l insects. Queen honeybees secrete “queen substance” from their mandibular glands. When an unfertilized queen leaves the colony, queen substance acts as an olfactory attractant for males. The same compound within the colony modifies the behaviour of workers, preventing them from rearing more queens, and also affects their physiology, disrupting the development of their ovaries.
Movement toward an odour source
Attraction to the source of an odour poses problems for all animals using the sense of smell. It had been supposed that animals simply moved up a concentration gradient, from an area of low odour concentration to an area of high odour concentration, ending near the source of an odour. However, consideration of the movement of odour molecules in air or water showed that, in general, such gradients do not exist under natural conditions. Wind flow varies in both direction and strength. In addition, during the day, when the ground is heated, rising and falling air movements contribute to turbulence. As a result, odour molecules, even when continuously released at the source, become dispersed as a series of wisps, similar to the way that smoke from a chimney becomes dispersed. As a consequence, a stationary animal or an animal moving toward an odour source in a straight line will encounter bursts of odour with relatively long intervals between bursts. This is true whatever the distance from the source, although at short distances bursts contain more peaks with high concentrations of odour molecules. Only by averaging the concentration over a period of time and distance is it possible to follow a gradient of odour. Some animals may do this, but insects (and probably many other organisms) use a different strategy. In these organisms an odour has the effect of switching on a behavioral program that uses some signal other than odour to locate the source. In many cases the other signal is wind direction, and the animal moves upwind, ultimately arriving at the source of an odour. This mechanism is called odour-modulated anemotaxis. It is used by male moths to locate females, by moths flying to a flower odour to obtain nectar, and by cabbage root flies flying toward a cabbage plant to lay eggs.
Wind direction may be determined by its mechanical effect on the body, and in insects this involves structures at the bases of the antennae and mechanosensory hairs on the head. The behaviour involved in moving upwind varies. Larval insects such as those of the desert locust walk directly upwind if they smell food after having been without it for some time, and adult golden rod beetles exhibit similar behaviour. Cabbage root flies, when they perceive the host odour, orient into the wind while still on the ground and then make a short, straight flight of perhaps one metre before landing. The arrival of a new puff of odour causes them to reorient to the wind and repeat the process. Thus, their movement toward the odour source involves a series of short flights. However, in many insects odour causes takeoff into the wind, followed by a zigzagging flight toward the source, much as a sailboat might tack into the wind. During most of the movement, the insect is flying across the wind with its body oriented obliquely upwind. As a result, it drifts sideways, as an airplane does in high winds. This sideways drift produces a flow of images of the ground across the insect’s eyes, and the insect adjusts its power output to maintain its general upwind movement. The same mechanism is used by nocturnal insects. However, in some day-flying insects such as tsetse flies, the flight toward an odour source may be much more direct, with the odour causing takeoff but flight being directed toward any moving object that is visible upwind.
Odour gradients, in which the concentration declines progressively with increasing distance from the source, probably do exist in very still environments such as those occurring in the soil. The soil-dwelling larvae of some insects that feed on roots, such as the corn root worm (the larva of a beetle), have been shown to move 