Acauloplacella (Papuaprium) immunis (Brunner von Wattenwyl, 1895)

Morris, Glenn K, Ingrisch, Sigfrid, Willemse, Fer, Willemse, Luc, De Luca, Paul A. & Klimas, Dita, 2025, Stridulation songs of some Tettigoniidae (Ensifera, Orthoptera) from Papua New Guinea, Zootaxa 5600 (1), pp. 1-81 : 7-12

Acauloplacella (Papuaprium) immunis (Brunner v. Wattenwyl, 1895)

( Figs 1–4 )

Specimens studied. PNG, nr Wau, W.E.I., 4 & 10 &12 & 17 viii 1981; coll. G.K. Morris (5 males, 1 female); two specimens recorded (81-1, 81-2) ( Depository NBC Leiden) .

Description. This material agrees with the redescriptions in Beier (1954, 1962), except for the white median spot on the posterior margin of the male pronotum ( Fig. 1 ) which extends a little both frontally and laterally, making it widely cross-shaped, rather than triangular. Male stridulatory organ not previously described, elongate, about 5.5 mm long and 2.0 mm wide, distally transversely truncate, inflated part of file of left elytron in dorsal view about 2 mm long, its base bulging; membrane of distal cells of stridulatory area dark brown in their centre with numerous superimposed tiny black points, similar to spots upon remainder of elytra; stridulatory file of left elytron in ventral view ( Fig. 2A ) of usual shape, shortest distance between most proximal and distal tooth 2.3–2.5 mm, basad very narrow and strongly curved with 13–15 teeth, the main part wider, only slightly curved in ventral view and faintly concave in profile, with 150–180 transverse teeth, parallel to each other and closely set ( Fig. 2C,D ). Fifteen to 18 teeth per 0.25 mm, distally increasing to 22–26 teeth per 0.25 mm; transverse, shallow, leaning basad, strong midtooth deflections ( Fig. 2C, D ).

Distribution. From scarce records (Beier 1963) and the limited additional material before us, the range of this insect reaches at least from extreme northeast West Irian through northeast New Guinea to southeastern Papua. By lack of acoustic data and particulars of the male stridulatory apparatus, it is an open question whether A. regularis ( Jong 1938) , from extreme northwestern West Irian, is conspecific with immunis as considered previously (Jong 1950, Beier 1962).

Crypsis and resonance. A. immunis was seen to adopt a cryptic daytime posture, flattening itself upon a supporting broad tree leaf (see Fig. 1 in Robinson 1973, Fig. 1 here). The insect extends its forelegs directly forward, the prothoracic femora resting against its genae and apposes its antennae between; femorotibial joints of the other limbs hide under the broadly ovoid tegmina which are pressed flat to the leaf surface. Body and tegmina colors match the colour of the perch. All of this adds up to a diurnal refuge for the katydid. Hiding the angular femorotibial joints may remove a detection cue used by predators ( Robinson 1973). The dorsoventral compression seen in other members of the tribe Phyllomimini to which A. immunis belongs suggests daytime concealment in this fashion may be a common strategy for avoiding predators.

Though many tettigoniids mimic the foliage on which they perch, this crypsis seems particularly marked in Phyllomimini of the African and Asian tropics and also in the Pterochrozini of the American tropics. But there is an interesting difference in the leaf mimicry exhibited by these two tribes: among Phyllomimini this camouflage involves dorsoventral compression, a flattening of the body, while in new world Pterochrozini the body is laterally compressed, extending the animal in the sagittal plane. Puzzlingly, both taxa associate these cryptic features with high-Q resonance stridulation.

A. immunis has a relatively straight file ( Fig. 2A ) with broad teeth, each tooth bisected midbreadth by a shallow bend ( Fig. 2 CD). The collective effect is an unbroken series of midtooth features, running the middle of the length of the file. Some Malaysian Phyllomimini , Tympanophyllum arcufolium and Onomarchus uninotatus , tribal relatives of A. immunis , also show a mid-breadth angle change in their broad file teeth, causing them to seem “fused in the middle” ( Heller 1995). A similar tooth form also occurs in three species of the pterochrozine Ecuadorean South American genus Typophyllum : T. mortuifolium , T. bolivari and T. nr. trapeziforme ( Morris et al. 1989).

Perhaps these bisected teeth enhance the insect’s sinusoid (high-Q) resonant stridulation by serving as a scrapershaping ‘guide’, functioning to mesh the teeth more intimately to the shape of the scraper. Such a guide function does not seem a requirement for making a high Q signal but those several species in which it occurs do make an extremely high Q. The function of songs with a dominant narrow frequency peak may have something to do with mate localization in a manner similar to the mechanism employed by some crickets.

Stridulation. A. immunis is a definitive example of resonance stridulation. Its call was a common nocturnal sound on the grounds of the Wau Ecology Institute, issuing from lower tree foliage and the higher parts of shrubs. Only a few males were perched low enough for their capture to be attempted from the ground. A. immunis calls were also a strong part of the night chorus in the Bulolo Gorge.

A. immunis ’ frequency carrier lies in the low audio range where humans hear well. There is a single very high-Q peak (Q 10 = 45). in the vicinity of 8 kHz ( Fig. 3E , Fig. 4 ). The marked tonality of this phyllomimine rivals the characteristically high-Q songs of new-world Pterochrozini such as Typophyllum mortuifolium ( Morris et al. 1989) . Phyllomimini also share remarkably accomplished leaf mimicry with Pterochrozini . But high-Q songs are not confined to these two exceptionally camouflaged tribes: the copiphorine Artiotonus artius ( Montealegre-Z et al. 2011) has a comparably high Q of 42.6 in the ultrasonic at about 40 kHz.

The songs of two males recorded at 17 and 21 °C, are the basis of the time-domain values given below. The dominant carrier is a single very high-Q peak at 8.6 kHz, lacking any significant harmonics ( Fig. 3E , 4 ). There is also no indication of frequency modulation ( FM) and no sign of a vestigial file on the underside of the right tegmen ( Fig 2B ). To make just one high-Q carrier frequency would seem to require vibration of just a single speculum, which suggests the absence of the harp as a functional speculum .

A. immunis ’ call is a train of 17–26 sinusoid pulses ( Fig. 3 ), the train lasting 3 to 6 s. At typical field temperatures these pulses arrive at a just countable rate. At 17°C a caged singer (specimen 1) produced three 7-s trains; each song separated from the next by 6 to 7 s of silence. The mean song duration for 10 songs of this same individual was 6.0 s.

Two songs are shown in Fig. 3A . Over the first third of the train the pulses are grouped temporally in pairs, while over the last two thirds they are given singly. But the repetition rate of both pairs and single pulses is about the same: 5–6/s at 21°C. Both measured specimens had this stereotyped arrangement of double pulses followed by singles. For each song the amplitude of the first double pulse was always substantially lower than those following. The amplitudes of the single pulses fall away toward the end of the call and the pulse period lengthens ( Fig. 3A ). An individual showed some variation in the number of double pulses (5 to 9) and single (9 to 18) between songs.

A pulse was sampled from each of 10 different songs made by specimen 1 singing at 17°C, giving a mean pulse duration of 18.2 ms. The mean carrier frequency, calculated over the entire pulse was 8.57 kHz ( Fig. 3E ), specimen 2. There is no frequency modulation during the pulses.A third harmonic, very weak, was detected at 26.1 kHz, more than 39 dB below the carrier, so likely of no biological consequence.

In comparison to other Tettigonioidea, the sound level is rather low: 85.5 dB (impulse, hold) dorsal 10 cm, for the earlier double-pulses, dropping by about 3 dB for the later single pulses.

Figures

FIGURE 1. Acauloplacella (Papuaprium) immunis, habitus of adult male, dorsal aspect, on vertical glass in daytime cryptic posture.

FIGURE 2. Scanning electron micrographs (SEMs) of A. immunis strigin parts: A) Left tegmen ventral aspect, straight file of broad teeth; B) Oblique foreshortened view of wide transverse vein backing scraper (arrow) on underside of right tegmen; C, D) File teeth close-ups show mid-breadth inflections and gullet nodules.

FIGURE 3. A. immunis acoustic analysis: A) Two songs, trains of sinusoidal pulses, first several pulses paired in time; B) Pulse pair at higher resolution; C) Single pulse at still higher resolution; D) Portion of pulse time-resolved, strongly sinusoid wave train; E) Fourier transform spectrum, single extraordinarily high-Q spectral peak near 8 kHz.

FIGURE 4. A. immunis frequency-domain waterfall display of pulse; time advances into page.