims are forced to wade through thepollen masses that have accumulated at the spathe's bottom there theyfind an exit hole formed by local gaping of the spathe rims. The femalespathes lack such an aperture. After potentially depositing pollen onthe stigmas during their efforts to escape, gnats are detained untildeath. Except for an autogamous subspecies of A. flavum Schott, the fewmonoecious taxa of Ariseama are protogynous and possibly self-sterile.In monoecious A. tortuosum (Wall.)Schott, exit forming is delayed untilpollen release. Three Arisarum species that are suspected to deviate intheir floral syndrome are discussed along with araceous generapossessing similar one-way pitfall mechanisms. (C) 2000 The LinneanSociety of London.