THE ROLE OF RESIN IN ANGIOSPERM POLLINATION: ECOLOGICAL AND CHEMICAL CONSIDERATIONS

Members of at least two unrelated genera of plants, Dalechampia (Euphorbiaceae) and Clusia (Guttiferae), attract pollinators by secreting resins from floral structures. Bees that pollinate these flowers collect and use the resin in nest construction; these include Euglossa, Eulaema, Eufriesea, Trigona and, Hypanthidium in the neotropics, and Heriades in Africa. Floral resins are slow in hardening as compared with many other plant resins; this facilitates collection and storage by bees, hence probably enhancing the attractiveness of the flowers. Floral resins are probably of great utility and dependability and may be an especially important resource to certain bees, which in turn are major pollinators of many species of tropical plants. Bees appear to forage floral resin using a "strategy" of energy efficiency; large bees collect resin only from copious sources, smaller bees collect resin from sources with small amounts as well as from sources with large amounts of resin. Resin secretion in flowers may have originated as defense against herbivores and secondarily assumed the role of pollinator reward. IN THE PAST FEW DECADES there has been a growing awareness among pollination ecologists of the diversity of rewards offered pollinators by flowering plants (Simpson and NeS, 1981). Most early investigations focused on the roles of nectar and pollen in attracting pollinators. Some of the first investigations of alternative reward systems were of relatively restricted, often specialized systems, including the Yucca-Tegeticula relationship (Riley,1892), Ficus-Agaonidae relationship (McLean and Ivimey-Cook, 1956; cf. Janzen, 1979, and Wiebes,1979), and several "odor flowers" (e.g., Ceropegia; L. Muller, 1926). Vogel's studies (1969, 1974) of oil-producing flowers were the first to show that reward systems other than nectar and pollen are both common and widespread, especially in the tropics. Although the use of resins by bees for nest construction is widely known, it has been largely assumed that these materials were only collected from wounds in resinferous plants or from vegetative structures that have protective coatings of resin (Iwata,1976; Krombein,1967; l Received for publication 13 December 1982; revision accepted 27 March 1984. Portions of this work were supported by grants from University Research Expeditions Program (UREP, University of California, Berkeley) and NSF Grants DEB7724263 and DEB78-24218. I thank D. H. Janzen, C. D. Michener, S. F. MacLean, P. B. Reichardt, D. W. Roubik, D. S. Seigler and three reviewers for helpful comments on the manuscript. Grigarick and Stange, 1968; Stephen, Bohart and Torchio, 1969; Zucchi, Sakagami and de Camargo, 1969; Michener, 1974). H. Muller (1879) was the first to publish an observation of bees collecting resins from floral structures. Additional observations by Cammerloher (1931), Skutch (1971) and Armbruster and Webster (1979) indicate that at least two unrelated genera and perhaps several hundred species of tropical plants offier resin as floral rewards to pollinating bees. The purpose of this paper is to bring together scattered observations on the pollination of resin-producing flowers and discuss the resin-reward syndrome in light of chemical, physical, energetic, and ecological considerations. USE OF RESIN BY BEES-Many insect taxa have been reported collecting plant resins for use in nest construction. Among these taxa, by far the most diverse and abundant are members ofthe superfamily Apoidea (bees), especially the families Apidae and Megachilidae (Table 1). Female euglossine bees (Euglossini, Apidae) collect resin with their mandibles and transfer it to the corbiculae of their hind legs, where it is transported in the same manner as pollen (Armbruster and Webster, 1979; Dressler, 1982). Similar behavior is reported for meliponine bees (Meliponini, Apidae) and honey bees (Apis; Apini, Apidae) (Schwarz, 1948; Michener, l 974). As a consequence, bees in the family Apidae generally do not collect resin and