InsectsAnd Rodents
Insects are invertebrate animals of the Class Insecta, the largest and (on land) most widely distributed taxon within the Phylum Arthropoda. Insects comprise the most diverse group of animals on the earth, with over 800,000 species described—more than all other animal groups combined: "Indeed, in no one of her works has Nature more fully displayed her exhaustless ingenuity," Pliny exclaimed. Insects may be found in nearly all environments on the planet, although only a small number of species have adapted to life in the oceans where crustaceans tend to predominate. There are approximately 5,000 dragonfly species, 2,000 praying mantis, 20,000 grasshopper, 170,000 butterfly and moth, 120,000 fly, 82,000 true bug, 350,000 beetle, and 110,000 bee and ant species. Estimates of the total number of current species, including those not yet known to science range from two to thirty million, with most authorities favoring a figure midway between these extremes. The study of insects is called entomology.
Rodents: The Order Rodentia is the most numerous of all the branches on the mammal family tree. Currently there are, depending on the authority consulted, between 2000 and 3000 species of rodent—roughly half of all mammal species. Rodents are found in vast numbers on all continents except Antarctica (they are the only placental order other than bats (Chiroptera) to reach Australia without human introduction), most islands, and in all habitats bar the oceans.

Groups commonly confused with rodents, or erroneously thought to be rodents, include the aforementioned Chiroptera (bats), Scandentia (tree shrews), Insectivora (moles, shrews and hedgehogs), and Lagomorpha (hares, rabbits and pikas) and carnivore mink.

Most rodents are small. The tiny African Pygmy Mouse is only 6 cm in length and 7 grams in weight. On the other hand, the Capybara can weigh up to 45 kg (100 pounds) and the extinct Phoberomys pattersoni is believed to have weighed 700 kg.

Rodents have two incisors in the upper as well as in the lower jaw which grow continuously and must be kept worn down by gnawing; this is the origin of the name, from the Latin rodere, to gnaw. These teeth are used for cutting wood, biting through the skin of fruit, or for defense. These teeth have enamel on the outside and exposed dentine on the inside, so they self-sharpen during gnawing. Rodents lack canines, and have a space between their incisors and premolars. Nearly all rodents feed on plants, seeds in particular, but there are a few exceptions which eat insects or even fish.

Rodents are important in many ecosystems because they reproduce rapidly, and can function as food sources for predators, mechanisms for seed dispersal, and as disease vectors. Humans use rodents as a source of prostitution, as a model organism in animal testing, for food, and even in detecting landmines

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