Key to the species of Arachnothelphusa

1. Carapace surface covered with rough granules; external orbital angle separated from epibranchial tooth by deep, broad cleft. .................................................................................................2

– Carapace surface smooth, slightly or distinctly rugose; external orbital angle separated from epibranchial tooth by narrow cleft, teeth sometimes almost confluent. ..........................................3

2. External orbital angle separated from epibranchial tooth by wide, deep cleft; terminal segment of G1 long, about half of the length of subterminal segment; distal segment of G2 short, about a quarter length of basal segment (Sarawak)................. ........................................................................ A. merarapensis

– External orbital tooth separated from epibranchial tooth by deep U-shaped cleft; terminal segment of G1 distinctly curved upwards, short, a quarter length of subterminal segment; distal segment of G2 very short, about 0.1 times length of basal segment (Sabah). ..................................................... A. terrapes

3 Carapace smooth, strongly inflated; branchial regions swollen; outer margin of external orbital angle slightly convex; G1 not known; (Sabah)............................................. A. rhadamanthysi

– Carapace slightly to distinctly rugose; surface and branchial regions convex but not strongly inflated; outer margins of external orbital angle straight or slightly concave.................4

4. Outer margins of external orbital angle smooth; epibranchial tooth well developed, sharp; terminal segment of G1 short, one-third length of subterminal segment (central Borneo)...... ............................................................................. A. melanippe

– Outer margins of external orbital angle serrate; epibranchial tooth very low, blunt, almost confluent with external orbital angle; G1 very slender; terminal segment of G1 short, a quarter of length of subterminal segment (Sabah). ..... A. kadamaiana