The sensory physiology of the human louse Pediculus humanus corporis de Geer (Anoplura)

(i) Sensory responsesThe reactions of the body louse to temperature, humidity, smell, contact and light have been tested in an arena divided into two halves.Temperature. A temperature of 29–30° C. is preferred before 32° C. or 27° C. As the alternative temperature rises above 32° C. or falls below 27° C. the avoidance becomes increasingly strong. Different individuals vary in sensitivity.These results are in accordance with those observed in a linear gradient of temperature, in which the lice collect chiefly in the region from 28 to 31° C.The response is always to air temperature; there is no response to radiant heat from objects at 20–45° C.Humidity. The louse is generally indifferent to humidity over the range from 10 to 60 or 75% r.h. Higher humidities are avoided. But when offered two humidities the choice is greatly influenced by the conditions experienced by the louse in the immediate past; it avoids any change; hence different individuals may show quite different responses. Moreover, when offered the choice of very moist air (95 % r.h. or over) and very dry (47 % r.h. or under) the louse becomes more readily adapted to the moist air and begins to avoid the dry.Smell. The louse prefers cloth that has been in contact with human skin to clean cloth or cloth smelling of dog or rabbit. The smell of other lice and of their excreta is also attractive. Many substances serve as repellents; a refined petroleum with a very faint odour has been chiefly used for the experiments.Contact. When offered smooth and rough materials the louse chooses the latter. It moves more rapidly on smooth materials and does not come to rest so readily. It shows little response to air currents unless very strong, when they are avoided.Light. The movements of the louse are arrested or retarded by sudden exposure to a bright light, and sometimes it may show avoiding movements. But the movement of the louse towards dark places is mainly a response to directed light received by the horizontally placed eyes. Slight differences in the light received from different directions exert a much greater effect if the louse is exposed to a low level of general light intensity.The movement of the louse towards relatively small dark objects is probably a manifestation of the same response.