The spontaneous activation of Aedes aegypti, and their responses to external stimuli (especially changes in temperature, humidity and carbon dioxide concentration) have been analyzed in a way that allows the controlling neural mechanisms to be inferred. In general, the flying insect turns back on leaving a favorable air stream, but does not turn on entering one. The normal attack program reduces to four basic steps: a general search initiated by a small rise in the CO2 level, no turn when the insect enters the "host stream" (i.e., the convective or other air current whose temperature and/or relative humidity has been modified by the host), an induced turn on leaving the host stream, and a visual landing. Repellents resemble CO2 in activating previously resting mosquitoes. If, however, the host stream contains a repellent vapor, the insect is made to turn on entering the host stream and is prevented from turning on leaving it. Thus the repellent vapor reverses the normal response to the host stream in two ways that are vital to success in locating a warm-blooded host. It also tends to inhibit probing. The antennal setae through which the various environmental influences are perceived can be inferred from a quantitative consideration of the responses.