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Most psocids (SO-sids) are small, ranging from 0.04 to 0.4 inches (1 to 10 millimeters) in length. They are usually brownish or whitish with black markings, but some tropical species are brightly colored with distinctive markings. The large and distinctive head has small to bulging compound eyes, with each eye made up of multiple lenses. The antennae (an-TEH-nee), or sense organs, are long and threadlike. The chewing mouthparts are directed downward and include parts that are sharp and pointed. The front of the head is usually swollen to make room inside for special muscles that control the part of the mouth known as the sucking pump. The thorax or midsection, especially the first segment, is usually narrower than the head or abdomen. Most adult psocids have four wings that are fully developed and are held like a roof over the body when at rest. Some species have wings that are reduced in size or absent altogether. The legs are usually slender, but in some species the back legs are swollen to help with jumping or crawling backward. The relatively large abdomen is eleven-segmented.
Psocids are found on all continents, including Antarctica. There are 4,408 species of psocids worldwide, mostly in the tropics. About 260 species occur in the United States and Canada.
Psocids live in a wide variety of habitats on land. In spite of the common names that include the word "louse," these insects do not live on other animals. They are most common on dead or living leaves, on stone or bark surfaces, and in leaf litter. Some species prefer living in caves; others are known to bore into wood. A few species of psocids are common in homes and buildings, especially where food is stored.
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