Jeffrey Caldwell recently posted two interesting papers about clues that nectar plants provide to pollinators (in this case, moths) that indicate presence of nectar: carbon dioxide and humidity. No reason why these should not obtain for day-flying leps as well.
Caldwell writes on DesertLeps:
>>It is quite costly in terms of energy expenditure for moths to hover, and this research indicates that humidity emanating from the nectar supply of a flower is one of the cues that hawkmoths are able to detect and key in on, helping them search for food more efficiently:
The discovery is touted as that of “a previously unknown sensory channel that is used in plant-animal interactions”. The white-lined sphinx (Hyles lineata, Sphingidae) apparently can detect miniscule differences in relative humidity, namely such as the invisible plume of humidity emanating from a flower offering nectar.
The abstract — “Floral humidity as a reliable sensory cue for profitability assessment by nectar-foraging hawkmoths”– by Martin von Arx, Joaquin Goyret, Goggy Davidowitz, and Robert A. Raguso, an article of the Proceedings of the National Academy of Sciences of the United States of America, is here:
Full text of the article here:
and again about CO2 levels:
This paper says “adult herbivorous Lepidoptera have especially well-developed CO2 organs” — the Carolina sphinx (Manduca sexta, Sphingidae) and other adult-feeding moths have “large labial-palp pit organs that sensitively detect” carbon dioxide.
Datura wrightii, a favorite nectar source, emits a big whiff of carbon dioxide when it first opens and is full of nectar, possibly sort of a silent dinner bell for the moths.