Scopiorinus mucronatus (Saussure & Pictet, 1898)

To the unaided ear the songs of S. impressopunctatus and S. mucronatus are highly similar. Each insect produces a succession of ticks reminiscent of a watch. A tick rate of 1.4/s was measured for a 5. impressopunctatus male at 22 C; a S. mucronatus male ticked 1.2/s at 26 C. The ticks of S. mucronatus (Fig. 6) last perceptibly longer than those of S. impressopunctatus (Fig. 5) and there is a rasping quality which presumably reflects the distinct amplitude modulations within each tick. S. carinulatus song (Fig. 4), contrasts strongly, being a fluttering sustained buzz of recurring pulse train groups. The flutter, as perceived by the human ear, results from the relatively substantial silent interval interposed between PTGs. Carrier frequency analysis reveals a narrow spectral peak, at testing to the function of a high-Q resonator in sound generation. Especially is this so for S. impressopunctatus and S. mucronatus whose ticks, resolved in oscillograms (Figs. 5, 6), have a highly sinusoidal wave form. On the assumption that these are pure sine waves, one calculates a carrier frequency from the high resolution oscillogram which agrees closely with the central peak location of the analyser spectrograms. In such a calculation for S. im pressopunctatus, based on 42 successive waves from the central region of one pulse, carrier frequency was 26.9 kHz.

This unusually high sinusoidal carrier frequency is generated by an equally unusual wing morphology. The 4 major morphological modifications can be illustrated by comparing S. impressopunc tatus with P. camellifolia (Fig. 7). P. camellifolia emits a broad spectrum (low-Q) calling song with a low (5 kHz) carrier, via a train of rapid-decay pulses. In Scopiorinus (1) the mirror is disproportionately small. P. camellifolia has a (dorsal midline) pronotal length of 5.8 mm (n= 1), S. impressopunctatus 5.0 mm (n= 1). The longest dimen sion of the mirror frame, the cantilever length, (Morris & Pipher 1967) is 4.6 mm in P. camellifolia; but in S. impressopunctatus it is only 1.4 mm! Thus while the pronotum of Scopiorinus is about 13% smaller than that of Pterophylla, its mirror is about 70% smaller. In Scopiorinus (2) the mirror membranes of right and left tegmina are thick and leathery. This constrasts with the thin transparent membranes of P. camellifolia. A scanning photo shows a very bright region in Scopiorinus' mirror centre (Fig. 7c). This is apparently the result of increased emission from an effectively doubled gold coating, i.e. the membrane is thin enough in the mir ror centre that the coatings of the two sides of the specimen lie close together and behave as a single thickened coat. Compare this cir cumscribed thin region to the more extensive bright area of P. camellifolia (Fig. 7b). In Scopiorinus (3) the veins that support the file and scraper are massive (Fig. 7c, d, e), suggesting the development of exceedingly strong forces during stridulation. To each side of the tail of Scopiorinus* file, the tegmen is deeply incised so that this portion of the forewing projects 'handle-like' across the midline (Fig. 7a). There has been an elevation of the mirror upward, away from the plane of the dorsum, giving more volume to the air chamber beneath the tegmina. This chamber is bounded anteriorly by the immensely swollen file. An abruptly terminated bar has been formed from vein Cu 2, providing a lateral wall for this chamber. In Scopiorinus (4) the teeth of the file are much broadened and appear to be recessed into the file buttress at their edges (Fig. 7f). This would perhaps secure the scraper more firmly between two file teeth, keeping it aligned parallel to the long dimension of the tooth, minimizing scraper twisting. Broadened teeth of this sort occur in Metrioptera sphagnorum F. Walker, a decticine endemic to Canada, and coincide with the half of the file used by this insect to generate a 33 kHz sinusoidal sound (Morris & Pipher 1972).

The sound level measured for a specimen of S. impresso punctatus is surprisingly low at 84 dB (6 cm) (Table 1). The two Monte Verde species at 88 and 96 dB (5 cm) are more typical of Tet tigoniidae. A remarkable feature of Scopiorinus is its low song-to-silence ratio, i.e. the high proportion of its singing time spent not broad casting. A S. impressopunctatus tick lasts about 21 ms, including low-amplitude portions preceding and following the intense emis sion. At the aforementioned tick rate of 1.4/s, the song period = 714 ms. For each 21 ms tick there is 693 ms of silence. In other words the singer signals for about 3% of the available time. To put this in perspective consider another emitter of sinusoidal pulses, and one which adds to its down time by chirping rather than trilling: Gryllus campest ris L. At a calling song chirp rate of 3.5/s (Eisner & Popov 1978), and with each chirp composed of 3, 18 ms duration, pulses (Huber 1963), in 1 s G. campestris produces 3.5 X 3 X 18 = 189 ms of signal. This is a duty cycle ratio of 189/811 = 23%. Scopiorinus spp. were never detected stridulating in the field. Although these insects emit by far their greatest energy in the ultrasonic range they do possess a faint audible song component. One might have expected to make some use of sound in stalking and capturing specimens, the normal practice of those who hunt katydids. In fact specimens were only detected visually, in the beam of the flashlight. Later in the laboratory, after long isolation from their normal surroundings, these specimens produced their calling songs. Singing in these species is subject to a very low disturbance threshold.

[1]

Viittaukset