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Both the existence of jointed appendages and the functional segmentation of their bodies are characteristics of the phylum Arthropoda. Chitin, a robust and water-resistant polysaccharide, makes up their exoskeleton. This phylum contains the five subphyla Crustaceans, Hexapoda, Chelicerata, Trilobitomorpha, and Myriapoda among its members.

The term "hexapod" describes a class of animals that have three pairs of legs and a consolidated thorax (six legs). More than three pairs of legs are present in the majority of other arthropods. The flies, then.

Flies are not classified under the subphylum Chelicerata because it also comprises arachnids, daddy longlegs, horseshoe crabs, and scorpions. Flies are not classified under the subphylum Crustaceans because it also contains some zooplankton, crabs, isopods, lobsters, crayfish, and barnacles.