Key to Aquatic Arthropods
	25a	(16)	Body divided into three distinct regions: a head with mouthparts, eyes and antennae; a thorax with legs; and an abdomen without jointed appendages except for terminal ones. Thorax always with only three pairs of legs, and with or without wings. Insects ~ Class HEXAPODA	[95]
	25b		Body appearing otherwise: usually divided into two distinct parts: cephalothorax and abdomen; or only the head a distinct part and a segmented region behind the head. Number of appendages variable, but more than six which are legs (include claws if present, but not leg-like mouthparts), or less than six. Without wings	[26]
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	26a	(25)	Body with two distinct regions (cephalothorax and abdomen): cephalothorax with mouthparts and legs; abdomen with or without appendages and appearing segmented or not	[27]
	26b		Body otherwise (include here microcrustaceans -- aquatic forms under 3 mm in length)	[28]
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	27a	(26)	Cephalothorax with four (4) pairs of legs, no antennae; usually with eight, simple eyes, and fang-like mouth parts. Terrestrial. Spiders ~ ARACHNIDA	[50]
	27b		Cephalothorax with five (5) pairs of legs (counting claws but not leg like mouthparts), antennae, two eyes (usually compound and on stalks). Abdomen usually having paddle-like appendages. Prawns, shrimp, crabs ~ Class CRUSTACEA, Order DECAPODA	[74]

CRUSTACEANS  The Class Crustacea is an extremely diverse group within the Phylum Arthropoda, including many forms, such as copepods, cladocerans, and ostracods (Fig. 2), which are essentially microscopic (under 2 mm) in size. Others, such as prawns and crabs are the largest invertebrates found in Hawaiian streams and estuaries. All have a chitinous exoskeleton, mandibles (chewing mouthparts), and two pairs of antennae. The thorax is usually combined with the head (forming the cephalothorax) within a common covering called a carapace. Some insect larvae (Class Insecta) may superficially resemble crustaceans. For a partial list of aquatic Crustacea known from Hawaiian fresh and brackish waters see CPIE.

Figure 1. Ostracodes or "seed" shrimp.	Figure 2. Microcrustaceans: 1) copepod; 2) ostracod; 3) cladoceran.

	28a	(26)	 Key to Aquatic Crustaceans Very small crustaceans (under 3 mm). Body typically composed of an enlarged cephalothorax with eyes or eyespots and jointed appendages, and a smaller, segmented abdomen like Fig. 2 - 1), with or without appendages (usually at least with terminal appendages); OR a body with jointed appendages enveloped by a bivalved shell (Fig. 1)	[37]
	28b		Small to large crustaceans (2 to 200 mm). Body typically composed of a head and more or less equal thoracic segments (with legs); OR a cephalothorax (with mouthparts and legs) and segmented abdomen, usually with appendages	[29]
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	29a	(28)	A carapace covers the cephalothorax, which has at the front antennae and mouthparts, then five (5) pairs of walking legs (including claws). Behind the carapace is a segmented abdomen, usually about as long as or a little longer than carapace, but may be much smaller and folded against the underside ~ Order DECAPODA	[74]
	29b		Carapace short, exposing most thoracic segments; OR carapace lacking	[30]
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	30a	(29)	Carapace essentially lacking	[31]
	30b		Carapace present, but short, leaving 4 or 5 thoracic segments not covered	[Note D]
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	31a	(30)	With three pairs of uropods. Body compressed laterally (side to side). Sand fleas, hoppers, amphipods ~ AMPHIPODA	[85]
	31b		With one pair of uropods. Body cylindrical or compressed top to bottom	[32]
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	32a	(31)	First gnathopods chelate ~ TANAIDACEA	[Note D]
	32b		First gnathopods subchelate or simple ~ ISOPODA	[89]
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	Figure 1. Acorn barnacles (Chathamalus) expose by the falling tide.
	33a	(14)	Body enclosed in a bilterally symmetrical wall of six calcareous plates cemented to substratum, the aperture (opening in the middle) closed by calcified valves (Fig. 1). Acorn barnacle ~ Suborder BALANOMORPHA	[34]
	33b		Cemented to substratum, but form an asymmetrical "box" of four plates, or if bilaterally symmetrical, then having a wall of only four plates; OR attached by a long, fleshy peduncle	[Note D]
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	34a	(33)	Rostral plate overlapped by adjacent plates ~ Family CHTHAMALIDAE Chthamalus intertextus Darwin
	34b		Rostral plate overlapping adjacent plates ~ Family BALANIDAE Balanus sp.
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	37a	(28)	Carapace forming a bivalved shell enveloping all or most of the body [see Fig. 2 - 2) & 3)]	[38]
	37b		Carapace typically covers head and thorax; not bivalved. Abdomen segmented and not enclosed by carapace [see Fig. 2 - 1)]	[80]
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	38a	(37)	Carapace distincly bivalved, with a hinge along the dorsum (back); carapace covering all but antennae and limbs which may project out of shell for locomotion (see Fig. 1). Seed shrimp ~ Subclass OSTRACODA	[Note B]
	38b		Carapace folded, not hinged. Head not covered by carapace, but usually separated from it by a distinct ventral notch. Water fleas ~ Subclass CLADOCERA	[Note B]

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Note B:
These small crustaceans are not well known in Hawaiian fresh water environments, although they may be commonly encountered. Lack of reference material and small size make identification to species very difficult.

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Note D:
Your identification is leading to a group with NO known fresh or brackish water representatives in Hawai`i. The identification may be correct for the location where you made the collection or observation (perhaps near the ocean), but the species is marine and not covered in these keys. If the collection was made in freshwater, you should back up several couplets and reconfirm your decisions.