Phylum Arthropoda - The Arthropods
The group Arthropoda includes many diverse organisms. What they have in common is a hard exoskeleton and stiff jointed appendages such as legs, mouthparts, and antennae. This group includes the insects, spiders, water mites, crayfish, copepods and other similar organisms.
General Life cycle - With such a large group of organisms it is not surprising to find a lot of diversity in their life cycles. In general, these animals mature in one of two methods. Either they begin as a larvae that looks very little like the adult, then pupate and emerge in the adult form, or they go through a series of nymphal stages that look increasingly like the adult form, until they are mature. Some of the arthropods are aquatic for their entire lives while others have a terrestrial stage of their life cycle.
Identification - Because of the size and diversity of this group, there are many individual characteristics to look for when identifying types of arthropods. Water mites and spiders have 8 legs (in mature forms) and lack antennae. Aquatic insect adults have six legs and usually have wings and one pair of antennae. Insect larvae can be wormlike or have six pairs of legs and sometimes exhibit wing pads. Crustaceans have two pairs of antennae, branched appendages, and pairs of mandibles and maxillae.
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