1972 - The Vasculum

47
VASCULUM VOL. LVII. No. 3. 1972
diapause. In addition the rate of larval growth with temperature follows an almost identical relationship to that
shown by Agabus bipustulatus which suggests an overwintering larva.
Of the remaining two species of Agabus found in the tarn Agabus chalconatus was only identified
from nine males, no females having been captured; both this species and Agabus sturmii are locally common
throughout Britain. The larvae of the latter were captured regularly in the tarn and overwintered at much the same
time as Agabus bipustulatus. Although adults of this species never laid eggs in the laboratory, suggesting either
adult diapause or the absence of the correct substrate for oviposition, it appears likely that there would be diapause
in the third larval instar. This species is smaller than Agabus bipustulatus and the larval size ranges do not not
overlap which in itself will prevent competition for similar sized prey.
It is interesting to note how the group of closely related species occurring in the same water body
regulate their life histories differently by spatial separation of larvae between plankton and substrate, in size
difference, or in the season when present.
The remaining species include Rhantus bistriatus, described by Galewski (1963) as being the most
typical representative of the genus occurring in shallow impermanent sedge or grass choked ponds.
Trichoptera
All the Limnophilus spp. of caddis occurring in the tarn have long adult flight periods and according to Crichton
(1971) these species are all likely to exhibit adult diapause in that no oviposition takes place until late in the
season. In addition P. Hiley (personal communication) states that these species eventually deposit their eggs on
dry stones below top water level and the larvae begin to develop in the egg mass, but then remain inactive until
they are covered by the returning water surface.
Phrygranea grandis, which occurred regularly in the south-east end of the tarn has a flight period from late April
to early July and not only do adults require to deposit eggs into water but also larvae are found throughout the
year, according to Bray R. B. (1969). This species is apparently unsuited to life in all but the perennial parts of the
tarn and is likely to have been eliminated in the catastrophic drying of 1971.
Hydracarina
The five species occurring in sorne numbers in the tarn are Arrenurus buccinator, Arrenurus membranator,
Limnesia koenike, Piona coccinea, and Pionacercus leukarti, According tol C. L. Hopkins (1962) these are all
species of thick vegetation in small ponds or reedswarnp. In Tarn Dub the first three species occur in all months of
the year whilst the last two are present from March to September only.
Hirundinea
Of the two species of leech present Erpobdella octoculata dominates. Surveys by K. H. Mann (1955) suggest that
smaller water bodies with calcium contents below 7mg/litre are usually inhabited by a leech population in which
Erpobdella octoculata dominates Helobdella stagnalis
Mollusca
The Gasteropod fauna of the tarn is typical of the soft water "slum" fauna described by Boycott (1936). Limnaea
truncatula and Planorbis spirorbis being typical of areas that dry up in summer, whilst Limnaea glabra is a
calcifuge and Planorbis leavis is an inhabitant of soft water areas.
Summary
Elements of the fauna, flora and water chemistry of Tarn Dub, an upland pond subjected to periodic drying out,
are described. The species list for the area