Thrips are very tiny insects, measuring only 0.5-1.5mm (0.06-0.2"). Thrips are so small that they feed on individual fungal spores or use their unique, asymmetrical mouthparts to pierce and feed on individual plant cells. They have chewing mouthparts but the right mandible has been lost and the left one is used as a piercing tool (thrips are the only insects that have such asymmetrical mouthparts). Most species feed on plant sap but some live in leaf litter and feed on fungi; some are actually predatory, feeding on other insects. The ordinal name Thysanoptera comes from the Greek words “thysanos” (fringe) and “'pteron” (wing). This “fringe” is created by setae, a design unique to the Thysanoptera that greatly increases the surface area of their wings. Interestingly, they are often reluctant fliers with a predisposition towards flying before thunderstorms; hence they are called “thunder-flies” in some places. They are easily blown about on the wind and use it for dispersal. Winged and wingless forms may occur within the same species, the wingless forms tending to be more common in autumn. Most species of thrips over-winter as either adults or pupa. They are described as holometabolous insects (exhibiting complete metamorphosis) even though the nymphs look like small, wingless adults. Thrips undergo an extended metamorphosis in which the final, immature stage (consisting of 2 or 3 inactive pupa-like instars) is quiescent, non-feeding, and sometimes even enclosed in a silken cocoon. This stage of development, usually called a pupa, has aroused a great deal of speculation by some entomologists who claim that thrips represent a missing link between hemimetabolous and holometabolous development. A close examination of Thysanopteran pupae reveals that they do not undergo any internal transformation. It may well be that these "pupae" are an example of convergent evolution instead of a “missing link”. Debates aside, it is known that thrips often have short life cycles; from egg to adult, it may take just three weeks. Because of this, they can easily reach plague proportions and may occur in such large numbers that they become pests in commercial agriculture. Damage from extensive feeding can cause stunted and deformed fruits and seeds. While some thrips are pests and known carriers of plant diseases, others are important pollinators of the flowers they feed on. One species of thrips (Liothrips urichi) has been successfully used to control a pest plant (Clidemia hirta) in Fiji. Most fossil thrips have been found in amber, where they are fairly common. The oldest fossil thrips are from the Permian period, about 280-225 million years ago. The word "thrips" is both singular and plural (there is no such thing as a "thrip"). There are about 5,000 described species.