Some species of ants (Formicidae) and stinging wasps (some Sphecidae, Bradynobaenidae, Rhopalosomatidae, and all species of Mutillidae) possess stridulatory organs without resonance structures. The stridulatory organs occur in both sexes between the 3rd and 4th, 4th and 5th, or 5th and 6th abdominal tergites. Some sphecid wasps (genus Pseudoplisus) makes use of all three positions. The main part of the stridulatory organ is an area of 60 to 200 very regular ripples with species-specific ripple distances of 1.2 to 5 micrometres. To prevent mechanical destructions of the ripples during sound production the force is spread over a width of more than 100 micrometers. The signals are optimised to serve as warning signals. Frequently, wasps and ants present the acoustical signals together with optical (aposematic colouration) and chemical signals (allomones). The potential predators (e.g. lizards, mammals) are phylogenetically very different. Therefore the acoustical signals must have a broad frequency spectrum. The mechanisms of the production of such spectra were investigated by laser vibrometry. The signals were analysed by cepstrum and by an adaptive optimal-kernel time-frequency representation according to Baraniuk & Jones (IEEE Transaction on Signal Processing, 2361-2371 (1995)) [The work was supported by DFG research grant Ts 53/1-1 and Ts 53/1-2].