Insects, spiders, and their relatives are all arthropods (ar-thro-pawds), or animals with a hard outer skeleton and several pairs of jointed limbs. Arthropods form the largest group of animals on Earth. They are found almost everywhere, from the deepest ocean trenches to the tallest mountain peaks. There are nearly one million species of insects known, nearly half of all the different plants and animals combined. There may be as many as ten to thirty million insect species total. Even a "typical" backyard may be the home to several thousand species of insects and spiders. Estimates of species numbers vary because scientists still know so very little about them. Millions of new species await discovery, especially among insects and mites on land and crustaceans in the ocean.
It is estimated that for every human being alive today, there are as many as two hundred million individual insects. Just the total weight of all the ants in the world, all nine thousand different kinds, is twelve times greater than the weight of all the humans on the planet. Despite their amazing numbers and the fact that they are found virtually everywhere, insects and other arthropods are still very alien to us, as if they were beings from another planet. They move on six or more legs, stare with unblinking eyes, breathe without noses, and have hard skinless bodies made up of rings and plates, yet there is something strangely familiar about them, too. Arthropods have to do all the things people do to survive, such as find food, defend them-
selves from their enemies, and reproduce. They also rely on their finely tuned senses to see, touch, hear, smell, and taste the world around them.
Because of their numbers and the fact that they eat almost everything that is plant, fungus, or animal, arthropods have a huge impact on all the species sharing their habitats. They pollinate flowers, disperse seeds, recycle dead organisms, and bury animal waste. Plant-feeding species provide a natural pruning service that keeps plant growth and populations in check, while flesh-eaters control the populations of other animals. They, in return, are an important food source for fishes, reptiles, amphibians, birds, mammals, and other arthropods.
Many different kinds of scientists study the lives of insects, spiders, and their relatives. Entomologists (EHN-tih-MA-luh-jists) examine the lives of insects, while arachnologists (uh-rak-NA-luh-jists) look at spiders and their relatives. Myriapodologists (mi-RI-ah-po-DAL-luh-jists) focus their attentions on millipedes, centipedes, and their kin. Invertebrate zoologists (in-VER-teh-breht zu-AH-luh-jists)and some marine biologists study marine crustaceans, sea spiders, and horseshoe crabs. It is the work of all these scientists that has provided the information found on these pages.
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