‘Die musikalischen Acallen:’Beobachtungen zur Stridulation bei den Cryptorhynchinae von den Kanaren - Cryptorhynchinae-Studie 13.

This study describes songs, morphology of the stridulatory apparatus and mechanisms of sound production for 12 species of the weevil genera Acalles, Dichromacalles and Onyxacalles (Curculionidae: Cryptorhynchinae) from the Canary Islands. SEM studies reveal a pronounced stridulatory sector consisting of 30-40 parallel ridges and a subreticulated portion at the inner surface of both the elytra. Cuticular emergences on the 8th abdominal tergit function as plectrum. Video-sequences clearly reveal movements of the plectrum against the elytral stridulatory sector, illustrating the mechanism already proposed by Wollaston [Wollaston 1860a]. These movements generate a rattling noise of species-specific intensity. A thorough acoustic analysis reveals a broad frequency spectrum between 1,5 and 12 kHz for all species, with a species-specific lower limit, shifted towards deeper frequencies for species with powerful stridulation. Songs consist of elements [Lautelemente] between 10 and 50 ms duration, separated by longer pauses between 30 and 200 ms. Elements consist of needle-shaped pulses of only 0.2 ms duration, which generate the broad frequency spectrum [Fig. 9]. Duration of elements and pauses are species-specific, in spite of considerable variation. Elements and pauses are shortest for Acalles lepidus (15 and 36 ms, respectively), while Acalles muelleri with 30 and 180 ms, and Acalles sonchi with 37 and 60 ms exhibit the highest values. In addition, these two species show the loudest stridulation, while sounds produced by species such as Onyxacalles ringeli can barely be heard. Stridulation is elicited by touch, and therefore is probably a defense reaction.