For example, "Thrips tabaci" damages crops of onions, potatoes, tobacco, and cotton. Some species of thrips create galls, almost always in leaf tissue. These may occur as curls, rolls or folds, or as alterations to the expansion of tissues causing distortion to leaf blades. More complex examples cause rosettes, pouches and horns. Most of these species occur in the tropics and sub-tropics, and the structures of the galls are diagnostic of the species involved. A radiation of thrips species seems to have taken place on "Acacia" trees in Australia; some of these species cause galls in the petioles, sometimes fixing two leaf stalks together, while other species live in every available crevice in the bark. In "Casuarina" in the same country, some species have invaded stems, creating long-lasting woody galls. While poorly documented, chemical communication is believed to be important to the group. Anal secretions are produced in the hindgut, and released along the posterior setae as predator deterrents In Australia, aggregations of male common blossom thrips have been observed on the petals of "Hibiscus rosa-sinensis" and "Gossypium hirsutum"; females were attracted to these groups so it seems likely that the males were producing pheromones. In the phlaeothripids that feed on fungi, males compete to protect and mate with females, and then defend the egg-mass. Males fight by flicking their rivals away with their abdomen, and may kill with their foretarsal teeth. Small males may sneak in to mate while the larger males are busy fighting. In the Merothripidae and in the Aeolothripidae, males are again polymorphic with large and small forms, and probably also compete for mates, so the strategy may well be ancestral among the Thysanoptera. Many thrips form galls on plants when feeding or laying their eggs. Some of the gall-forming Phlaeothripidae, such as genera "Kladothrips" and "Oncothrips", form eusocial groups similar to ant colonies, with reproductive queens and nonreproductive soldier castes. Most insects create lift by the stiff-winged mechanism of insect flight with steady state aerodynamics; this creates a leading edge vortex continuously as the wing moves. The feathery wings of thrips, however, generate lift by clap and fling, a mechanism discovered by the Danish zoologist Torkel Weis-Fogh in 1973.