Edward S. Ross 



Sex, Drugs, and 

 Butterflies 



For male milkweed butterflies, a dead, withered leaf 

 may have a chemical allure no pretty flower can match 



by Michael Boppre 



Observe the butterflies, sombre black fel- 

 lows. . .flying in a crowd round a shrub with 

 thick silvery-looking leaves. It is the 

 Toumefortia Argentifolia, a tree that I see 

 on almost every seashore that I have visited 

 throughout the Pacific... A branch is bro- 

 ken, and the leaves are hanging dry and 

 wilted. The butterflies settle on the dead 

 leaves in swarms, almost pushing and 

 jostling one another to get a good place. No- 

 tice that it is the withered leaves and flowers 

 that they prefer, and seem to become half- 

 stupid in their eagerness to extract the pecu- 

 liar sweetness, or whatever it is, that the 

 leaves contain. 



Since these observations were pub- 

 lished in 1890 by C. M. Woodford, in A 

 Naturalist Among Headhunters, other re- 



ports in the scientific literature have de- 

 scribed butterflies apparently sucking at 

 dead parts of Toumefortia trees and a 

 number of other, unrelated plants. For 

 nearly a century, these reports were a great 

 puzzle to naturalists and scientists: first, 

 because dead plants are dry and butterfly 

 mouthparts are designed to suck up liq- 

 uids, and second, because only male but- 

 terflies were seen at the dead leaves. Only 

 in the last few decades have scientists in 

 Australia, Europe, and the United States 

 pieced together an explanation involving 

 complex interactions of sexual communi- 

 cation and chemical protection. 



The butterflies Woodford watched were 

 members of the genus Euploea (com- 



Left: Male crow and blue tiger butterflies congregate on a bundle 

 of dried Heliotropium plants, in search not of food but of 

 pyrrolizidine alkaloids. Numerous other insects, such as the 

 snouted tiger moth, above, are attracted to the dried parts 

 of plants containing these protective compounds. 



Michael Bopprd 



monly known as crows) in the milkweed 

 butterfly subfamily, Danainae. Other fam- 

 ily members include the tiger, queen, and 

 monarch butterflies. Males of all danaines 

 possess hairy glandular organs. Nine- 

 teenth-century naturalist Fritz Miiller pro- 

 posed that all these "pencils, tufts or 

 manes of hair," which he found in a vari- 

 ety of forms in the males of many butterfly 

 species, were odoriferous organs serving 

 "as an excitement to the opposite sex." 

 The proof came nearly one hundred years 

 later In the mid-1960s, smdies by Lincoln 

 Brower (now at the University of Florida, 

 but then at Amherst College) and his co- 

 workers showed that male Florida queen 

 butterflies locate females visually and, 

 once they are within close range, emit 

 chemicals from these glandular organs, or 

 hairpencils, to seduce them. Such chemi- 

 cal sexual stimulation is widespread in 

 butterflies and moths, but the danaines ex- 

 hibit one of the most elaborate chemical 

 communication systems known among 

 the Lepidoptera. (The American monarch, 

 Danaus plexippus, is an exception. In the 

 mating strategy of this species, chemical 

 communication plays a minor role. Male 

 scent organs are much reduced and rarely 

 employed in sexual interactions, which ap- 

 pear to the human eye more like rape than 

 seduction.) 



During courtship, a danaine male hov- 

 ers above a female. He exfiudes his hair- 

 pencils (usually hidden inside his ab- 

 domen) close to her antennae and then 

 expands them, often for just fractions of a 

 second. In many species, the sudden pro- 

 trusion and expansion of the hairpencils 

 deUvers tiny, pheromone-laden particles to 

 the female's antennae, which are lined 

 with olfactory receptors. Without ade- 



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