Stick Insects

J. T. Salmon, Zoology Department, Victoria University College

Few people there must be who have not at some time or other come lace to face with one of those most peculiar of insects popularly known as ‘Stick Insects’. These grotesque animals, which at close quarters wear an almost human quizzical expression, are normally denizens of tropical lands, but strangely enough they are very common throughout the length and breadth of New Zealand.

Tropical forms of stick insects are generally winged, but all the New Zealand forms, some 19 species in all, are apterous and occur from North Auckland to Stewart Island, on the lowlands and on the highlands, up to heights of 4,000 to 5,000 feet. Some dwell on scrub, others in the forest, while high-mountain forms are usually found on tussock, sedges or Dracophyllums. Of cryptic colouration and concealing habit, they are seldom seen except when moving, and are obtained by collectors usually through the beating or sweeping of foliage. When disturbed stick insects are generally cataleptic, and will release their hold on foliage and fall or slide down the stems of a plant to the ground, or some other solid resting place, where they will remain motionless for hours with the fore legs held outstretched and parallel beyond the head, and with the middle and hind legs directed posteriorly and pressed against the sides of the body. When at rest amongst foliage, these insects either hang below branches and twigs or arrange themselves in such a way that their elongate bodies merge with the surrounding branch system so as to resemble a twig; hence the popular name of stick insect. All New Zealand species are protectively coloured so that their colour and colour pattern blends with their surroundings. This is particularly so with one species of Clitarchus which normally rests with its body pressed flat against the trunk or branches of manuka and kanuka trees. Except to the trained eye this species would pass unnoticed on the bark until it moved, so beautiful is its camouflage.

All stick insects are foliage feeders, but except for odd occasions they do little visible damage to shrubs or trees, and cannot be classed as economic pests. Occasionally, however, large numbers are found defoliating a single tree, sometimes in a private garden. Outbreaks of this nature arise from a culmination of factors such as the continued presence of a gravid female insect in that locality during the preceding season, favourable and mild climatic conditions, and luscious growth of the plant. Usually they last for one season only, the insect numbers being rapidly reduced with the onset of early winter frosts and cold weather.

The male stick insect is easily recognised, having a body shorter than and very much thinner than that of the female; some males are mere page 78 match-sticks', and they are always difficult to identify correctly when separated from the appropriate female. But in many species of stick insects males do not exist; or, if they do, they occur only at long intervals of several years. The females continue to reproduce by parthenogenesis. For this reason most species are known from the females only. With one exception, no males have ever been found belonging to the New Zealand species of the genus Acanthoxyla; all these species exist as females only, reproducing parthenogenetically. That one exception is A. senta found only on Great Island of the Three Kings group, North Auckland. To the genus Acanthoxyla belong all the common medium-sized green and brown spiny stick insects of New Zealand which range over the lowlands of the North, South and Stewart Islands. The very large brown spiny stick insects found in forest trees in many parts of New Zealand, and measuring up to 14 in. in length, belong to the genus Argosarchus.

Returning to the breeding habits of these insects, in those New Zealand species in which males do occur both fertilized and unfertilized eggs are laid by the female, and both hatch. The fertilized eggs produce both males and females but the unfertilized eggs produce females only. The common smooth-bodied New Zealand stick insects belonging to the genus Clitarchus behave in this way, and will reproduce either parthenogenetically or sexually, though males of the species are almost as common as females. Clitarchus hookeri can produce two broods during the spring-summer season by parthenogenesis, and from a single original female kept in captivity I have continued to breed successive parthenogenetic generations for five years.

When mating occurs the male stands upon the back of the female and passes its abdomen down and round one side of the female's abdomen to reach the genitalia below. In this position the male is carried around by the female, often for several days or even one to two weeks at a time, each insect feeding independently as the female moves from place to place. The coitus is periodically interrupted while some eggs are laid, but during this egg-laying period the male does not necessarily leave the back of the female.

With insects kept in captivity it is not unusual to find males of one species attempting to mate with females of another species, or even another genus.

Although foliage feeders, each New Zealand species of stick insect has very definite food requirements. All species will eat manuka, but only Clitarchus hookeri can be successfully brought through its entire life history on a diet of manuka alone. Species of Acanthoxyla or Argosarchus confined on manuka will thrive for a few weeks, but then slowly die unless fed some of their specific food plant. In the case of species of Acanthoxyla rata or pohutukawa leaves are essential, or possibly also rimu and totara. With Argosarchus, Myrtus bullata, the ramarama is the essential food plant.

Newly-hatched stick insects, unless given their appropriate food plant within 24-48 hours of hatching, will always later die. even though they may live for some considerable time and grow, on the wrong foliage diet. page break

Plate I
Acanthoxyla geisocii on totara tree, illustrating cryptic colouration and form.
J. T. Salmon photo.

page 80 Certain introduced plants, such as willows, cedars and garden roses, appear to provide adequate food requirements for Acanthoxyla sp. A colony of New Zealand Acanthoxyla prasina was introduced into England accidentally in 1910, near Paignton, and is still thriving there on Japanese cedar. Both A. prasina and. Clitarchus hookeri have found their way from New Zealand to the Scilly Isles, where they are established in the gardens of Tresco Abbey.

Stick insects will feed during the daytime, but for the most part they are nocturnal, remaining concealed in foliage during the day. They walk fairly rapidly with a peculiar ambulatory or swaying motion, and individuals will travel considerable distances, often over open country, for no apparent reason other than an urge to migrate.

One of their most fascinating habits is the performance of the ‘dance’, a peculiar motion in which the body is swayed from side to side by a flexing motion of the legs at the trochanteral-femoral and femoral-tibial joints. This may continue for half an hour or more without stopping. It starts and stops for no apparent reason, and has no apparent purpose. I have seen more than a hundred young stick insects standing on the glass of a breeding cage, all performing this dance together and in unison — an extraordinary sight. I have also seen lone individuals in the field going through the same antics, usually at a little before sunset.

Most stick insects in New Zealand occur in two forms, a green form and a brown form. They always hatch green coloured, but may change to brown at the second moult. The adult insects cannot change colour, though green ones do tend to go a brownish green a week or so before dying. All species undergo four ecdyses during growth to maturity, and some can live for periods up to two years.

Stick insects are sometimes found with one or more legs shorter than the others. This arises from the accidental loss of a leg which is later regenerated. A regenerated leg is always shorter than the normal leg, its actual length depending on the instar in which it was lost. For instance, if it is lost in the first instar it will be only slightly shorter than normal when the insect is full grown; if it is lost in the second or third instars it will be correspondingly so much shorter; a leg lost after the last moult cannot be regenerated, and remains as a stump. Legs lost are usually broken off at the coxal-trochanteral joint which covers over with a small black cap, underneath which the new leg gradually grows. As growth proceeds, the cap extends until a small loop or hook-like structure results, within which can be seen the perfect leg in miniature. This persists in this state until the next moult, and when the insect emerges from its old skin the loop has been replaced by a perfect though smaller leg.

It is not unusual for a leg to be lost during moulting, and in fact stick insects suffer a severe mortality rate during moulting as they sometimes fail to emerge properly from the old skin and become strangled, as it were, by it.

Stick insects belong to the Orthoptera, Family Phasmidae. They are absolutely harmless and are quite amenable to handling, though easily injured by crushing. page break

Plate II
Regeneration of legs in Acanthoxyla prasina. of the loop containing the regenerating leg. Fig. 1: First stage, black cap formation. Fig. 2: Second stage, formation Fig. 3: Third stage, the new regenerated leg appears after moulting.
J. T. Salmon photos.