Functional morphology and evolution of a Carpenter's plane-like tool in the mandibles of termite workers (Insecta, Isoptera)

The left mandible of termite workers possesses just in front of the molar plate a characteristic ``premolar tooth'' that, in most species, is partly or wholly hidden under the mandible. The position, structure and size of this tooth were observed and compared from a functional point of view in 46 wood-feeder species belonging to all termite families and in 33 soil-feeder species belonging to 4 different clades of Termitidae. In all wood- and other plant matter-feeder species observed the premolar tooth resembles the chisel of a carpenter's plane-like device. It is suited to cut superficial fragments out of the wood before ingestion. In all soil-feeder species observed the premolar tooth has lost one or several functional features that characterise wood feeders. It assists the other teeth in the gathering of soil particles towards the mouth before ingestion. In the left mandible of termite workers, the premolar tooth thus shows clear morphological adaptations to the species' diet.