Figure 8.8. Heating of Xanthosoma
robustum inflorescences (solid circles);
open circles show an inflorescence on its
second evening; squares show the ambient
temperature. Also indicated are the
approximate times at which scarabs began
arriving at the newly opened inflores-
cence and at which the second-night
inflorescence released its pollen.
found that Xanthosoma robustum peaks around 40°C
even when the ambient temperature is as low as 8°C.
However, after reaching its peak, the inflorescence
cooled more rapidly in the refrigerator than at normal
air temperatures. These inflorescences had been cut
from the plant several hours earlier, but neither the
timing of heating nor the temperature reached was
affected,
A marking technique that allowed individual iden-
tification of each recaptured scarab (Goldwasser et al.
1993) showed that few scarabs move to the nearest
available inflorescence; most fly to more distant in-
florescences, passing over eligible ones nearby. This
tends to maximize outcrossing and enhance gene
flow. However, in three years of marking and recap-
turing scarabs on both sides of the Continental Divide,
I never found a scarab on one side that had been
marked on the other. These scarabs visit other aroids
besides Xanthosoma robustum; individuals may
switch among it, Syngonium sp., and Philodendron
sp. from one night to the next. This lack of host speci-
ficity would reduce the efficiency of pollination.
Three other types of visitors are associated with
this pollination system: (1) "interlopers," which ex-
ploit the interaction to the detriment of the aroid or
scarabs (Goldwasser 1987); (2) predators, which con-
sume other visitors to the inflorescences; and (3) com-
mensal species, which have little effect on the other
species. Interlopers include mites of the family Macro-
chelidae, which are present at any inflorescence that
has scarabs. The mites ride from one plant to the next,
20—30 of them on a single scarab. Between rides, the
mites drink nectar inside the chamber. Sucking bugs
of the family Miridae are consistent visitors to the
inflorescences of many aroids and sit on the soft parts
of the spathe, sucking plant juices. The mirids also
reproduce on the inflorescences; in Monteverde,
about a third of the three dozen individuals typically
present are nymphs. Single females of some species
of flies arrive at newly opening inflorescences before
other visitors arrive, walk around laying eggs, and
leave. The larvae grow inside the inflorescence with-
out causing evident damage, but then emerge as adults
by tunneling out and leaving holes in the spathe that
may increase the vulnerability of the developing fruits
to attack by fungus or animals (Madison 1979).
Spiders are the most striking of the predators at
aroid inflorescences. They are uncommon visitors to
Xanthosoma robustum, but deter or remove other
visitors so that inflorescences at which a spider is
present tend to be bare and free of damage. Other
predators include adult and larval staphylinids (rove
beetles) and the predatory larvae of syrphids (hover
flies). The syrphid larvae remain on the same plant
between flowerings and crawl up to each successive
inflorescence when it opens, growing larger each time.
Many other species seem to be commensal, includ-
ing small wasps, drosophilids and other flies, earwigs,
thrips, and psocids (lice). Several dozen drosophilids
typically sit on the upper part of the spathe; earwigs,
thrips, and psocids stay inconspicuously down in the
chamber and may feed on detritus. The consistent
presence of nonpollinating visitors at aroid inflores-
cences in Monteverde and elsewhere, and the involve-
ment of several life stages for many of them suggest
that some may be specific in their choice of aroids.
The pollination interaction between scarabs and
aroids provides resources that some species exploit
directly and a consistent setting for interacting with
other species. The number and variety of visitors270 Plant-Animal Interactions