Passion fruit is one of the most exotic flavors in the world of botanical delicacies. The sweet tangy flesh is used to flavor ice cream, cheesecake, and mixed drinks around the globe. Rich in vitamin C and lycopene, eating passion fruit is said to sooth a queasy stomach according to Andean lore. As Bug of the Week continues its winter sojourn to warmer parts of the globe, our next stop is south Florida to find vines of the passion fruit and the spectacular zebra longwing butterfly. I had the opportunity to enjoy the antics of the remarkable zebra longwing, Heliconius charitonius, as it visited flowering trees and shrubs along a trail through a swampy forest. Tropical butterflies in the genus Heliconius, such as the zebra, are notable for their longevity. These beauties live up to six months. Zebras and their kin evolved an interesting strategy to gather nutritious pollen used to sustain their long lives. The zebra flies a well-defined route through the forest visiting flowers that present a fresh batch of pollen and nectar each day. This behavior is called traplining and is employed by many tropical pollinators including bees, hummingbirds, and bats.
Zebra butterflies have excellent eyesight and patrol a regular route each day to obtain pollen from flowers along their trapline.
After collecting a gob of pollen on its long, straw-like proboscis, the butterfly secretes specialized enzymes to release the amino acids and other nutrients in the pollen. The nutrients are absorbed through the membranes lining the proboscis and used to produce eggs and maintain the zebra’s high level of activity zooming about the forest. In addition to visiting flowers laden with pollen, the zebra also searches for different species of plants in the passion vine genus, Passiflora. These tropical vines bear the magnificent passion flower and following pollination, the passion fruit. Several species of passion vine are used by the immature stages of the zebra and other Heliconius butterflies as a source of food. These voracious caterpillars consume great quantities of leafy tissue on a daily basis. As a group, passion fruit plants are protected from most leaf-munching caterpillars and other vegan insects by a veritable witch’s brew of highly toxic chemicals including alkaloids, a family of toxins that include strychnine and nicotine, and cyanogenic glycosides, chemicals that release cyanide upon entering the body. However, using some clever metabolic machinery, the specialized Heliconius butterflies have turned the tables on passion fruit plants and now feast on their leaves with impunity. With so much munching by the larvae of the zebras and their kin, one wonders how the vines of Passiflora survive.
It seems that the mobile and eagle-eyed Heliconius butterflies locate passion vine plants by the shape of their leaves. To fool these clever herbivores, tropical passion vines have evolved leaves that vary dramatically in shape. In this way it is more difficult for butterflies to zero in on any one leaf shape as they search for food for their young. Some species of Passiflora have taken this game of deception one step further. They evolved small structures on the leaves and petioles resembling the eggs of Heliconius butterflies. Larvae of Heliconius are known to be cannibalistic and female butterflies may avoid placing eggs on a leaf if it is already occupied by another caterpillar or by an egg about to hatch. By creating a structure that resembles an egg, the passion vine hangs a sign that says “no vacancy” to the female butterfly looking for a spot to lay eggs. This remarkable act of mimicry helps the passion vine escape the ravages of very hungry caterpillars.
This hungry zebra caterpillar makes short work of a leaf.
References used in the preparation of this Bug of the Week include Caterpillars of Eastern North America by David L. Wagner, Butterflies East of the Great Plains by Paul Opler and George Krizek, and Coevolution of Animals and Plants by Lawrence Gilbert and Peter Raven.