British & Irish Stick-Insects of Cornwall
by Malcolm Lee
Introduction
This century has seen many additions to our native fauna.
Some of the most unusual are the New Zealand stick-insects,
three species of which survive in the South West of Britain,
and one in South West Ireland: the Prickly Stick-insect Acanthoxyla
geisovii, the Unarmed Stick-insect Acanthoxyla inermis
(which is the species found in Ireland), and the Smooth Stick-insect
Clitarchus hookeri. A fourth species, the Laboratory
Stick-insect Carausius morosus, is often reared in
schools and private homes as pets, and discarded eggs and
insects may lead to short lived colonies outdoors, but these
cannot survive frosts and are usually killed off by our winters.
The first insect to be recorded was the Prickly Stick-insect,
located in Paignton, Devon in 1909, and Tresco, Isles of Scilly
in 1943. The Scilly colony may be as old as the Paignton one,
since a consignment of New Zealand plants was imported onto
Tresco in 1907, and some of these were sent to Paignton the
same year. The next insect to arrive, although the last to
be identified, was the Unarmed Stick-insect, which was certainly
in Treseder's Nursery in Truro in the 1920s, and may have
been there somewhat earlier. The Smooth Stick-insect was first
recorded on Tresco in the 1940s, but was probably present
much earlier.
They arrived here on New Zealand plants, most likely as
eggs in the soil. There were probably several importations.
Apart from those at Tresco and Treseder's, stick-insects at
Falmouth in 1981 were traced to an import by Scott's at Merriot,
Somerset in the 1970s, where insects were later seen in the
nursery.
Life Cycle
All three species have a similar and most unusual life cycle.
They breed parthenogenetically, that is, eggs develop without
the need for fertilization by a male. Indeed, males of the
Prickly and Unarmed Stick-insects are unknown, even in their
native New Zealand, and probably do not exist. Males of the
Smooth Stick-insect are not uncommon in New Zealand, but none
have been found here.
The life cycle begins with the hatching of the eggs, mainly
in late April or early May. They hatch to become miniature
adults about ½" long (12mm), and grow by shedding
their skin and expanding before the new one hardens. Some
10 weeks after hatching, and five or six moults later, they
become adults. A few weeks later they begin to lay eggs, typically
2 or 3 a day but some have been known to lay up to 9 eggs
in a day. In their adult life of 3 or 4 months they may lay
many hundred of eggs in a warm summer and autumn, although
cooler weather will greatly reduce this number.
Prickly Stick-insect
Prickly Stick-insect Acanthoxyla geisovii
photograph taken by Malcolm Lee
This is an unmistakable insect with a body covered in numerous
black spines. Adults have a body about 4" long (100mm)
and may be either green or brown, green being the more common
colour.
FOODPLANTS: This is one of the few insects which feed on
both evergreen and deciduous plants, having been found on
Cryptomeria, Thuja and Chamaecyparis, as well as Bramble Raspberry,
Pittosporum, Myrtle and Fuchsia.
LOCATIONS: It can be found on two of the Scilly Isles. On
Tresco in the Abbey Gardens, where it was first discovered
in 1943, and fairly widespread on the island. Also on St Mary's
near Old Town Church and by McFarlane Downs. On mainland Cornwall
at St. Mawes (where Tresco insects were released in the Riviera
Gardens c1959 and are now widespread in local gardens), Feock,
St Just-in-Roseland, Trethem, west Truro, and Veryan. Also
in Devon at Paignton, Torquay, Galmpton, and Ermington.
Prickly Stick-insect tetrad distribution at
Nov. 2003
Unarmed Stick-insect
Unarmed Stick-insect Acanthoxyla inermis
photograph taken by Malcolm Lee
Relatively smooth, with a few tiny bumps on the body. Adult
insects have a body typically 4" long (100mm), but an
insect with a 5" body (125mm) was found in Port Isaac
in 1992. This is the longest insect ever found outdoors in
the UK. Insects can be either green or brown. Whilst green
forms are generally a uniform apple green, brown forms can
be most varied. These range from light straw, mahogany red
to brown and purple blotched, and all shades in between. Mahogany
red insects have also been observed to change colour, being
red in the daytime and pale at night. Interestingly, in their
native New Zealand, the mahogany red, and brown and purple-blotched
forms are unknown, as is their ability to change colour.
FOODPLANTS; Mainly roses, bramble, currant, potentilla,
and leylandii hedging, but also dahlia, hypericum, red valerian,
and probably a wide range of plants.
Unarmed Stick-insect tetrad distribution
at Nov. 2003
LOCATIONS: Until 1985, all non-spiny stick-insect reports
on mainland Cornwall were treated as the Smooth Stick-insect.
Research by Paul Brock on insects from several locations showed
they were the similar Unarmed Stick-insect. It is most likely
that all mainland insects are the Unarmed, but this has not
been positively confirmed at every location. These are the
known mainland sites: Truro, particularly around the former
site of Treseder's nursery (first reported 1979, but seen
at the nursery since the 1920s), several sites around the
Helford River (1987, but may have been there since the 1920s),
also at Camborne, Constantine, Falmouth, Feock, Mawnan Smith,
Mevagissey, Mylor Bridge, Penryn, Probus, Port Gaverne, Port
Isaac, Rosewarne, Tywardreath and Veryan Green. Many of these
colonies originated from Treseder's plants, or via Falmouth
insects, which were collected in 1981 and given to a butterfly
breeder in Penryn. Their eggs hatched in spring 1982 and nymphs
were distributed to schools throughout Cornwall. Discarded
school insects and eggs may give rise to more colonies.
This is the species found in SW Ireland, particularly around
the Bay of Kenmare in County Kerry. It was first recorded
on Rossdohan Island in the early 1960s, but may have been
there much earlier. Several large gardens around the Bay of
Kenmare were laid out with New Zealand plants from Treseder's
in Victorian times.
Smooth Stick-insect
This is very similar to the Unarmed Stick-insect, but somewhat
smaller (80mm long). It can be identified as its cerci are
pointed, not rounded. It has no opercular spine, and has a
near continuous black line on its body. Its known food plants
are Brambles and roses, also ferns and callistemon. Outside
New Zealand just two colonies are known, and both are on the
Isles of Scilly; the Abbey Gardens Tresco, and in the north
of St Mary's.
Laboratory Stick-insect
As this species is so often kept in schools and private
homes, discarded insects and eggs may form short-lived colonies
almost anywhere. They will be killed off by the first frost.
This is smaller than the New Zealand species, being typically
only 3" long (70mm). It also lacks the cerci at the end
of the abdomen, and has long antennae, almost as long as its
legs.
Comparing the end of the abdomen with this key may identify
the three non-spiny stick-insects; -
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