C-9-10

CONVERGENT EVOLUTION IN HAWKMOTH-POLLINATED PLANTS? CHEMICAL ANALYSIS OF FLORAL ADVERTISEMENTS AND REWARDS.

Robert A. Raguso
Center for Insect Science, University of Arizona, Tucson, AZ 85721-0077,USA.


Hawkmoths (Lepidoptera: Sphingidae) are important pollinatorsof tubular, night-blooming flowers worldwide, especially in wet tropicaland xeric subtropical habitats. Such flowers are typified by palecoloration and sweet, penetrating fragrance as advertisements and copiousnectar as floral rewards. Foraging hawkmoths are thought to use botholfactory (distance or releasing cue) and visual (close range) stimulito find these flowers. I used GC-MS, HPLC and a hand-held refractometerto analyze the chemical composition of floral scents and nectars from aguild of hawkmoth-pollinated desert plants from Arizona, representing 13species from 7 angiosperm families. Floral scent chemistry variedconsiderably between species, with regard to complexity (class, numberand relative abundance of volatiles), site of emission (different floralorgans and vegetative tissues) and rates of emission per flower. However, differences in emission rates are negated when standardized forfloral mass, suggesting that volatile emissions “per plant” are comparablebetween species. There were no universally present scent compounds or classes,although oxygenated monoterpenoids and aromatic esters were frequentlyidentified; indeed, some species with yellow flowers were nearlyscentless. Absence of strong chemical convergence may reflect (1)a functional redundancy in distance attraction or release of feeding behaviorby different classes of scent compounds, (2) phylogenetic constraints onscent production, and/or (3) learned associative cues used by experiencedmoths to differentiate between variable nectar sources. Floral rewardswere more similar between species, featuring relatively dilute nectars(20-37% dissolved solides, by refractometer) that were dominated by sucrose(55-90% of sugar content), but also contained non-sugar constituents. These nectars are similar to those favored by hummingbirds, but differsubstantially from the hexose-dominated nectars used by bats (high volume,dilute) and bumblebees (low volume, highly concentrated).