The Mode of Action of Repellent Chemicals Against Blood-sucking Flies

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Abstract
Recent conflicting theories regarding host finding by blood-sucking flies and the mode of action of repellents are examined and discussed. The claim that repellents owe their activity to infra-red absorption in a narrow waveband (449–467 cm.−1) was examined by measurement of published curves for infra-red absorption by repellent, non-repellent, and doubtful chemicals. Peaks of absorption at this waveband in comparison with adjacent Wave numbers are three times as common in repellent as in non-repellent materials, but in comparison with absorption over the entire spectrum studied all groups of materials show a trough of absorption at this narrow waveband. No supporting evidence could be found for the claim that mosquito repellents masquerade as water vapour by virtue of their infra-red absorption spectra, nor for the suggestion arising out of this, that such repellents should be attractive at low humidities. Repellency of indalone was found to increase as humidity decreased.Regardless of the role molecular vibration may play in olfactory perception, the usefulness of infra-red radiation to blood-sucking flies seeking their hosts and to man seeking repellents for the flies remains to be demonstrated.