Flies belong to the order Diptera, a large, complex order of insects. Most members of this order have two wings (one pair) as adults. However, there are a few wingless dipterans. Dipterans vary greatly in size, food source preference, and in the developmental stage that parasitizes the animal or produces pathology. As adults, dipterans may intermittently feed on vertebrate blood or on saliva, tears, or mucus. These dipterans are referred to as periodic parasites and may serve as intermediate hosts for helminth parasites or for protozoan parasites. They may also alternately feed both on feces and on food and may possibly serve as vectors for bacteria, viruses, spirochetes, chlamydiae, etc. As larvae (maggots), dipterans may develop in the subcutaneous tissues of the skin, respiratory passages, or GI tract of vertebrate hosts and produce a condition known as myiasis.