had shrivelled up or were in fruit all of a sudden. We were amazed, since we had had so
much rain not long before, but, of course, on these light soils water retention is very poor
and it heats up fast in the hot sunshine. We found colossal quantities of Clustered Clover,
almost all of it in fruit and dried up, and some Corn Spurrey by a horse-grazed field, and
other plants of passing interest such as Vervain, Trailing St John's-Wort, Knotted Clover and
Grass Vetchling. We surmised that the dry winter had perhaps not allowed rapid growth of
the perennials, but the wet May had allowed the annuals to have a wonderful time.
In early July, on a cool and windy evening, we joined a visit to Davy Down in Thurrock
to learn about bats and glow-worms. Brannetts Wood is ancient woodland, and the Mardyke
wanders across grassy water-meadows. Nearby is a pond where Marsh Frogs and Water
Voles live. I have seen Water Voles there several times in broad daylight, but this evening
they were not to be seen. Marsh Frogs we heard, making a loud croak.
Three people had bat boxes, which I learnt have tunable frequency sensitivity in the MHz
range, which pick up the pulsed squeaks of the bats and then lower the output frequency
so we can hear the noises. The frequency picked up indicates the species of bat. We
'heard' several Pipistrelles at about 45-50MHz, and several Serotines at about 25MHz,
though three more have been recorded here on other warmer nights. Apparently the size
of the bat goes roughly inversely to the frequency of sound emitted, just as small women
usually sing soprano but big men usually sing bass. We saw the Serotines but the Pipistrelles
were so small and quick that I never saw one, though others did. Each makes regular
squeaks for a short while, then they get faster as the bat homes in on something. It sounds
like blowing a raspberry as the bat is actually catching the insect. We also heard crickets
on the bat boxes, and apparently they were all Roesel's Bush-crickets, which are common
at Davy Down in the damp grass.
On the way back the Glow-worms were shining, and we saw three, all in thick vegetation
below the woodland. We were told that, had the evening been warmer, there would have
been many more. The females make the glow and sit in vegetation trying to attract a mate.
The actual beetle is about 1cm or so long, and its rear part, about one third of its total
length, glows. The adult females cannot feed or fly, and the glow takes quite a lot of
energy, so they cannot glow for too long or there is not enough energy left to lay eggs
after mating. This whole evening was fascinating, and I learnt a great deal about all these
interesting animals.
As July proceeded the weather became hotter and hotter, not just in south Essex but also
over much of Europe. And promised thunderstorms never dropped any rain, while the
weather forecasters were saying there was no end in sight to the hot weather. Everything
was brown, trees wilting, insects giving up, everything parched. The hottest ever July
day in Britain was recorded on 26th, and we all wished that instead of holidays booked
by the Mediterranean, we should have gone to Orkney or Shetland instead, where the
temperatures were around 20°C and the grass was green.
Essex Field Club Newsletter No. 51, September 2006 7