Large Macrocyclic Lactones as Specific Cuticular Lipids of Eueides Butterflies

Linear long-chain hydrocarbons are common constituents
of the outer
cuticle of most insects. These cuticular hydrocarbons form the first
barrier to the environment, preventing desiccation, but also play
important roles in communication among many insects, transporting,
for example, information on species, sex, or physiological state. Eueides is a genus of heliconiine butterflies, a group known
for their complex mimicry systems. We report here the identification
and synthesis of large macrocyclic lactones from the cuticles of five Eueides species, which largely replace the hydrocarbons.
These lactones have chain lengths of 23–34 and ring sizes of
17–29. They occur in Eueides but not in other
closely related genera such as Heliconius, Dryas, Dryadula, Agraulis, or Dione. Each species has a specific mixture of
macrocyclic lactones. These unique bouquets may be used in species
recognition, as visual cues may be unreliable in mimicry rings. We
identified 43 different compounds using GC/MS, GC/IR, and enantioselective
synthesis via ring-closing metathesis and Jacobsen hydrolytic kinetic
resolution. The absolute configuration could not be determined because
these lactones were not separable by gas chromatography on chiral
phases.