Thrips are minute insects whose chief distinguishing features are four wings fringed with hair-like cilia and sucking mouthparts. The order name, Thysanoptera, means fringe-winged. Thrips are best known as pests of plant crops. They use a single mandible to punch a hole in a leaf or flower and then use syringe-like stylets to suck out the juices which damages or destroys the plant’s cells in the process. There is a Greenhouse thrips that damages hothouse plants and an onion thrips that attack onion crops. About 60% of species attack green leaves or flowers. Some of these species also act as vectors for plant diseases, such as tomato wilt virus. Most of the remaining 40% of species live on leaf litter, a few species live on fungi and another few are predatory, killing other small insects.
Thrips are simple insects with incomplete life cycles that are probably closely related to true bugs (Hemiptera), book lice (Psocoptera) and true lice (Anoplura and Mallophaga). Thrips probably evolved from fungus or detritus feeders and only secondarily took up a plant-sucking lifestyle. All their near relatives have chewing rather than sucking mouthparts.
Thrips like warm, dry weather and are mostly found in tropical and subtropical regions of the world. They can feed and grow in cooler conditions but often cannot raise the energy to fly. When conditions are right, they will climb en masse to the top of vegetation and fly away in swarms. There are wingless species as well but they are light enough to be carried away by breezes and so also disperse. Swarms of predatory thrips have been known to bite humans.
Thrips live short, quick lives, maturing from egg to adult in as little as a few weeks. When conditions are suitable, they can produce several generations in a year. The two sexes are similar but males are usually smaller. Some thrips have an ovipositor and so can place their eggs inside plant tissues. Species without an ovipositor lay their eggs under bark or in leaf litter. Development is intermediate between simple (eggs hatch into mini-adults) and complete, with a full metamorphosis between larval and adult forms. The first two instars are larvae-like with no external wings. The third and fourth instars in some species are inactive, do not feed and have external wings, so the third instar is called the prepupa and the fourth is a pupa. The pupal stage is even sometimes encased in a cocoon. The following stage after the pupa is a full-fledged, winged, sexually mature adult.
The order is divided into two suborders, the Terebrantia and the Tubulifera by the shape of the last abdominal segment and the development of the ovipositor. They also have different developmental patterns. These are further sudivided into 9 families and there are about 5500 described species with many more unknown species in tropical areas. The thrips fauna of Africa and South America are still virtually unstudied.
Acknowledgements: I used information from several texts, the most important of which is Borrer and Delong An Introduction to the Study of Insects. I also use a number of websites to get up to date information such as the CSiIRO Australia, tolweb.org and a number of University sites.
