Cornell University, discovered, male 

 queen butterflies lacking this type of com- 

 pound are much less successful in getting 

 accepted by a mate. These findings sug- 

 gested that the chemicals played an impor- 

 tant role in the lives of the butterflies, but 

 no one knew just what that role was or 

 where the dihydropyrrolizines were com- 

 ing from. 



The answers to these questions began to 

 come in the mid-1970s, from scientists 

 working independently (John Edgar, with 

 the Commonwealth Scientific and Indus- 

 trial Research Organization in Australia) 

 and collaboratively (Jerrold Meinwald and 

 others at Cornell, and Dietrich Schneider 

 and me at the Max Planck Institute for Be- 

 havioral Physiology). We now know that 



adult male milkweed butterflies utilize 

 certain secondary plant compounds, 

 known as pyrrolizidine alkaloids (PAs), as 

 chemical precursors for synthesizing 

 dihydropyrrolizines. (Secondary plant 

 compounds are chemicals that are not part 

 of the plant's essential molecular makeup 

 but that frequently have a defensive func- 

 tion and lead to better survival.) 



The butterflies use their sense of smell 

 to locate the dry, withered, or damaged 

 parts of certain plants that contain 

 pyrrolizidine alkaloids. After landing on 

 an appropriate plant, the butterflies walk 

 about, probing the surface here and there 

 with their proboscises. Eventually they 

 settle down at one spot and release drops 

 of fluid on the plant. They tiien reimbibe 



The crow caterpillar, left, may gather and 

 store certain noxious plant compounds, 

 such as cardenolides, while feeding on its 

 host plant. Below: The chrysalis of a friar 

 butterfly has a strong metallic luster, the 

 effect of light reflecting off many thin 

 layers in the cuticle. 



Photographs by Michael Boppr6 



the fluid mixture and, with it, some of the 

 plant's PAs. Butterflies often congregate in 

 small groups and fight over spots previ- 

 ously wetted by others. What Woodford 

 saw a century ago was undoubtedly such 

 an incident, for Toumefortia trees contain 

 pyrrolizidme alkaloids. (Other PA plants 

 include Crotalaria, or rattlebox, in the pea 

 family; Senecio, or groundsel, in the aster 

 family, and Heliotropium in the borage, or 

 forget-me-not, family.) 



These alkaloids occur in Uving as well 

 as dead plants, but in live tissue, the com- 

 pounds are sealed within cell vacuoles, 

 where the butterflies cannot detect them. 

 If, however, a leaf has been damaged by, 

 say, leaf-feeding beetles, it may attract 

 male milkweed butterflies, which, chick- 

 enlike, scratch at it with their legs, creating 

 fresh tears in the plant tissue and thus 

 gaining access to the alkaloids within. 



Using pyrrolizidine alkaloids purified 

 from plant extracts, we have demonstrated 

 that the butterflies are after flie PAs and not 

 any other plant compounds. And flieir in- 

 terest in these chemicals is independent of 

 any nutritional requirements: their sole 

 reason for visiting PA-containing plants is 

 to gather the alkaloids. These butterflies, 

 then, visit two groups of plants: those they 



29 



