The use of olfactory repellents, especially against biting insects, is an important protective measure. It is also a matter of common knowledge that certain insects are able to locate their mates or their prey largely through the agency of attractive scents. Concerning these latter, Dethier remarks that, “we recognize the fact that no one attractant alone performs the service of guiding an organism to its proper habitat, or mate, or food. The desired end is achieved by a complex array of stimuli working in harmony. In chemical attractants, however, one finds precision guidance. The action of others, as light, temperature, humidity, is comparatively gross, approximating as it does usually, a vicinity. The extreme value of chemical attractants to an organism lies in the specificity and accuracy of operation.” (1).

No abstract

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.

The vapor pressures of a number of solutions of H2S in water have been measured at temperatures between 5° and 60 °C. A new type of glass diaphragm manometer having several advantages is described, and a bibliography of flexible glass manometers is given. The results show that Henry's law is not strictly obeyed and that previously reported values may require correction. Discussion of the results is reserved for a later paper.

The upwind guidance of flying insects was considered in a previous paper (Wright, 1958) where a homing mechanism was suggested that did not require visual contact with the ground. The response of Drosophila to attractive scents presented in various ways has now been studied. The results show that visual contact with the ground is in fact essential, and other factors of the guidance system have been clarified.