DEET was historically believed to work by blocking insect olfactory receptors for 1-octen-3-ol, a volatile substance that is contained in human sweat and breath. The prevailing theory was that DEET effectively "blinds" the insect's senses so that the biting/feeding instinct is not triggered by humans or animals which produce these chemicals. DEET does not appear to affect the insect's ability to smell carbon dioxide, as had been suspected earlier.
However, more recent evidence shows that DEET acts as a true repellant that acts because it is unpleasant to the insect: A type of olfactory receptor neurons in special antennal sensilla of mosquitoes has been identified that is activated by DEET as well as other known insect repellants such as eucalyptol, linalool, and thujone. Moreover, in a behavioural test DEET had a strong repellant activity in the absence of body odor attractants such as 1-octen-3-ol, lactic acid, or carbon dioxide and female and male mosquitoes showed the same response
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