glycosides and cyanohydrins. The high diversity of
cyanogens among passionflower species could be an
evolutionary response to herbivory by heliconids.
Heliconids, however, apparently adapt to the ever-
changing cyanogen regime by evolutionary changes
in their hydrolytic enzymes and by sequestration
of cyanogens. Heliconid caterpillars have obviously
coevolved with passionflower. They are pretty good
passionflower hackers.
Heliconid butterflies have adapted to a life cycle on
their host plant. Females lay small numbers of eggs
in globular yellow clusters directly on passionflower
leaves, favoring young shoots. When eggs hatch, the
caterpillars are conveniently sitting on their food
source. The plant is therefore under selection pressure
to somehow prevent the adult female butterfly from
locating, selecting, and laying eggs on its leaves.
Detailed studies by Lawrence Gilbert and W. W. Benson
and colleagues have demonstrated diverse passionflower
defenses that extend beyond defense compounds.
Passionflower produces extrafloral nectaries that attract
various species of ants and wasps. These insects help repel
heliconid caterpillars. Some passionflower species are
protected exclusively by ants, some by wasps, and some
by both. At least one study has shown that caterpillar
survival is much lower on Passiflora with attending ants:
caterpillar mortality rate was 70% on ant- attended plants,
compared with 45% on non- ant plants.
Some Passiflora extrafloral nectaries appear, to the
human eye, to mimic heliconid egg clusters. Perhaps
that is also how they appear to the Heliconius butterfly
eye. These passionflower vines typically have young
leaves spotted with a few conspicuous yellow globs,
the egg mimics. Female heliconids will not lay eggs on
a leaf already containing egg masses, and the mimic
egg masses presumably prompt the female to continue
searching. Lawrence Gilbert believes the mimic eggs
to be a recent evolutionary development in the plant-
insect arms race, because only 2% of passionflower
species have them.
Leaf shape varies within a species of passionflower,
and passionflower leaves often resemble those of
other common plant species growing nearby, showing
leaf mimicry. Perhaps heliconids may be tricked by
the similarity of appearance and thus overlook a
passionflower. This speculation depends, of course, on
the butterfly using visual cues to locate passionflower
vines. If the insect depends principally on scent, such
leaf mimicry would seem to be useless.Plate 11- 42. Heliconid butterflies (this one is Heliconius doris)
are found throughout the Neotropics and have been the
object of much coevolutionary study. Photo by John Kricher.Plate 11- 43. A Broad- billed Motmot (Electron platyrhynchum)
with prey, a large caterpillar. Birds are strong predators of
caterpillars and butterflies and exert strong selection pressures
on the evolution of these insects. Photo by Nancy Norman.Plate 11- 44. Passionflower vines (Passiflora) are the only plants
the caterpillars (larvae) of heliconid butterflies feed on.
This photo shows a plant and its inflorescence. Photo by John
Kricher.chapter 11 evolutionary arms races: more coevolution, more complexity 197