1. Propagation and reception of communicative sound signals were studied ethologically in two species of Cydnidae (Tritomegas bicolor L.,Canthophorus dubius Scop.). The rivalry alternation or rival song evocation was chosen as criterion for successful communication. 2. Sexually mature males in the presence of a female begin rivalry alternation within 2 min after a short courtship. Amputation of abdominal trichobothria does not essentially change the acoustical communication ability. 3. Two males in separate cages do not emit coordinated sounds (heterophonic rival song) unless the frames of the cages are in direct contact (T. b.)(Fig. 3) or signal transmission is secured through plant stems or similar material (C. d.). 4. Single males, stimulated with tape-recorded courtship (M-2) and rivalry (M-E) signals through a piezoelectric transducer, answered with M-R sounds only if the crystal was in contact with the frame of a cage. An air gap of a few millimeters between transducer and cage suffices to prevent the normal acoustical reaction (Figs. 4, 5). 5. Animals (T. b.) without trichobothria or tarsi do not show any difference to the control animals in this experiment (Fig. 4a-c). Only animals without legs did not answer (Fig. 4d). 6. From these experiments the following conclusions are drawn: a) The bugs of the fam. Cydnidae communicate acoustically primarily or exclusively by substrate-borne signals. b) The abdominal trichobothria are not the main mechanoreceptors in their acoustical communication system. c) The critical receptors are probably located in the legs at a site other than the tarsi.