Couplet 6

6a:  Mentum expanded, fused laterally to head capsule, covering ventral mouthparts completely when mandibles closed, mentum extending anteriorly beyond other mouthparts to form cutting edge; outer angle of fore tibia with large, inwardly curved uncus; body cylindrical; antenna moniliform; head, pronotum and elytra with deep canaliculate grooves.

Rhysodidae   p. 401

6b:  Mentum not fused laterally to head capsule or extending beyond other mouthparts, maxilla and labium with at least palpi visible (Figure 26-4); outer angle of fore tibia with straight or outwardly curved teeth or spines (Figure 26-4); head, pronotum, and elytra without deep canaliculate grooves; body form and antenna variable.