Specialized use of pollen vectors by Caesalpinia gilliesii, a legume species with brush-type flowers

The pollination of Caesalpinia gilliesii (Fabaceae) , a legume species with long, brush-type flowers, was studied in wild populations in Argentina. A total of 341 hawkmoths (Sphingidae), belonging to 10 species, with tongues ranging from 1.5 to 13.5 cm in length, were captured by means of light traps. Hawkmoths could be classified in four significantly different tongue-length classes or guilds: short (15.35 ± 1.11 mm), medium (30.23 ± 2.24 mm), long (55.85 ± 4.98 mm) and very long (95.61 ± 13.63 mm). A total of 133 of the hawkmoths, belonging to seven species, carried pollen of Cae. gilliesii , mainly on their venter. Successful contact with anthers while drinking nectar depended upon hawkmoth tongue length. The relative frequency of pollen-carriage and the mean tongue length of each species were positively and significantly correlated. Considering a null hypothesis of nonspecialization, in which hawkmoths carrying pollen are distributed at random among individuals of the four guilds, long-tongued hawkmoths carried Cae. gilliesii pollen more frequently than would be expected by chance. In contrast, short- and medium-tongued guilds carried pollen less frequently than is predicted by random expectation. Individuals with short tongues accessed nectar from below without touching the anthers, whereas very long-tongued hawkmoths, with tongues longer than the length range of the flower reproductive organs, can probably reach nectar without touching the fertile organs. Results show that a pterotribic pattern of pollen deposition on the body of long-tongued hawkmoths is operating in this species. This suggests that brush-type blossoms are not necessarily unspecialized, despite popular belief.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 88 , 579–592.