Colour and size variation within a population of Brachaspis collinus (Hutton) (Orthoptera: Acrididae)

Abstract

There are at least two types of biological variation, i.e., “group, or geographical variation” referring to differences between populations and "individual variation” referring to differences among individuals in a single population. Variation among individuals has been of interest in descriptive zoology and especially to the taxonomist as a reaction to the ‘type’ species concept. Moreover, the occurrence of individual variation and its significance to the species is an important part of ecological and evolutionary study. A population can adapt itself to a varying environment either by producing phenotypes with a wide tolerance and plasticity, or by producing a wide variation of individuals, with certain individuals sacrificed to the advantage of the population.

Geographical and individual variation in size of animals is common, especially among insects. Widespread species which show similar adaptation to similar conditions are often considered in terms of "ecological rules”. One of these is Bergmann's Rule concerning the size of individuals and temperature. Briefly, Bergmann's Rule predicts larger sized individuals in cooler parts of a species range. The validity of the rule has been shown for many birds and mammals but reports on its application to poikilotherms conflict. Bigelow (in press), recorded a significant clinal increase in size with increase in altitude within certain populations of two species of New Zealand Alpine Grasshoppers (Acrididae), viz: Brachaspis nivalis (Hutton) and Sigaus australis (Hutton) but size variation has not been investigated in populations of other New Zealand species of grasshoppers. Brachaspis collinus (Hutton), a fairly large brachypterous species, occurs above 3,000' in the mountains from N.W. Nelson to Arthur's Pass in the South Island. It was therefore of interest to determine the relationship of size and altitude in B. collinus to further test the applicability of Bergmann's Rule to New Zealand Grasshoppers.

The widespread occurrence of geographical variation among populations in colour and pattern has been noted by Mayr (1963) and by many others. This variation has been recorded for adult specimens of New Zealand grasshoppers by Bigelow (in press), who also notes a colour polymorphism in the adults of B. collinus in the more southern parts of its distribution. However, the extent of the variation of adults and the variation of juveniles in this species requires further study. This study has been concentrated on one population of B. collinus in an attempt to determine the extent and possible significance of the individual variation.