The occurrence of an additional instar in the development of Chorthippus brunneus (Orthoptera: Gomphocerinae)

In two populations of Chorthippus brunneus in the Breckland district of East Anglia an additional instar is interposed between instars II and III during the development of female nymphs. The IIa nymph occurs with a frequency approaching 100%, in both heavily and very lightly grazed grasslands.

The IIa nymph is intermediate in morphological characteristics between the second and third instars. It is characterized by having wing buds similar to those of a second instar except with more extensive venation, while the genitalia are more like those of a third-instar nymph. The pronotum length: head length ratio is approximately 1·0 and there are 16 or more annuli in the antennae.

Members of the population taken from the very lightly grazed grassland and raised in the laboratory on grass that had been heavily grazed by rabbits omitted the IIa instar and so passed through only four stadia during their development. In contrast, members of the population taken from the heavily grazed field site continued to include the IIa instar in their development when fed in the laboratory on grazed grass.

Comparisons with other studies of this species show that there is geographical variation with respect to numbers of instars passed through during development. The additional instar has been found only in East Anglia where the warmer sunnier climate, which is semi-continental in the Breckland district, leads to earlier hatching. This allows more time for an extra instar to be included in the development.

The ecological implications of the observed variation in life history are discussed.