These words relate to a large and varied division of invertebrate animals. Bug and wog are names loosely applied to any very small creeping, crawling, jumping or flying creature that is generally thought of as a nuisance. Wog is the more informal and is even applied to the completely unrelated bacteria and viruses that cause disease, especially a cold which is "going around": Sorry I can’t go ?I’ve got the wog . Bug is the least specific word in the group, but arthropod is the inclusive, zoological term. Arthropods have no backbone; they are characterized by jointed legs or appendages, segmented body parts and usually, hard outer coverings. The true bugs belong to a special order of arthropods whose members have biting or sucking mouth parts, such as bedbugs, water bugs and plant bugs. Beetles also have biting mouth parts and have hard, horny fore wings that cover and protect their hind wings. Bugs and beetles form part of the largest class of arthropods , the insects . There are close to a million kinds of insects , all having bodies divided into three sections, each of which carries a pair of legs, making six legs in all. This characteristic distinguishes insects from spiders , which have eight legs and which, with scorpions, mites and ticks, are placed in another separate class of arthropods , the arachnids . The class of crustaceans includes shrimps, crabs, lobsters, barnacles and crayfish, living mostly in or near water. Crustaceans are characterized by tough outer shells that are shed by moulting as they grow larger.