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The Fantasmagoric Australian Giant Prickly Stick-Insect
Extatosoma tiaratum(MacLeay)

By Kevin Wright, DVM

Introduction

Extatosoma tiaratum (MacLeay), commonly known as the giant prickly stick-insect, Macleay’s spectre or (colloquially) the spiny leaf-insect, is a magnificent Australian phasmid. Females commonly attain a length over 145 mm (a 160mm female was recently found in Kuranda) and a weight, when gravid, in excess of 13 g. Males are slightly smaller but weigh much less, typically weighing 1.5-2.0 g. This species is clad in a range of hues, from light beige to sombre mahogany and covered with spines and other protuberances. Whilst this is effective camouflage that serves to break up its outline against a background of leaves and twigs, these anatomical adaptations and overall size of the adult specimens make the Australian giant prickly stick-insect a striking centrepiece display in the home or public insectarium.

The giant prickly stick-insect seems to be well-established in European home collections. This is not the case in the United States of America, where the species is perceived as a potential agricultural pest capable of becoming established in the warmer states. In the US, possession of giant prickly stick-insects (and many other non-native invertebrates) is regulated by federal agencies and is, therefore, restricted to institutions rather than private hobbyists.

Overview

Giant prickly stick-insects show remarkable sexual dimorphism. The mature males bear functional wings and are slender-bodied with a thin abdomen to aid in flight. The mature females have vestigial wings and large pendulous abdomens that become distended upon maturation and ovulation. Both sexes prefer to hang upside-down on twigs and branches and slowly and methodically climb around their enclosure. The tail is markedly curved in the female so that when a stick-insect is forced to walk on the ground the tail hangs forward over its head, similar to a scorpion on the alert. The male stick-insect has a tail that is far less curved and may lay flat when it is facing up on a vertical perch.

Parthenogenesis, the asexual production of viable eggs from virgin females, is fairly common in stick-insects. In fact, there are species in which males have never been collected or subsequently reported. Giant prickly stick-insects can reproduce parthenogenetically, in which case all the offspring will be female, or by sexual reproduction, which results in male and female offspring. Once mature, a female giant prickly stick-insect may live from six to around eighteen months, with males often having a shorter life. During its lifetime, a female may lay several hundred eggs, sometimes more than 1000.

The males have an insatiable sex drive and will often mate, or attempt to mate, in lieu of eating. While most males and females will continue to eat whilst mounting or copulating, in some instances it may be wise to periodically separate the male to encourage him to eat and put on a bit of weight between matings.

Captive maintenance and diet

Giant prickly stick-insects are relatively easy to house. A colony of one male and three females can be kept in an enclosure as small as 30 x 60 x 30 cm, although larger enclosures are suggested. I personally keep my insects well ventilated but others would argue that because they are often found in rainforests in the wild, they, therefore, prefer it humid. Some hobbyists use screen-sided enclosures but glass or acrylic aquariums with a mesh or slatted top work well. Supplemental lighting is not necessary in a well-lit room and excessively bright conditions may be stressful to the giant prickly stick-insect. Temperatures should range between 22-26°C and the humidity should stay around 30-40% by day, with a brief period of much higher humidity following misting to provide drinking water. The top of the substrate should dry out within two hours of misting. If it stays too humid, giant prickly stick-insects become susceptible to dermatological infections by bacteria and fungi. In very large enclosures, a mosaic of microclimates with different light intensity, temperature and humidity may be provided to encourage more normal behaviours, as the stick-insects may move around and choose the best environment for its particular needs.

Moistened peat moss can be used as a substrate. This should be lightly moistened and then squeezed until it retains little water, but clumps when compressed. The peat should be 2-5 cm deep and allowed to thoroughly dry out periodically. Do bear in mind that using peat (particularly if it is too wet) can become mouldy or infested with small creatures. For this reason some hobbyists prefer to use newspaper, changing it weekly, and others use no substrate at all, preferring to wipe up the feces and uneaten food with a moistened paper towel or sponge.

Cage furniture should include several branches of varying diameters in a horizontal position or slightly inclined. Some hobbyists include a small jar of water to hold the browse and keep it fresh while others simply lay the browse across the perches or hang it from clips attached to the side of the enclosure by suction cups. Whatever approach you use, it is important that it is easy for you to service the enclosure and keep it clean. If it is difficult to spot-clean and remove feces or uneaten food, or to change the browse or spray mist the enclosure, you are likely to procrastinate servicing the insects as often as necessary to keep the colony healthy.

In some countries there is the potential for the giant prickly stick insect to colonise and potentially pose a threat to the local ecology. The climate in the UK is too cold for this to pose a problem but if you do live in a warmer climate, you should take measures to ensure this does not happen. It is particularly important to dispose of material from the insects’ enclosure in a responsible manner. When you do change the substrate, or spot-clean, you should freeze the debris for at least 72 hrs to kill any undetected eggs that it might contain to prevent accidental introduction of the giant prickly stick-insect outside its native range. An alternative to freezing is to spread the debris on a cookie sheet to a depth of 1 cm and leave it in an oven heated to 80°C for at least 45 minutes.

Giant prickly stick-insects eat a variety of readily available food plants. Rose leaves are often readily accepted, as are mulberry leaves and blackberry leaves. Other reported food plants include bramble, several species of eucalyptus, raspberry, oak, Pyracantha, and hawthorne. Other food plants are likely to be accepted and some rearers have recently had success with Japanese knotweed. If you try to feed an untested food plant, you should establish a second small colony comprised of young specimens and feed them the plant for several weeks to months. If the test colony specimens gain weight and shed properly (and even better, lay fertile eggs!) then you may assume this plant species is a safe and nutritious food item - ignoring this advice may result in the loss of an entire colony due to unexpected toxicity or malnutrition!

Fresh leaves are the proper food, although dried out leaves will be eaten as a last resort. However, the low water content of wilted and dried leaves may result in constipation, a potentially fatal disorder (see below). Fresh leaves frozen in water may be thawed and some specimens will eat them. The long-term consequences of feeding frozen leaves appears undocumented but may result in some nutritional deficiencies, as water-soluble vitamins and other nutrients degrade with freezing. It is essential to collect the food plants only from sources known to be free of pesticides, herbicides and other toxic compounds. I highly recommend keeping hardy roses or bramble and Pyracantha yourself so that you have a steady supply of safe food for your stick-insect colony. You also should be wary of allowing your colony size to outgrow your browse capability. If you cannot guarantee fresh browse on a regular basis, you should not keep stick-insects.

I’ve noted an interesting behaviour in hungry giant prickly stick-insects. If the available browse has dried and no fresh browse is available, the hungry stick-insects will descend to the ground. If they are kept with dry peat moss as a substrate, they will start to manipulate the dry peat moss with their forelegs and mouth and eventually form a ball approximately 1.5-2 cm in diameter. The ball is quite solid and may incorporate bits of dried leaves that have fallen to the moss. Some may be seen chewing on the edge of the ball for several minutes. The balls are eventually abandoned and the stick-insect will ascend into the branches to await fresh browse. I am unaware of other reports of this behaviour in this or other stick-insect species.

Breeding

Copulation is an extended affair and may last upwards of an hour or more. The male can be quite persistent and will often remain mounted on the back of a female for days before and after copulation. This mate-guarding has been shown to increase the number of offspring sired by the male in a female in other insects, but I am unaware of this theory’s validation in the Australian giant prickly stick-insect.

Eggs are often oviposited with considerable force and may startle the owner by hitting the side of the enclosure. A primed female may lay over a dozen eggs in the course of an evening and several hundred (sometimes in excess of 1000) eggs in her life. The eggs are typically 5-6 mm long x 3-5 mm wide.

Novice stick-insect hobbyists may confuse the male’s spermatophore with an egg. If the spermatophore is not successfully implanted in the female it may drop to the ground instead. It is typically bright white to ecru and glistens when fresh. The eggs, on the other hand, are mottled gray and brown and have an “operculum” on one end surround by a dark brown to black collar. The eggs are very variable in pattern, with no two being exactly alike. The spermatophore is also smaller than the egg. A final distinction is that you will only find one spermatophore per male, while several eggs may be laid in the same time frame.

The eggs typically take about nine months to hatch. They may be left in the adult cage, or removed to incubate artificially. They should be kept at temperatures between 22-26°C and under fairly dry conditions, although some hobbyists spray mist the eggs weekly. Excessively moist conditions may cause the eggs to develop a coating of mold; some eggs appear to tolerate external fungi and will hatch while others may die. Whether or not the eggs die may be due to the types of fungi growing on the egg, rather than any inherent variation in egg viability.

Nymphs are quite active upon hatching and must be kept in an enclosure with a fine enough mesh, or ventilation slat, to prevent their escape. They are typically voracious and grow quickly. If nymphs do not appear to be eating well, it may be that the food plant leaves are too tough. If this is the case, the leaves should be slightly mashed or cut with scissors to allow the juices to leak through. Alternatively, a different species of food plant may need to be offered instead.

Handling

Giant prickly stick-insects prefer to hang underneath their perches. If it is necessary to remove them so that their enclosure can be thoroughly cleaned, they may be transferred to a screened top placed across a five-gallon bucket. They will typically hang there for several minutes or more, although agitated males may take flight. Be sure to have any fans turned off before opening the enclosure to prevent disaster falling upon a wayward male! It is also wise to make sure that no other dangers lurk nearby, such as the family dog or cat, a hot stove or an escape route.

If you must handle the giant prickly stick-insect, beware! Their spines can puncture human skin and might break off the tips in the skin. If maneuvered gently to hang onto the hand this is unlikely, but if you attempt to invert it to walk on your hand some giant prickly stick-insects become agitated and are likely to back up and scratch your skin. Other giant prickly stick-insects are nonplussed as to whether they are hanging or walking right-side up on your hand. Thin gloves should be worn if the stick-insect needs to be physically restrained, it may be coaxed into a clear plastic tube of sufficient diameter for a close visual exam.

Problems

Giant prickly stick-insects require dry conditions to avoid bacterial and fungal infections of the exoskeleton. Excessively dry conditions, however, can result in dehydration and subsequent constipation. Constipation typically is signaled by an absence of fecal material produced over a twelve to twenty-four hour period. The appetite may also decrease markedly. If untreated, the tip of the tail may turn dark brown to black and the discolouration may lengthen cranially along the adjoining abdominal segments. Dry fecal material may be visible at the anus. If the constipation has not resulted in permanent damage to the intestinal tract or secondary infection, treatment consists of lightly misting the stick-insect and allowing it to drink until full.

Placing a shallow pan of water just beneath the stick-insect, positioned so the tip of its tail soaks in it, will help soften the fecal material at the anus and in the distal portion of the intestine. The constipated stick-insect will often climb into this position on its own if the overhanging branch is close enough to the pan of water. Within twenty-four to forty-eight hours, the constipation will be relieved in uncomplicated cases and the stick-insect will resume eating and defecating normally. If the constipation does not resolve with this treatment, a moistened cotton swab and fine tip forceps may be used to tease the protruding feces out of the anus, but it is likely that a stick insect thus affected has had severe damage to the intestine and may die.

Stick-insects molt as they grow. This is typically without incidence, although some stick-insects may fail to emerge from their old skin and may die. If conditions are too dry, a stick insect may emerge but a leg may remain stuck within the old skin or injured or pulled off during the abnormal molt. Legs will re-grow in young stick-insects but the final length of the leg depends on the number of moults between the injury and maturation. It is fairly common to see adult stick-insects with one or more legs much smaller than the others due to amputation or injury of the limbs while young.

Related species

There are currently four Extatosoma species. The most commonly seen in culture is the leaf-mimic, Extatosoma tiaratum, first seen in the UK in the 1960s, and now one of the most popular species of captive stick-insect. It originates from New South Wales, south-east and north Queensland (Australia) and cultured stock almost certainly comes from insects reared in north Queensland in the 1950s.

From the same areas originates the lichen-mimic, E. bufonium, which is now being reared by enthusiasts. In common with another lichen mimic, E. carlbergi, from Papua New Guinea and Irian Jaya, this species differs from E. tiaratum in having small, non-overlapping expansions of abdominal segments five to seven.

The leaf-mimic Extatosoma popa, from Irian Jaya and Papua New Guinea grows to around 116-150 mm and has a pale v-shaped mark on the mesonotum (the upper surface of the second, and usually longest, segment of the thorax). A paper by Brock (in press) changes the current classification and includes the first description of the male of E. popa, as well as keys to males, females, eggs and newly-hatched nymphs. This species is now also increasingly being reared by enthusiasts.

In Conclusion

In summary, the giant prickly stick-insect is an attractive, hardy and prolific phasmid that is well established in captivity. Its needs are easily met if you have access to pesticide and herbicide-free food plants. Unfortunately, the ease with which it is kept can be a draw-back in some countries (not, thankfully, the UK), as it is feared as a potential agricultural pest in warm climates. If you do live in a country where this could be a problem, check your local regulations before attempting to acquire this species. If it is legal for you to own, I highly recommend giving this species a try.