For the time being, at least one passionflower species,
Passiflora adenopoda, may have a firm advantage in
the coevolutionary arms race. Its leaves have a dense
covering of minute, hooked spines, called trichomes,
resembling a bed of nails. The trichomes impale the
soft- skinned caterpillars, and once a caterpillar is
stuck, it starves. Score one for the plant.
Trichomes occur on other plant species, and at
least one butterfly species, Mechanitis isthmia (an
ithomiine, not a heliconid), has adapted to thwart
trichome defense. Mechanitis caterpillars, which feed
on plants of the nightshade family, avoid impalement
by spinning a fine web to cover the trichomes, so they
can safely move over the leaf surface to feed on the leaf
edges. The arms race continues.
Why Are Heliconid Butterflies Obvious?
Heliconid butterflies are among the most obvious,
strikingly patterned butterflies of the tropics (plate 11-
45). They fly slowly, almost delicately, and are very easy
to see along forest edges as well as in interior rain forest.
When atop a plant, many of these butterflies glow
brilliantly, as if lit up. Why are they so conspicuous?
Consider the potential risk to the insect. Most tropical
bird species feed heavily on insects. For a butterfly
group to be colored like neon signs, practically saying
“eat me,” seems like a death wish.
But being conspicuous serves as a warning. Recall that
coral snakes are dangerous and obvious. If heliconid
butterflies are distasteful to would- be predators, a
selection pressure favoring aposematic coloration
becomes likely. Once a bird has eaten an unpalatable
insect, bright coloration would tend to facilitate its
learning avoidance behavior. It has been demonstrated
that birds are capable of remembering butterfly warning
coloration and will avoid unpalatable butterflies once they
have experienced them. Lincoln Brower demonstrated
that birds avoid eating milkweed- feeding butterflies, all
of which are bright orange, once they have experienced
the indigestion that follows (plate 11- 46). Milkweeds
contain high volumes of cardiac glycosides that
obviously help protect the plants. Heliconid butterflies
are likely to store the plant defense compounds they
take in as caterpillars (or to use them as precursors to
synthesizing their own unique defense compounds) and
thus become unpalatable to predators. In other words,
the chemical defense of the plant becomes, in turn, the
chemical defense of the insect.
In an experiment conducted by W. W. Benson
in Costa Rica, the wing coloration pattern of the
unpalatable butterfly Heliconius erato was altered,
and the new patterning was one that predatory birds
would not have seen before. A control group identical
to normally occurring specimens was also established.
Equal numbers were released of the altered, uniquely
patterned individuals and the normal butterflies of
the control group. Significantly fewer of the altered
individuals were recaptured by the researchers, an
indication that fewer had survived. Some alteredPlate 11- 46. The bright orange Monarch Butterfly (Danaus
plexippus), here feeding on aster flowers, is the best- known
of the milkweed butterflies and the one Lincoln Brower used
in his experiments to demonstrate that Blue Jays (Cyanocitta
cristata) avoid Monarchs after being sickened by eating them.
Photo by John Kricher.Plate 11- 45. Even from a considerable distance, Heliconius
butterflies are obvious. This one is either H. melpomene or H.
erato; they are very difficult to distinguish. Notice how the wing
pattern seems to focus the eye on the most vulnerable part of
the insect, seemingly daring a predatory bird to attack it.
Photo by John Kricher.
198 chapter 11 evolutionary arms races: more coevolution, more complexity