Michael Robinson and Thane Pratt.
The Phenology of Hexacentrus mundus (F. Walker) at Wau, Papua New Guinea (Orthoptera, Tettigoniidae).
Psyche 82:315-323, 1975.

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THE PHENOLOGY OF HEXACENTRUS MUNDUS
(F. WALKER) AT WAU, PAPUA NEW GUINEA
(ORTHOPTERA, TETTIGONIIDAE)
When at Wau, Morobe District, Papua New Guinea, during 1970-1971, the senior author was impressed by the very distinctive nature of the stridulation of a common tettigoniid Hexacentrus mundus (F. Walker). No attempt was made to study the species at this stage but it was encountered frequently during other re- searches and males appeared to be stridulating throughout the entire year. Phenological studies of the spiders at Wau (Robinson & Robinson, 1973; Robinson, Lubin & Robinson, 1974) showed that adult spiders of several species were active and reproducing through- out the year and gave rise to the suggestion that tropical arthropods may be less seasonal at middle altitudes than they are in the low- lands. Nearly all the sparse data on the phenology of tropical in- sects come from lowland study areas (see review in Robinson, Lubin & Robinson ibid.). We therefore decided to census the number of singing male H. mundus that were audible from three line transects located at around 1066 meters altitude. We carried out this survey, weekly, for one year (January 1974 to December 1974). We orig- inally chose to census H. mundus because its song sounded unique among the orthopteran stridulations at Wau; it turned out to be a lucky choice since the insect is almost certainly predatory and there- fore more interesting for comparisons with the spider data that are already available from this locality. Stridulation is essentially a sexual activity and the census should provide an index of repro- ductively active individuals. It showed that males were active in every one of the months of the year.
The species
The species was identified by Prof. I. J. Cantrall, Museum of Zoology, Ann Arbor, Michigan, to whom we are most grateful. lsmithsonian Tropical Research Institute, P.O. Box 2072, Balboa, Panama Canal Zone.
2Wau Ecology Institute, P.O. Box 77, Wau, Papua New Guinea. Manuscript received by the editor September 15, 1975.



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316 Psyche [September-Decemb
Figure I.
Male of fiexamirvt mnfiss feeding on srnalt acridiid. Not, that the prey is being held down by an anterior leg. Length ca. 60 mm.



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Psyche . [Septwsher-December
Figures I and a show the general appearance of male and female adults. The insect J'S grass green in color at all developmental stages and there is a quite distinct sexual dimorphism in the general shape of the resting adult. The dimorphism largely derives from a differ- ence in the shape of the tegtnina. Both sexes have large, widely , spaced and projecting compound eyes, as have most predatory katy- dids. The tibiae of the anterior legs have rows of strongly developed spines with which the animal holds and restrains live prey (see Figure i ). As far as we were able to determine the insert has no specialized cryptic or mimetic posture (unlike some of the other tet- tigoniids at Wau; see Robinson, 1973).
We originally considered the possibility that the species was preda- tory after an examination of its morphology. Later we found an
adult male eating a large jumping spider that was obviously, not car- -
don. This was the only instance we saw of predation in natural conditions (we never saw the species eating plant material at any stage). In captivity the insect caught and ate a variety of living insects and was particularly adept at catching calliphorid flies. These were caught if they settled close to the resting katydid; we did not see any stalking behavior. The katydid made a rapid forward lunge and then trapped the flies against the substrate beneath the spinous anterior tibiae. We saw nothing that was homologous to a mantid (or reduviid) strike, i.e., the anterior less were not extended and then rapidly folded around the prey in a pincer grip. Figure I shows a male Hexacentrus feeding on an acridiid nymph which was caught after it had been left (by us) in a damaged condition in front of the katydid.
Figure 3. Sonagram of interrupted stridulation. Frequency range 0-16 KHz. Time base 0.6 (ii-condi.




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19751 Rob'mson and Pratt - Phenology of Hexacentrus 319 During the day the insect responded to the movements of flies by orienting towards them. The impression gained was that the animal was responding to visual stimuli.
If feeding occurs principally at
night (when stridulation occurs), visual prey detection seems less likely. We do not know that the animal is active only at night. In captivity it certainly feeds by day and in the wild it assumes posi- tions, by day, on vegetation from which it could capture prey. The stridulation
The males stridulate at night from the upper surface of leaves in secondary growth and in forest- and roadside-edge clearings. The stridulation is bisyllabic, "metallic," and repeated over lcng periods of time. Figure 3 shows sonagraphic representation of the sound. It is based on a recording made at 1% inches per second on a Uher 3000s tape recorder. Some harmonics are visible, but the frequency range of the principal stridulation is clearly visible in the sonagram. Striduating males stand with their tegrnina partly opened, i.e., the two tegmina are not touching along their posterior margins (which normally lie along the midline of the body when the insect is not in flight or singing).
The song of Hexacentrus mundus has all the characteristics of a calling song as defined and described by Dumortier ( 1963). The census
The census was carried out on one day per week between 2000 and 2100 hours, between 1.5-2 hours after nightfall, irrespective of the weather conditions. Locations of the transects involved are shown diagramatically in Figure 4. They were always censused in the same order, I, 11, and 111. The technique was very simple. One of us simply walked along each transect ti the end and back (tran- sects I and 111) or around twice (transect 11), counting all the sing- ing Hexacentrus that were audible on the two journeys. Individuals that were singing during both journeys were scored once only. From January to August (inclusive) the census was carried out by M.H.R. ; T.P. carried out the remainder of the year's study. The transects differed in vegetational structure Transect I was along an unpaved road flanked by uncut verges ( 1-1.5 meters wide) and bordering coffee plantations for most of its length ( 100 meters). Transect I1 (ca. 200 meters) was around the outer perimeter of a garden which abutted coffee plantation around the entire length except for the short stretch of road shown in Figure 4. Transect I11 (14 meters) was along a footpath connecting ~o&e 4 with the



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320 Psyche , [September-December
Table 1. Census results.
---
Transects Average Trs.asects Average
of all of all
I I1 111 transects I I1 I11 transects
January 2 2 1 July 4 1 1
2 2 - 3 2 0
3 2 1 2 1 2
4 3 1 3 1 2
Monthly 4 3 1
totals; 15 12 4
12 5
5
averages 3 2.4 0.8 6.2
3 1.25 1.25, 5.5
February 4 2 1 August 2 1 1<
4 3 - 1 - 1
3 1 1 2 1 1
4 1 1 2 1 1
Monthly 1 - -
totals; 15 7 3 8 3 4
averages 3.7 1.75 0.75 6.25 1.6 0.6 0.8 3.0 March 2 1 2 September 3 2 1
4 3 3 3 1 4
2 2 2
2 1 4
Monthly 4 2 2 5 2 5
totals; 12 8 9 13 6 14
averages 3.0 2.0 2.25 7.25 3.25 1.5 3.5 8.25 April ,3 2 1 October 3 1 3
4 2 2 5 2' 4
3 2 2 4 2 3
3 2 1 4 3 4
Monthly 2 3 1
1 - -
totals; 15 11
8
17 8 14
averages 3.0 2.2 1.4 6.6
3.4 1.6 '2.8 7.8
May 3 3 1 November 4 2 5
2 2 1 4 2 7
Monthly
4 2 3
totals; 5 5 2
12 6 15
avera g es 2.5 2.5 1.0 6.0 4.0 2.0 5.0 11.0 June 3 1 2 December
4 1 2
4 2 1 - - -
2 - 1
Monthly 1 2 2
totals; 10
5 6 4 1 2
averages -2.5 1.25 1.25 5.25 2.0 0.5 1.0 3.5 main building complex of Wau Ecology Institute. This footpath passed through coffee plantation edge for approximately one third of its 'length. The remainder of this transect ran through tall grass. All three transects were equally exposed; transects I and I1 were surrounded by trees and bushes.
The weekly census totals and monthly- averages are shown in Table $1.
Three month's censuses are incomplete. May is for only two weeks (MHR was absent from Wau), November is based on three weeks, and December is based on two weeks only (TP left



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to
Golden
Ridges
to
Mt.
Kaindi
Robinson and Pratt- Phenology of Hexacentrus 321
1
t ransect s /
\^ kunai grass
-/
Figure 4.
Sketch map of the location of the transects. W,au; MHR left in September). We have gra,phed the monthly averages for each transect and the average for the combined monthly totals and these are shown in Figure 5.
The sampling system that we used was not only simple but arbi- trary. We did not attempt to determine whether the results were influenced by the weather conditions obtaining during the census period. We would guess that we were sampling a fairly high pro- portion of the males within earshot of the transects but that this proportion did not remain the same, necessarily, from census to census. We got the impression that fewer insects were singing on cold clear moonlit nights. Dumortier ( 1963 ; 638-9) lists a number of climatic variables that may affect insect song (either as exciters or inhibitors). For these reasons we would not place too much reliance on the magnitude of the variations in the census results. The direction of the variations may reflect some general trends, since the lowest counts occur towards the middle of the Southern Hemisphere winter when rainfall and temperatures at Wau are low (see Robinson, Lubin & Robinson, 1974: 132-3).



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Figure 5. Graph of monthly average number of male Hexacentrur d u s stridulating on trmsects I, 11, and 111. Average for all transects marked A.
The most important conclusion that we draw from our data is that they are a reliable indication of year-round activity by a mod- erately large carnivorous insect. We strongly suspect that katydids; in temperate regions will prove to be markedly seasonal in singing activity, reflecting the fact that .adults are not present throughout the year. It would further seem probable that in temperate regions the larger the insect the shorter the portion of the year "occupied" by adults and we would expect that large katydids would be more seasonally restricted (as adults) than smaller species. (This should be generally true of temperate region arthropods that do not possess a pupal stage or cannot otherwise spread their development over more than one year, assuming it takes longer to reach a larger adult size, and that the overall period of each year available for active- life is more or less equally restricted for all arthropods living under



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Robinson and Pratt -- Phenology of Hexacentr-23 the same climatic regime.) Fulton's (1951) data for 19 species of North American orthopterans show that most of thefee sing at some stage during the period June to November. If largish insects such as H. mundus exist on a year-round basis, as secondary consumers, it is possible that primary consumers may be similarly (relatively) aseasonal.. Sustained coexistence (Robinson, Lubin & Robinson, 1974: 158) of organisms over long periods of the year may be a situation with a high potential for generating species diversity. Thus, at the simplest level, it is possible to visualize the evolution of a predator specializing in large orthopterans only in a situation where, as at Wau, large orthopterans are present on a year-round basis.
The work was carried out at Wau Ecology Institute, which sup- ported T.P. in part. We are grateful to Dr. J, L Gressitt for use of the facilities at Wau. Donald Windsor prepared the sonagram. DUMORTIER, B.
1963. Ethological and physiological study of sound emissions in arthro- pods. In Busnel, R. G., ed., Acoustic Behaviour of Animals, Elsevier Publishing Co., Amsterdam, London, New York, pp. 583-584.
FULTON, B. B.
The seasonal succession of orthopteran stridulation near Raleigh. North Carolina. J. Elisha Mitchell Sci. Soc. 67: 87-95. M. H.
The evolution of cryptic postures in insects, with special refer- ence to some New Guinea tettigoniids. Psyche 80: 159-165. M. H., Y. D. LUBIN AND B. ROBINSON.
Phenology, natural history and species diversity of web-building, spiders on three transects at Wau, New Guinea. Pacific Insects 20: 117-163.
M. H. AND B. ROBINSON.
The ecology and behavior of the giant wood spider Nethila maculata (Fabricius) in New Guinea. Smithson. Contr. 2001- 149: 1-76.




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