• chemical signals;
  • Diurideae;
  • floral odour;
  • GC-EAD;
  • Ichneumonidae;
  • pollination

Pollination by sexual deception is among the most intriguing of orchid pollination syndromes. Odours are well established as the primary stimuli for sexually attracting the male insect pollinators in these orchids. We applied gas chromatography with electroantennographic detection (GC-EAD) to investigate chemical communication between the sympatric, but morphologically distinct, orchids Cryptostylis erecta and C. subulata and their pollinators. Cryptostylis is unusual among sexually deceptive orchid genera in that all five Australian species share the same pollinator, the ichneumonid wasp Lissopimpla excelsa, but hybrids are unknown. We show that volatile odour compounds are not produced in detectable amounts in either species. Floral extracts containing many low-volatility compounds showed considerable differences in composition between C. erecta and C. subulata. By contrast, GC-EAD revealed the male wasp pollinators are electrophysiologically responsive to the same GC peak on two different kinds of GC column in both orchids. This leads us to predict that a single compound is the sexual attractant in all Australian Cryptostylis. The apparent conservation of chemical signals among distinct species contrasts with that of other sexually deceptive orchids that are often morphologically similar but reproductively isolated by their different chemical signals. © 2004 The Linnean Society of London, Botanical Journal of the Linnean Society, 2004, 144, 199–205.