Pearl Bodies as Ant Food: An Ecological Role for Some Leaf Emergences of Tropical Plants

Pearl bodies, single or multicelled leaf emergences, are characterized by a lustrous, pearl-like appearance; a spherical or club-like shape with a basal constriction; ease of detachment from the plant; storage of relatively large quantities of lipid; axd a limited size range, usually from 0.5 to 3.0 mm in length. They are broadly distributed in the Dicotyledonae and have been reported in 19 families and 50 genera, all of tropical or subtropical affinity. Several lines of evidence implicate these structures in plant-ant association; these are: at the generic level, the presence of pearl bodies is often associated (48%) with other ant-related features of the plant such as specializel domatia and extrafloral nectaries; when domatia or extrafloral nectaries are present, pearl-body production is parallel in time and space; and ants are known to collect pearl bodies from leaves and return them to their nests. These observations and correlations suggest that pearl bodies play an important role in many tropical plant-ant mutualisms. RECENT FIELD STUDIES OF PLANT-ANT INTERACTIONS have refocused attention on an old controversy: the role of extrafloral nectar (EFN). Evidence indicates that EFN production defines the pattern of ant foraging on the plant (Bentley 1977a, O'Dowd 1979, Pickett and Clark 1979) and can result in interference by attending ants with plant herbivores and seed predators as well as the maintenance of a favorable plant-nutrient environment (Bentley 1977a, Inouye and Taylor 1979, Kleinfeldt 1978). Where there is a high degree of reciprocal specialization in plant-ant interaction, e.g., A cacia-Pseudomyrmex; Cecropia-Azteca; and Macaranga-Crematogaster, a different food resource has been emphasized. In these associations, food bodies (Beltian, Mullerian and Beccarian bodies, respectively) are harvested by the ants and play an essential role in the well-being and reproduction of the ant colonies (Janzen 1966, 1969; Rickso'n 1980). Pearl bodies, similar in many respects to these ant-food bodies, are produced on the leaves and shoots of many tropical species. While the Beltian bodies of Acacia cornigera are actually modified leaf tips (Rickson 1969) and show developmental differences from pearl bodies as a whole, the anatomy and development of the ant-food bodies of Macaranga spp. and Cecropia splp. are consistent with descriptions of pearl bodies (Meyen 1837, Holmgren 1911). In fact, earlier researchers describe these ant-food bodies as "Perldrusen" (Meyen 1837), "perlules" (Rouppert 1926), and "parlharen" (Holmgren 1911). As a corollary to the role of EFN secretion suggested by Delpino (1886), Penzig (1892) hypothesized that pearl bodies serve as ant-food and represent a parallel and co'mplementary food resource to EFN. As was aptly pointed out by his contemporaries, e.g., Holmgren (1911), Penzig unfortunately provided no evidence for his interesting hypothesis. I bring together here the literature describing the characteristics and distribution of pearl bodies and couple it to the substantial research accrued over the last century on EFN and plant-ant interactions. Evidence, both direct and circumstantial, supports Penzig's contention that these leaf emergences are nutritionally directed and function as ant food. DEFINITION AND TERMINOLOGY OF PEARL BODIES Meyen (1837) first reported "pearl glands" and described them as single or multicelled emergences on shoots and leaves with a characteristic spherical shape and lustrous aspect. Following his initial observations they have been variously termed pearl bladders (De Bary 1877), pearl hairs (Holmgren 1911), and bead bodies (Wheeler 1910). Since there is no evidence that these leaf emergences are secretory, I term them "pearl bodies" consistent with Meyen's initial description but without the misleading description as glands. Pearl bodies are of heterogeneous origin and differ in morphology and development. They range from single to multicelled (fig. la-f) and are derived from both epidermal and subepidermal tissues (Penzig 1892). Nevertheless, they share many common features such as a characteristic, limited size range 1Present address: Department of Biological Sciences, The Flinders University of South Australia, Bedford Park, South Australia 5042, Australia. 40 BIOTROPICA 14(1): 40-49 1982 This content downloaded from 157.55. 39.201 on Sat, 23 Apr 2016 05:26:53 UTC All use subject to http://about.jstor. org/terms