Phasmid Study Group

For almost 100 years, stick-insects have become naturalised in south west England, and around the Bay of Kenmare in south west Ireland. There are three New Zealand species, two of which have become widely distributed, and one is found only on the Isles of Scilly. A Mediterranean species has also been found on the Isles of Scilly in the last few years. Although it cannot survive outdoors here, one other species is commonly kept as pets, and discarded insects could turn up almost anywhere to be confused with these naturalised species.

Unarmed Stick-insect Acanthoxyla inermis

This is the most commonly seen species, accounting for nearly two thirds of records. Almost all the sites are
in Cornwall, but there is a small one just across the Devon border, in Plymouth. The strongest colonies are around Truro, Falmouth, and Mevagissey. This is also the species found in south west Ireland.

It was certainly present at Treseder's Truro nursery in the 1920s, but could have been there somewhat earlier. It would have arrived on plants imported direct from New Zealand by Treseder's, who were the first to import Australasian plants from late Victorian times and also supplied the large gardens in south west Ireland.

This is our longest UK insect, with the adult body measuring up to 125mm, although about 100mm is more usual. It has a smooth body, with just a few tiny bumps on its back. They can be either green or brown, with green being the more common colour.

Prickly Stick-insect Acanthoxyla geisovii

This species is found in two main areas; in Devon around Torbay, and in Cornwall around St Mawes. It is also widespread in the Isles of Scilly on Tresco and St Mary's.

It was first found in Devon in 1909, the earliest UK stick-insect record. On Tresco, although not seen there until 1943, the colony must have arrived on plants imported direct from New Zealand before the war broke out in 1939. Colonies on mainland Cornwall originated from the deliberate release of Tresco insects in a St Mawes garden c1959.

This species is readily identified as it has a body covered in small black spines. It is almost identical in size to the Unarmed Stick-insect, and can similarly be green or brown in colour.

Smooth Stick-insect Clitarchus hookeri: This is the third New Zealand species. It is only found in the Isles of Scilly, on Tresco and St Mary's, where it probably first arrived on imported plants around the same time as the Prickly Stick-insect. It is slightly smaller than the other New Zealand species, typically 90mm.

Mediterranean Stick-insect Bacillus rossius: In 2002 and in 2004, a small colony of this species was located on Tresco. It is a native of southern Europe, where it is principally found near the Mediterranean coast. It is a similar size to the Smooth Stick-insect.

Indian or Laboratory Stick-insect Carausius morosus: This species is the most common stick-insect in culture and is readily available in pet shops. It is easy to keep, but can be embarrassingly prolific. Each insect can lay up to a thousand eggs, many of which will hatch out in a matter of months. Whilst it cannot overwinter in our climate, it is occasionally found outdoors in summer and autumn as a result of the release of surplus insects. It is smaller than the other species, usually only 70mm.

Life Cycle: The naturalised stick-insects seen in the UK are all female, and reproduce by laying fertile eggs without the need for a male; a process known as parthenogenesis. Indeed, Acanthoxyla males have never been found in New Zealand, and probably do not exist. They all have a similar life cycle, with eggs hatching out in spring to form miniature adults about 12mm long. Development is rapid, as they are fully grown by mid summer. A few weeks later they start to lay eggs, typically 3 or 4 a day. These are simply dropped below the plant the adult is feeding on. Adults only live three or four months, with few surviving much into winter. A Overwintering of adults is not important for the survival of the colony, as it is the eggs that ensure the next generation. When these hatch the following spring, the emerging nymphs climb the first stem they see, which is likely to be the plant the parent fed on the previous year. Bramble is always a favourite foodplant, although our stick-insects will survive on many plant species, including evergreens like Leylandii hedging.

Identification of Adult Phasmids

Phasmids are best identified as adults, when the key features are fully formed. The Prickly stick-insect is identified from its black spines. Those species without spines are all very similar, but can be identified by comparing the end of the abdomen with these images.

1: Unarmed Stick-insect: rounded cerci.

2: Smooth Stick-insect: pointed cerci longer than last abdominal segment, with only a small gap separating them.

3: Mediterranean Stick-insect: pointed cerci half as long as last segment, and they are widely separated (nb. small nymphs of A. inermis may look similar). A
further key indicator is the antennae, which are very short, being only around a quarter the length of the front leg.

4: Laboratory Stick-insect: No visible cerci. Antennae almost as long as the front leg.