SUMMARY1Superficially similar mechanisms of pedipalpar stridulation occur in the generaPandinusandHeterometrus.The basic differences are pointed out and it is shown that part, at least, of the anatomical bases for both mechanisms occur in another scorpionid genus,Opisthophthalmus.2Contrary to previous opinions on the subject, it was found that the stridulatory movements inPandinusandHeterometrusare not the same; inPandinusthey are defensive while those described forHeterometruscorrespond to the movement of pedipalpar attack.3It is pointed out that these two different movements produce friction in different places: the attack movement causes most friction over surfaces which produce the sound inHeterometrus, the defensive movement at the points which have become stridulatory surfaces inPandinus.4It is shown that there is anatomical preadaptation to cheliceral stridulation within the genusOpisthophthalmus.