This study describes the specialized brood-site-based pollination syst
em of Peltandra virginica Kunth (Araceae) with the chloropid fly Elach
iptera formosa Loew and provides experimental evidence that gender-rel
ated changes in floral odor composition synchronize pollinator behavio
r with the blooming sequence. P. virginica is protogynous and does not
self-pollinate because of a strong temporal separation in sexual func
tion, and it is dependent upon insects for pollination because the spa
the completely surrounds the spadix during the pistillate stage. Field
observations conducted in central New Jersey and southeastern Pennsyl
vania showed that E. formosa is closely associated with P. virginica i
nflorescences. Within the floral chamber, E. formosa adults feed on po
llen, mate, and find oviposition sites while the larvae complete their
development. Although drosophilid and syrphid flies were collected an
d reared from inflorescences of both sexual stages, only E. formosa em
erged from pistillate-stage inflorescences and adult E. formosa rapidl
y transferred fluorescent dye particles between inflorescences of both
sexual stages in laboratory enclosures. These findings indicate that
this fly species is the primary pollinator at our study locations. Fie
ld censuses demonstrated that although E. formosa visited P. virginica
inflorescences of both sexual stages with equal frequency, the female
flies preferentially oviposited within pistillate-stage inflorescence
s. Analysis of floral volatiles with gas chromatography-mass spectrome
try showed that only the spathes emit the floral fragrance and that th
e composition of the floral volatiles changed during the transition fr
om pistillate to staminate stage. A particularly noticeable change occ
urred in the emission ratio of the two primary floral odor components,
4,5,7-trimethyl-6,8-dioxabicyclo[3.2.l.]octane and an unidentified ch
emical analog with molecular weight of 142. The relationship between f
loral volatile composition and fly oviposition behavior was evaluated
using sham inflorescences placed in the P. virginica study population.
The sham inflorescences were constructed by covering spadices dissect
ed from mature inflorescence buds with spathes excised from different-
age inflorescences. After a 2-hr-long exposure period, sham infloresce
nces fitted with pistillate-stage spathes contained more E. formosa eg
gs than the sham inflorescences with staminate-stage spathes. However,
the highest number of ovipositions occurred in sham inflorescences fi
tted with spathes excised from mature inflorescence buds. The fragranc
e emitted by these spathes was composed almost entirely of the two pri
ncipal odor components. The decline in ovipositions observed in both r
eal and sham inflorescences corresponded to an increase in the emissio
n ratio of 4,5,7-trimethyl-6,8-dioxabicyclo[3.2.1.]octane to the molec
ular weight 142 analog. This suggests that gravid flies searching for
oviposition sites used the ratio of the two primary floral volatile co
mponents as an inflorescence gender or age recognition cue.