Insects are invertebrates and are referred to as the class Insecta. Insects are six-legged animals with an exoskeleton (hard part is on the outside) and three main body sections (head, thorax, and abodmen). Most insects also have wings, though not all fly. Insects are the most numerous and most widely-spread within the phylum Arthropoda, and are the most diverse group of animals on the earth, with around 925,000 species described—more than all other animal groups combined. They may be found in nearly all environments on the planet, although only a small number of species can live 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 described to date. 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. Arachnids are a class (Arachnida) of joint-legged invertebrate animals. They may be easily distinguished from insects by the fact that they have eight legs whereas insects have six. Arachnids are further distinguished by the fact they have no antennae and no wings. They have a two-segmented body, made up of a cephalothorax and an abdomen, the cephalothorax being derived from the fusion of the cephalon (head) and the thorax. Arachnids are mostly carnivorous—many are venomous; others are parasites. Arachnids usually lay eggs, which hatch into immature adults. Arachnids are chiefly terrestrial arthropods, comprising some 65,000 to 73,000 named species, including spiders, scorpions, harvestmen, ticks, and mites. Arachnids are named after the mythological figure Arachne.