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Tuatara: Volume 9, Issue 2, November 1961

Some Observations on New Zealand Cave-Wetas

page 80

Some Observations on New Zealand Cave-Wetas

Wetas form a group of large insects belonging to the Order Orthoptera. They are divided into the two families Henicidae Karny 1937 and Rhaphidophoridae Kirby 1883. The Henicidae. which consists of tree-dwelling and ground wetas, has already been discussed by Salmon (1956), and all remarks in this paper are confined to the Rhaphidophoridae or cave-wetas.

Cave-wetas belong to a group of moisture-loving insects, many of which have become cave-dwellers. They have a world-wide distribution, and are particularly well represented in New Zealand where they occur throughout the whole country, especially in limestone regions and greywacke tunnels. Many species occur in native bush, living in rotten logs and under bark and stones; while others inhabit the old, damp, deserted burrows of petrels and tuataras on a number of islands off the coast of New Zealand. They have even been found in the basements of houses. Because of this variety of habitats, it is assumed that they were originally bush-dwelling insects which sought protection in any damp, dark place that was convenient. Environmental conditions of almost complete darkness, cool temperature and high humidity are required by these insects.

They are commonly found near the entrances of caves, often hidden in groups behind projecting rocks, or in crevices, with the young usually congregating nearer to the mouth of the cave than the adults. Usually they hang head downwards from the walls or ceiling. Although they may live together in large colonies in caves, there is no social life, cave-wetas being solitary in their habits and behaviour.

In caves, activity is going on to a greater or lesser degree all the time. When the light intensity outside the cave decreases, the insects inside move towards the entrance. This is particularly marked if weather conditions are those especially favoured by them. Thus activity outside the cave appears to be initiated by rapidly changing light intensity, and is accompanied by the urge to search for food. On dark, starlight nights, or damp nights, wetas are most numerous outside caves, mainly because conditions are then closest to those page 81 inside caves. Thunderstorms prevent them from emerging, possibly because of the brilliant flashes of lightning. Moonlight also raises the light intensity well above the optimum requirements for the insects, with inhibiting effects on their activity. In the bush, cave-wetas have a far more pronounced diurnal rhythm.

Examination of the eyes and appendages of several species belonging to the genus Gymnoplectron Hutton, collected from both large and small caves, has shown that they do not vary perceptibly in size. Dissection of numerous insects has shown that the two lateral ocelli have become vestigial, while the median ocellus and compound eyes are still well developed and functional. With gradual loss of sight, the sense of touch has become correspondingly well developed, so that both antennae and palps enable wetas to move rapidly in the darkness, the antennae being waved around to detect any obstacles, while the palps feel the ground immediately in front of the insect.

Under laboratory conditions cave-wetas do not respond to sudden changes in heat intensity; but they are affected by temperature fluctuations in their natural environment.

Fig. 1: A typical cave-weta, Gymnoplectron fusca (Richards), from a gold-mine tunnel near Thames.

Fig. 1: A typical cave-weta, Gymnoplectron fusca (Richards), from a gold-mine tunnel near Thames.

Cave-wetas are primarily scavengers, living on a diet of plant and animal material. Fungi and animal food can be obtained inside caves; but bryophytes and angiosperms are also eaten, and in many cases these must be sought in the surrounding bush. However, where a river flows through a cave, plant debris is washed into the cave, especially during floods and is left stranded on the walls or floating on the surface of the water, thus forming a plentiful food supply. In the bush, cave-wetas are often associated with ferns and rata. Cannibalism is known to occur, and the remains of adults may often be found strewn round the floors of caves. Cave-wetas are particularly vulnerable to cannibalism just after moulting, before the new skin has had time to harden, and also during the mating season.

page 82

The chief characteristics of the group are lack of wings, lack of auditory organs, possession of long, compressed tarsi without pads. small bodies and very long hind legs and antennae. The adult female is easily distinguished from the male by having a long, scimitar-like ovipositor projecting from the posterior end of its abdomen.

The complete life cycle usually takes a little over two years. About eight months are required between laying and hatching of the egg; but the length of nymphal and adult life is very difficult to assess. Adults commence to appear in December and January, but mating does not reach its peak until April. In caves, females oviposit in the soft mud on the walls. Palps and ovipositor are used to find a suitable spot, as the correct texture and depth of mud are a prerequisite for egg-laying, and many areas are tested before one is selected. Only one egg is deposited at each insertion, and usually only one or two eggs at the most are laid in any particular spot. All freshly made ovipositor holes are surrounded with a little ring of mud. The same area of mud is often used over and over again by different females, as examination of mud usually reveals eggs in different stages of development. Oviposition reaches its peak between April and June, with hatching reaching its peak in December and January. During development the compound eyes of the embryo become visible through the chorion. A short time before hatching a dark brown cap becomes apparent at the vegetal pole of the egg. In some cases the chorion becomes transparent and the embryo can be seen quite clearly through it. The number of pre-adult instars is variable both inter- and intraspecifically.

Cave-wetas are considered to be representatives of a very ancient group of insects, which have changed very little since they originated. They have a long fossil history, their earliest relatives having been found in the Triassic in Queensland, 190 million years ago. It is interesting to note that two European members of the group are the only survivors of an ancient subtropical fauna of European Orthoptera dating from the Tertiary. An engraving of a cave cricket on a fragment of bison's bone found in the ‘Trois Frères’ cave at Ariège in the French Pyrenees is the most ancient evidence we have of real cave fauna. The fact that New Zealand has been isolated from the rest of the world geologically speaking for many millions of years probably accounts for the large number of endemic species distributed throughout the country.

Key to the Genera of Rhaphidophoridae of New Zealand

1 Some femora with apical spines. — 2
All femora without apical spines. Pharmacus Pictet and Saussure
2 Fore femora without apical spines. Isoplectron Hutton
Fore femora with apical spines. — 3page 83
3 Middle femora with only one apical spine. — 4
Middle femora with two apical spines. — 5
4 Hind tibiae with two rows of alternating large and small spines. Talitropsis Bolivar
Hind tibiae with two rows of spines gradually increasing in size towards distal end. Macropathus Walker
5 Hind femora with two apical spines. Gymnoplectron Hutton
Hind femora with less than two apical spines. — 6
6 Middle tibiae with linear spines on ventral and lateral surfaces. Turbottoplectron Salmon
Middle tibiae with linear spines on ventral surface only. 7
7 Fore and middle tibiae with a single pair of apical spines. Neonetus Hutton
Fore and middle tibiae with two pairs of apical spines. 8
8 Hind tibiae with two pairs of apical spines. Weta Chopard
Hind tibiae with three or four pairs of apical spines. — 9
9 Hind tibiae with three pairs of apical spines. Pleioplectron Hutton
Hind tibiae with four pairs of apical spines. — 10
10 Metasternum with a blunt tubercle in the middle. Ischyroplectron Hutton
Metasternum without a blunt tubercle in the middle. — 11
11 Fore and middle femora without linear spines beneath. Pallidoplectron Richards
Fore and middle femora with 8-10 linear spines beneath. Paraneonetus Salmon

Literature Cited

Richards, Aola M., 1958. Revision of the Rhaphidophoridae (Orthoptera) of New Zealand. Part III—The Genera Pachyrhamma Brunner and Pallidoplectron n.g. Trans. Roy. Soc. N.Z. 85: 695-706.

Salmon, J. T., 1956. A Key to the Tree and Ground Wetas of New Zealand. Tuatara VI (1): 19-23.

* Now of School of Biological Sciences, University of New South Wales, Sydney.