Pollen feeding in butterflies

The unique pollen feeding behaviour of the closely related butterflies from the genera Heliconius and Laparus (Nymphalidae) represents a key innovation for their advanced life-history. These neotropical butterflies feed on floral nectar and pollen; the latter provides a vital source of nitrogen for adult cyanogenesis, extended reproductive life and nuptial gifts. The mouthparts and the flower probing behaviour show adaptations to pollen gathering and to the extra-oral extraction of amino acids from adherent pollen grains. Internal organs have not yet been examined in detail. The extraction process involves the use of a fluid of uncertain origin and an unknown mechanism for dissolving substances from pollen. The evolutionary origin of this intriguing and novel insect feeding behaviour is unresolved. The aim of this 3-year research project is to elucidate the mechanism and evolution of pollen extraction. In a comparative approach we want to (1) identify which body fluid is used for pollen processing, (2) determine whether there are adaptations of internal organs in pollen feeding butterflies, (3) show how pollen grains are altered during processing and therefore perhaps becoming lost for pollination, (4) demonstrate how amino acids (or other nitrogen components) are extracted from pollen on the outside of the proboscis and (5) draw conclusions on the evolutionary origin of the pollen processing behaviour by analysing proboscis cleaning behaviour in related butterflies. The study shall combine morphological methods (LM and EM-techniques), chemical analysis and experimental work in the laboratory of the University of Vienna with pollen sampling and field studies in La Gamba Biological Station (Costa Rica). Laboratory populations of butterflies (from the genera of Heliconius, Laparus and outgroup species) should help standardize chemical and behavioural tests which can be compared with results from various Heliconius species of natural populations. Understanding the mechanism of pollen feeding should provide insights into the novel feeding behaviour which opened a new adaptive field to butterflies and allow conclusions to be made on the evolution of the key innovations which are pivotal for the sophisticated behaviour of these insects.

The unique pollen feeding behaviour of Heliconius and Laparus (Nymphalidae) butterflies represents a key innovation for their advanced life-history. These neotropical butterflies feed on floral nectar and pollen; the latter provides a vital source of nitrogen for adult cyanogenesis, extended reproductive life and nuptial gifts. Previously the extraction process has not been studied in detail and hypotheses of the evolutionary origin of this intriguing and novel feeding behaviour in butterflies were missing . The 3-year research project resulted in the elucidation of the mechanism and evolution of pollen extraction. (1) Salivary fluid - not regurgitated nectar - could be identified as the body fluid which is used for pollen processing. (2) The salivary glands were found to be larger in pollen feeding butterflies than in related species. (3) Unique in butterflies, the saliva was shown to contain enzyms to digest proteins. (4) An experimental approach showed that pollen grains are damaged during processing. The pollen processing is now regarded to be an extra oral digestion which could be found for the first time in Lepidoptera. (5) A comparative behavioural study showed that pollen processing behaviour is a derived proboscis cleaning behaviour which was described for the first time in butterflies. The study combined morphological methods, chemical analysis and experimental work in the laboratory of the University of Vienna and the University of Texas (Austin) as well as field studies in the Tropical Research Station La Gamba (Costa Rica). Our results provided better understanding of the novel feeding behaviour which opened a new adaptive field to these butterflies and allowed conclusions to be made on the evolution of a key innovation which is pivotal for the sophisticated behaviour of these extraordinary butterflies.