Arthropoda is the phylum that includes all insects. Insects, on the other hand, have three pairs of jointed legs, segmented bodies, an exoskeleton, one pair of antennae, and (usually) one or two pairs of wings, unlike other arthropods such as lobsters, spiders, or millipedes. Insects can be found in almost every habitat, and there are currently 10 quintillion insects on the planet. Entomologists, or scientists who study bugs, have named one million insect species so far, but studies estimate that another four million are still unclassified. A mandible (or jaw) found in Scotland is estimated to be between 408 and 438 million years old. The oldest winged fossil has been discovered, dating back 330 million years.