In some insects the pupa is enclosed in a protective case, called the cocoon, built by the larva just before pupation. The wings begin as external bumps on the larva, and the adult emerges from the last molt without having undergone a pupal stage. The insect emerges from the egg as a miniature adult and the only futher changes are in size and in maturation of the reproductive organs. Among the well-known plant pests are the locust, armyworm , aphid, corn borer , coddling moth, tent caterpillar , Japanese beetle , gypsy moth , bagworm , and scale insect. Insect carriers of human diseases include the mosquito, housefly , tsetse fly , and flea .
Insects are the source of useful products such as honey , beeswax, silk , lac , and cochineal .
They are a major source of food for many animals, and some are eaten by humans in many parts of the world. The fruit fly has been the major experimental animal used in genetics.