Arthropods Phylum Arthropoda Insects and Their Relatives

Identification: Body segmented, segments usually grouped in 2 or 3 fairly distinct body regions. Paired, segmented appendages usually present. Body wall more or less hardened; forms an exo-skeleton (external skeleton) periodically shed and renewed. Similar phyla: Annelida (earthworms, leeches, various marine worms) have body segmented but with little or no differentiation of body regions. Appendages unsegmented or lacking. Body wall does not form an exoskeleton. Legless insect larvae differ from annelids in their internal anatomy (generally have tracheae and malpighian tubules, which annelids lack), and the body usually contains fewer segments (13 or fewer in insect larvae, ordinarily more in annelids).

Immature stages: Most arthropods other than insects undergo little or no metamorphosis and the young resemble the adults. A few (some crustaceans) have a larval stage markedly different from the adult; some (millipedes, some centipedes, some arachnids) have fewer legs in immature stages than in adult stages. Habits: Very diversified. Practically every animal habitat contains some arthropods, and different arthropods vary greatly in their feeding habits.

Importance: The importance of insects has been outlined briefly in the Preface (p. vii). Other arthropods are important in similar ways. Many crustaceans (lobsters, crabs, shrimps) are used as food by man, and their collection and distribution constitute a sizable industry. Insects are rarely used as food by man (at least in this country), though certain insect products (honey) are. Classification: Present-day arthropods are arranged in 2 subphyla, the Mandibulata and the Chelicerata, which differ principally in the number and character of the appendages. Each subphylum is further divided into classes.

No. of species: World, 840,000; N. America, 104,000.

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