ONTOGENIC CHANGES IN THE MICROHABITAT PREFERENCES OF DECTICUS-VERRUCIVORUS (ORTHOPTERA, TETTIGONIIDAE) AT THE EDGE OF ITS RANGE

The large ground-dwelling bush cricket Decticus verrucivorus is widespread in continental Europe, but declining at the northern edge of its range. This paper defines the species' habitat requirements in northern Europe by describing the distribution of each of the seven nymphal instars and adults in relation to vegetation structure within a chalk grassland in southern England. First instars were strongly associated with short, sparse turf, reflecting the oviposition behaviour of females. Subsequent early instars were also found in short, open turf, but at the fifth moult a distinct shift in distribution to dense grass tussocks occurred. Late instars and adults were strongly associated with these structures, probably to avoid vertebrate predators. Mean temperatures within tussocks were lower than in short turf, but these highly mobile stages actively thermoregulate by basking on the sides of tussocks. In contrast, areas of short turf may be crucial for the development of the smaller and less mobile early instars and eggs. It is concluded that a fine mosaic of closely-grazed turf, dense tussocks and intermediate structures must be created and/or maintained at the species northern-most sites, if its present distribution is not to contract.