A Common Resource - Gower - page 209

Dung excludes light for several months as it is decomposing and therefore kills the plants underneath;
although recolonisation soon takes place, the effects on the vegetation may last up to 18 months. A fringe
of taller vegetation also develops around the fresh dung, and prominent in this plant community are
nutrient-loving species such as perennial rye-grass and white clover.
Alongwiththeever-presentearthworms,whichactivelyfeedontheinterioranddrawdungdowninto
their burrows, these organisms more or less complete the breakdown of the dung, returning the nutrients
to the soil, and eventually only fragments of the crust remain. These provide an attractive resting place
used by a variety of insects for basking in sunny weather. One rare fly found on the Gower commons,
the hornet robber fly Asilus crabroniformis , actively chooses these desiccated fragments of dung to lay
its eggs. This is our largest robber fly and an impressive insect, being a convincing mimic of the hornet
whenseeninflight (Fig.115).Itistherefore unlikely tobeoverlooked bytheobservant naturalist, butthe
precise reason for this odd behaviour is still not fully understood. It may be that the dung is a much safer
environment at this stage as many of the predators have departed. There is no evidence, however, that the
newly hatched larvae of the robber fly remain in the dung. It has been suggested that they prey upon the
larvae of the common dor beetle; their flight periods are indeed closely synchronised, and the robber fly
usually appears around dung that has been used by these beetles. Repeated searches of dor beetle bur-
rows elsewhere in Britain, however, and examination of their larvae and pupae, have so far failed to find
any robber fly larvae. On maturity the larva pupates in the free-draining soils of the commons and then
emerges in midsummer as an adult to hunt for grasshoppers and other prey. The robber flies are powerful
animals with sharp piercing mouthparts that penetrate the exoskeleton of other insects, allowing them to
suck their prey dry. The adult tends to fly in an arc to take flying prey from behind, although it will also
strike at grasshoppers on the ground. 'The name robberfly aptly conjures up the image of the highway
robber,readytopounceontheunsuspectingvictim'(Stubbs&Drake,2001).Asthecowpatsinwhichthe
hornet robber fly lays its eggs have to be exactly the right age, the removal of cattle from an area for even
oneseasoncanleadtoitsextinction.Climatechangemayalsobehavinganeffectonitssurvival,because
the behaviour of adults is affected by temperature.