Rodent
While the largest species, the capybara, can weigh as much as , most rodents weigh less than . The smallest rodent is the Baluchistan pygmy jerboa, which averages only in head and body length, with adult females weighing only . Rodents have wide-ranging morphologies, but typically have squat bodies and short limbs. The fore limbs usually have five digits, including an opposable thumb, while the hind limbs have three to five digits. The elbow gives the forearms great flexibility. The majority of species are plantigrade, walking on both the palms and soles of their feet, and have claw-like nails. The nails of burrowing species tend to be long and strong, while arboreal rodents have shorter, sharper nails. Rodent species use a wide variety of methods of locomotion including quadrupedal walking, running, burrowing, climbing, bipedal hopping (kangaroo rats and hopping mice), swimming and even gliding.
Scaly-tailed squirrels and flying squirrels, although not closely related, can both glide from tree to tree using parachute-like membranes that stretch from the fore to the hind limbs. The agouti is fleet-footed and antelope-like, being digitigrade and having hoof-like nails. The majority of rodents have tails, which can be of many shapes and sizes. Some tails are prehensile, as in the Eurasian harvest mouse, and the fur on the tails can vary from bushy to completely bald. The tail is sometimes used for communication, as when beavers slap their tails on the water surface or house mice rattle their tails to indicate alarm. Some species have vestigial tails or no tails at all. In some species, the tail is capable of regeneration if a part is broken off.
Rodents generally have well-developed senses of smell, hearing, and vision. Nocturnal species often have enlarged eyes and some are sensitive to ultraviolet light. Many species have long, sensitive whiskers or vibrissae for touch or "whisking". Some rodents have cheek pouches, which may be lined with fur. These can be turned inside out for cleaning. In many species, the tongue cannot reach past the incisors. Rodents have efficient digestive systems, absorbing nearly 80% of ingested energy. When eating cellulose, the food is softened in the stomach and passed to the cecum, where bacteria reduce it to its carbohydrate elements. The rodent then practices coprophagy, eating its own fecal pellets, so the nutrients can be absorbed by the gut. Rodents therefore often produce a hard and dry fecal pellet. In many species, the penis contains a bone, the baculum; the testes can be located either abdominally or at the groin.
Largest organisms
The largest arthropod known to have existed is the eurypterid (sea scorpion) "Jaekelopterus", reaching up to in body length, followed by the millipede relative "Arthropleura" at around in length. Among living arthropods, the Japanese spider crab ("Macrocheira kaempferi") is the largest in overall size, the record specimen, caught in 1921, had an extended arm span of and weighed about . The heaviest is the American lobster ("Homarus americanus"), the largest verified specimen, caught in 1977 off of Nova Scotia weighed and its body length was . The largest land arthropod and the largest land invertebrate is the coconut crab ("Birgus latro"), up to long and weighing up to on average. Its legs may span .Insects, a class of Arthropoda, are easily the most numerous class of organisms, with over one million identified species, and probably many undescribed species. The heaviest insect is almost certainly a species of beetle, which incidentally is the most species-rich order of organisms. Although heavyweight giant wetas ("Deinacrida heteracantha") are known, the elephant beetles of Central and South America, ("Megasoma elephas") and ("M. actaeon"), the Titan beetle ("Titanus giganteus") of the neotropical rainforests or the Goliath beetles, ("Goliathus goliatus") and ("G. regius"), of Africa's rainforests are thought to reach a higher weight. The most frequently crowned are the Goliath beetles, the top known size of which is at least and . The elephant beetles and titan beetle can reach greater lengths than the Goliath, at up to , respectively, but this is in part thanks to their rather large horns. The Goliath beetle's wingspan can range up to .