Trigoniophthalmus alternatus (Silvestri, 1904) in Devon caves
Chris Proctor, 37 Grenville Avenue, Torquay, TQ2 6DS.
Many people will have at least a passing familiarity with the thysaneurans (three pronged
bristletails), the largest and most obvious of the apterygote (primitive wingless) insects. The
familiar silverfish Lepisma saccharina, an introduced species which occurs in houses is the
one most people will have come across. Seven native species are known in Britain and
Ireland, five of which have been recorded in Devon (Delaney, 1954, National Biodiversity
Network). The Devon species are Dilta littoralis, Dilta hibernica, Petrobius maritimus,
Petrobius brevistylis and Trigoniophthalmus alternatus. The Dilta and Petrobius species are
widely distributed in Britain, but Trigoniophthalmus alternatus is restricted to south Devon,
making it of considerable interest from our point of view. T. alternatus is at the northern edge
of its range here – elsewhere it is widespread in continental Europe (except Scandinavia) and
it also occurs in the northeast USA, where it is though it may have been introduced (Fauna
Europaea database, Wygodzinsky and Schmidt, 1980).
While the thysaneurans as a group are instantly recognisable, identification to species is not
so easy. However with care, field identification to genus is perfectly possible, and with
Trigoniophthalmus this is all that is necessary, as we have just the one species in Britain, T.
alternatus. In common with other thysaneurans, Trigoniophthalmus alternatus is quite a large
animal, with a body length (not including the tail) of up to 10mm. It is brown in colour with
irregular paler patches, and often adopts a peculiar
angular hunchbacked pose that is well shown in figure
1. This may prove to be a useful field character but
further observations are needed. The antennae are
about the same length as the body, or a little shorter
(they are longer than the body in Petrobius, and Dilta
has antennae much shorter than the body). However
the antennae are frequently broken, especially on
older individuals so this is not a reliable character. The
eyes are the most useful diagnostic feature.
Trigoniophthalmus has a pair of large, circular
compound eyes, with a pair of small subtriangular
ocelli, lying close together in front of them (figure 2).
Petrobius has large circular compound eyes, with large
slit like ocelli in front of them, almost as wide as the
compound eyes (figure 2). In Dilta, the compound eyes
are rectangular, with small ocelli in front of their outer
corners (figure 2) (Delaney, 1954, Wygodzinsky and
Schmidt, 1980).
Delaney (1954) gives Berry Head, Brixham as the only British site for Trigoniophthalmus
alternatus but provides no other details (other than that it is “very rare!”). Members of the
Cave Research Group subsequently found it in caves in the Buckfastleigh, Yealmpton and
Plymouth districts between 1965 and 1973 (Hazelton, 1967, 1978). There appear to be no
more recent published records. The National Biodiversity Network database lists only the
Berry Head grid square (SX 9456) with no details of date or finder – presumably Delaney’s
old record. In 2005, I investigated invertebrates in various Devon caves and found
thysaneurans at two sites at Brixham and Buckfastleigh, sufficiently far inland to suggest that
the species involved was not Petrobius – a common genus on coastal rocks, which can often
be found in sea caves. Large scale macro photos were obtained and at both sites the animals
proved to be Trigoniophthalmus alternatus. These two caves are new site records for the
species.
At both the Brixham and Buckfastleigh sites T. alternatus was found on dry, vertical to
overhanging rock walls in the cave threshold – the twilight region a few metres in from the
cave entrance. They seem to prefer spots in the deep threshold, near the limit of light
penetration, sometimes occupying shallow fissures, but frequently on flat rock. They are not
abundant at either site – only two or three have been seen on any one visit. The old Cave
Research Group records (Hazelton, 1967) give the habitat as the deep threshold at one site,
and the dark zone at another, though the latter cannot have been very far from the entrance
as the cave it was found in is only 13 metres long. The sites chosen are much the same areas
favoured by other characteristic cave threshold fauna such as the cave spider Meta menardi.
In 2005 T. alternatus was first seen at the Brixham cave in early January. A visit to the cave in
May to try to obtain better photos failed to locate any specimens. Many visits were made to
the cave near Buckfastleigh from May onwards (a topographic survey was carried out over
the summer of 2005) and although a lot of other fauna was found, T. alternatus was not seen
at this site until mid October. A visit to the Brixham site in late November revealed that it had
reappeared in that cave. The Cave Research Group records (Hazelton 1967, 1978) of T.
alternatus in caves date from the winter period between November and March. Taking these
records together, T. alternatus seems to be present in caves over the winter but disappears
over the summer months. This suggests that the species may use cave thresholds as
hibernation sites.
The habits of T. alternatus in the northeast USA are described by Wygodzinsky and Schmidt,
(1980). There, it lives almost exclusively on rocks, generally in disturbed areas such as old
walls, under rocks and in limestone quarries. They hide during the day and emerge at dusk,
appearing only for a short period after sunset, around twilight. “Hundreds of specimens” are
described as milling around on top of a stone wall at Cornell University at twilight. If the
animal has similar habits in this country, then in summer it must presumably disperse from its
hibernation sites to adjacent rock outcrops, where it could be searched for after sunset. In
addition to searching outcrops near caves, it would be well worth checking other rock
outcrops, walls etc – it is likely that T. alternatus hibernates not only in caves but also in small
cavities of mesocavernous dimensions among rocks, in screes, and in walls.
I would be very interested in any reports either of confirmed or suspected presence of T.
alternatus at any further Devon sites. Thysaneurans on coastal rocks and in sea caves are
likely to belong to the common coastal genus Petrobius, but at present if thysaneurans are
found at any inland cave in Devon, then T. alternatus should be suspected. Given its rarity,
specimens of T. alternatus should obviously not be collected from any of its Devon sites. As
noted above, the animal can be identified in the field either by taking a good macro photo
showing the eyes (although a reproduction ratio of at least 1:1 is really required) or by
examining the animal with a good hand lens or field microscope. Even if not identified to
genus, any record of thysaneurans in an inland cave would still be valuable. I can be
contacted at the above address.
References.
DELANEY, M.J. 1954. Thysaneura and Diplura. Handbooks for the identification of British
Insects, 1 (2). Royal Entomological society.
HAZELTON, M. 1967. Biological records for 1964, 1965 and 1966. Transactions of the Cave
Research Group,9 (3), 162-241.
HAZELTON, M. 1978. Hypogean fauna: biological records No. 16, 1972-1976. Transactions
of the British Cave Research Association, 5 (3), 164-198.
WYGODZINSKY,P. & SCHMIDT,K. 1980. Survey of the Microcoryphia of the northeastern
United States and adjacent provinces of Canada. American museum Novitates, (2701), 1-17.
Fauna Europaea database:
http://www.faunaeur.org
National Biodiversity Network:
http://www.searchnbn.net/index_homepage/index.jsp
List of figures
Figure 1.
Trigoniophthalmus alternatus on cave wall, Buckfastleigh, 15th October 2005.
Figure 2.
(A), head-on view of Trigoniophthalmus alternatus showing the compound eyes (grey) and
ocelli (black). Diagrams (B,C,D) show the arrangement of the compound eyes and ocelli in (B)
Trigoniophthalmus alternatus; (C) Petrobius maritimus; and (D) Dilta sp., all viewed head-on.
Diagrams (A,B,C) are drawn from photographs, (D) after Delaney (1954).