Tips: Only adult arthropods are included, and certain uncommon orders are not included. To use a key, read both descriptions in a couplet (for instance, 1a and 1b). Decide which sounds most like your critter, and move to the next couplet indicated. Should you reach a dead-end, use the numbers in parentheses to backtrack until you reach a couplet that you felt unsure about, and try following the other path. Some orders are found more than once in the keys, because the arthropods occur in different forms.

STEP	FROM	FEATURES	ORDER / CLASS
1a		Four pairs of legs. . . . . . . . . . go to 2
1b		More than four pairs of legs. . . . . . . . . . go to 6
2a	1a	Abdomen with distinct segments. . . . . . . . . . go to 3
2b	1a	Abdomen without distinct segments. . . . . . . . . . go to 5
3a	2a	Conspicuous "pincers" (pedipalps) in front of shorter legs. . . . . . . . . . go to 4
3b	2a	Without "pincers," long and slender legs	OPILIONES (Daddy-Longlegs)
4a	3a	Fairly large, the abdomen ends in a stinger	SCORPIONES (Scorpions)
4b	3a	Small, the abdomen does not end in a stinger	PSEUDOSCORPIONES (Pseudoscorpions)
5a	2b	Body clearly divided into two main parts (cephalothorax and abdomen)	ARANEAE (Spiders)
5b	2b	Oval-shaped body not divided into two main parts	ACARI (Mites & Ticks)
6a	1b	Seven pairs of tiny legs, some roll up into balls	ISOPODA (Pillbugs, sowbugs)
6b	1b	More than seven pairs of legs, they don't roll up into balls. . . . . . . . . . go to 7
7a	6b	Tiny bodies, 10 - 12 pairs of legs	SYMPHYLA (Symphylans)
7b	6b	Bodies often fairly large, with > 15 pairs of legs. . . . . . . . . . go to 8
8a	7b	Flattened bodies, with one pair of legs attached to each body segment	CHILOPODA (Centipedes)
8b	7b	Bodies usually tubelike, with two pairs of legs attached to each body segment	DIPLOPODA (Millipedes)

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