Ridley’s Stick Insect

About walking leaves and walking sticks

Insects that strongly resemble leaves or twigs in appearance are called walking leaves or walking sticks, stick insects, ghost insects or phasmids. Stick insects are considered the longest insects on Earth. The body length of some Southeast Asian species can exceed half a meter with legs outstretched. Most stick insect species occur in tropical and subtropical habitats, but some are also found in Canada, Japan, Europe or New Zealand.

Ridley’s stick insect is known from only a single male specimen collected in the island city-state of Singapore at the beginning of the 20th century. This insect species probably inhabited tropical rainforest. The English entomologist William Forsell Kirby was the first to describe Ridley’s stick insect scientifically in 1904 under the name Bactricia ridleyi in Notes on Phasmidae in the Collection of the British Museum:

Greenish brown; head short, narrowing behind, and with two compressed blunt horns between the eyes; distance between the horns and the antennae, at the side of the head (…). Antennae and legs long and slender, the latter nearly straight, and unarmed except for a sharp, flat, curved spine near the base beneath the middle femur (…).

Notes on Phasmidae in the Collection of the British Museum (Natural History), Annals and Magazine of Natural History, Series 7, Volume 13, 1904, Issue 78, W. F. Kirby.

The body of the found individual, which is now kept in the Natural History Museum in London, was about 12.5 centimeters long, with legs and antennae adding another roughly 8.3 centimeters. Since stick insects almost always show pronounced sexual dimorphism and females are larger than males, it can be assumed that this species could become even larger.

From the overview The Types of Phasmida in the Natural History Museum, London, UK, published in the journal Zootaxa in 2016, it can be seen that the holotype described by Kirby in 1904 came from Henry Nicholas Ridley, to which the species epithet ridleyi also points. Ridley was a British botanist who directed the Singapore Botanic Gardens from 1888 to 1911, where he also came across the then unknown stick insect species.

Ridley’s stick insect – fact sheet

scientific names	Pseudobactricia ridleyi, Bactricia ridleyi, Bacteria ridleyi
original range	Singapore
time of extinction	1904
causes of extinction	habitat loss, poaching
IUCN status	extinct

Habitat loss may have led to the extinction of Ridley’s stick insect

The IUCN names habitat loss and also the possibility that the species may have played a role in traditional Asian medicine and may therefore have been collected as likely causes of Ridley’s stick insect’s disappearance. Singapore in fact underwent rapid development during the 20th century, combined with enormous population growth. Between 1911 and 1957, for example, mass immigration from China and India caused Singapore’s population to quadruple. And even after that, population numbers continued to rise strongly: between 1985 and 1995, for example, they grew by an average of 1.8% per year.

Singapore’s population growth and economic expansion led to the loss of 95% of its original rainforest since 1819 within 183 years. Only a tiny fraction of 0.025% remains of the former 740 square kilometers of rainforest. This comes from a study (2003) by Australian environmental scientist Barry W. Brook, in which he investigated the extinction of flora and fauna as a result of Singapore’s deforestation.

The forests found in Singapore today were only planted after the rainforest that once covered the roughly 683-square-kilometer island state had been almost completely cleared. Because of the high population density, hardly anything of Singapore’s original vegetation has survived. One exception is the mangrove forests along the coastal areas.

To what extent Ridley’s stick insect may have played a role in traditional Asian medicine cannot be said. It is known, however, as entomologists Frank Hennemann and Bruno Kneubühler note on phasmotodea.com, that the eggs of some larger ghost insect species are eaten by the indigenous population of Borneo because of their high protein content and as a remedy against diarrheal diseases.

Could Ridley’s stick insect still exist?

Since 2018, the IUCN has listed Ridley’s stick insect in the Red List as ‘Extinct’, because by that time no further individual had been found for more than 100 years. Several intensive searches in Singapore’s remaining forests and in neighboring countries for this species or genus were unsuccessful.

The tribe Diapheromerini , which belongs to the order of phasmids or stick insects, contains 37 genera. One of these genera is Pseudobactricia, established only in 1999 by the British entomologist Paul D. Brock and represented monotypically only by Ridley’s stick insect. Most species of the tribe Diapheromerini are found in North and South America, which led the authors of the encyclopedia Singapore Biodiversity (2011) to suspect that Ridley’s stick insect was not actually endemic to Singapore. This is also supported by the fact that the only specimen was found in the Singapore Botanic Gardens. In other words, it is not unlikely that the animal was accidentally imported together with plants. Scientists may therefore have searched for animals of this species in the wrong place.

And there is another reason for hope when it comes to the survival of Ridley’s stick insect, because scientists have rediscovered at least three phasmid species believed extinct in recent years: Scott’s stick insect (Carausius scotti) from the Seychelles island of Silhouette in 1990, the tree lobster (Dryococelus australis) from the Australian islands of Lord Howe and Ball’s Pyramid and from New South Wales in Australia in 2001, and in 2009 Davidrentzia valida, which is also endemic to Lord Howe Island and New South Wales.

Interesting facts about ghost insects

One special feature of phasmids is that they do not undergo a complete metamorphosis, as we know it from other insects, where young animals differ greatly in shape and lifestyle from adults. In ghost insects there is no pupal stage and the nymphs already look very similar to the adults.

The young hatch from eggs that can look completely different depending on the species and often resemble plant seeds or the animals’ droppings. The advantage is that they are hard to detect on the ground. Ghost insect eggs are in some cases better suited for identifying species than the insects themselves. This also led to the development of ootaxonomy, a branch of zoology in which eggs are used primarily for species identification.

The often nocturnal ghost insects belong, incidentally, to the flying insects (Pterygota), and many species do in fact possess wings and can fly, although most winged species are not good fliers. Some phasmid species have only rudimentary wings or none at all; in others wing development depends on sex. All ghost insect species are herbivores. Some species are specialized on certain plant species or groups, while others are far less specialized.

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