d 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 along chemical gradients.
Many insects produce a sex-attractant pheromone, by which one sex attracts the other from a distance. Among moths, it is common for the female to produce a sex-attractant pheromone. For example, female gypsy moths, which are flightless despite having fully developed wings, and female bagworms, which do not have wings, depend wholly on the power of their sexual odour to attract a mate. Female moth sex-attractant pheromones are produced in glands in the moth’s abdomen. When the female is ready to mate, she exposes the glands and disperses the pheromone into the air. This behaviour, known as calling, typically occurs at a time of day or night that is characteristic of the mating pattern of the species.
Sex-attractant pheromones can sometimes have unfortunate side effects for the insect producing them, because they can be used by other organisms to locate the insects. For example, males of the stinkbug genus Podisus produce a pheromone that attracts females as well as other males and immatures. It also attracts female parasitic flies of the family Tachinidae, providing the flies with an easy way to find their hosts, on which they lay their eggs. In some instances other organisms produce some of the sex-attractant pheromones of moths to mislead the moths. Late-stage immature and adult female bolas spiders in the genus Mastophora are known to produce some of the same components of the sex-attractant pheromone produced by females of some noctuid moths. The spider is active at night and hangs from a horizontal silk line. It produces a vertical thread, which it holds with one leg, and secretes a viscous fluid that forms a globule at the lower end of the thread. Male moths are attracted by the odour of what appears to be a potential mate, and the spider, apparently stimulated by the vibrations of the moth’s wings, uses its leg holding the thread to hurl the viscous globule at the moth. If the globule hits the moth, the moth becomes trapped, and the spider immobilizes it with venom by attaching the