CONTACT CHEMORECEPTION RELATED TO HOST SELECTION AND OVIPOSITION BEHAVIOR IN THE MONARCH BUTTERFLY, DANAUS-PLEXIPPUS

Behavioural events during host selection by ovipositing monarch butter flies (Danaud plexippus (L.), Danainae, Nymphalidae) include tapping t he leaf surface with fore-tarsi and touching this surface with mid-tar si ('drumming') and antennae. Flavonoids identified from host plant ex tracts are known to stimulate oviposition. Scanning electron microscop y revealed the presence of contact-chemoreceptor sensilla on all appen dages that contact the leaf surface. This electrophysiological study w as conducted to identify the contact chemoreceptors that are sensitive to the known oviposition stimuli and are therefore probably involved in host recognition. Receptor cells of conspicuous sensilla grouped in clusters on fore-tarsi of females were sensitive to the behaviourally active butanol fraction of host plant (Asclepias curassavica) extract . However, these receptors generally had low sensitivity to three ovip osition-stimulating flavonoids identified from this fraction, but they were also sensitive to the butanol fraction of a non-host (Brassica o leracea). Chemoreceptors in sensilla of the tarsomers 24, of the mid-l egs also responded to the behaviourally active fraction of host plant extract and showed some sensitivity to two of the flavonoids that stim ulate oviposition. Similar results were obtained from receptor cells i n sensilla on the tip of the antennae. Most of these sensilla had cell s responding to the butanol fraction of A. curassavica extract but onl y 25% of them were also sensitive to one of the behaviourally active f lavonoids. These electrophysiological results, in combination with beh avioural observations, suggest that host selection in monarch butterfl ies relies on a complex pattern of peripheral sensory information from several types of tarsal and antennal contact chemoreceptors.