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Description and Life Cycle
Amphipods are elongate and more or less compressed laterally. They do not have a carapace (the hard covering of the thorax common in other crustacea), and seven (rarely six) of the thoracic segments are distinct and bear leglike appendages. The abdominal segments are more often or less fused, and so the thoracic segments make up most of the body (Borror et al. 1989). They have two pairs of antennae, with one pair usually very small. The eyes usually are well developed, but are sometimes reduced or lacking. Members of this order have chewing mouthparts (Smith and Whitman 1992).
Adult amphipods range from 5 mm to 20 mm (3/16 to 3/4 inch) in length. Talitrus sylvaticus reaches a length of 8 mm (3/8 inch), T. specificus are 7 mm in length and T.allaudi about 3.5 mm. Aquatic species are often whitish but are seen in other colors also. The color of terrestrial species varies from pale brown to greenish to brownish black when alive, but they often turn red when they die.
Amphipods are sometimes misidentified as springtails (Insecta: Collembola). Springtails are also very commonly found in moist areas, sometimes in vast numbers. However, springtails are not crustacea, but insect-like arthropods with only three pairs of legs, one pair of antennae and usually a furcula (a forked structure) on the 4th abdominal segment. The group of springtails that might be confused with amphipods also has a tubular structure on the last abdominal segment.
Eggs are deposited within a brood pouch on the underside of the adult female amphipod's body. The eggs hatch in one to three weeks. The young amphipods resemble the adults and leave the pouch during the next one to eight days when the female has her first molt during mating. The molt usually takes about one hour. And most species complete their life cycle (egg to adult) in one year or less (Smith and Whitman 1992).
Most species produce only a single brood of eggs, but in at least one aquatic species, Hyalella azteca, the females average 15 broods over a five month period. Hyalella azteca is common in aquatic systems and is used by scientists as an indicator of environmental health and water quality in streams, lakes, and other bodies of water.