Flies of many kinds have affected humans and human welfare for thousands of years. Some flies suck blood, others are scavengers. Many transmit disease organisms, some are pests of cultivated plants, some live at the expense of other insects, and others aid in the pollination of plants. Flies of importance fall into several groups. They are the house fly, and its relatives the flesh flies, bottle flies, blow flies, black flies, filter flies, horse flies, deer flies, and vinegar flies.
All flies have a complete metamorphosis with larva, pupa, and adult stages. The larva of the fly are commonly called maggots. Some are legless and dirty white in color. Larva feed differently and occupy different habitats than adults. The larva are enclosed in a heavy pupal skin or puparium. Once the adult female has emerged and mated, she selects the correct habitat and deposits her eggs. The habitat selected is species dependent and may also differ seasonally, geographically, and with regard to types of habitats available. The length of the life cycle is both species and environmentally dependent.