INTRAPOPULATION SIZE VARIATION OF FREE-LIVING AND TREE-BORING COLEOPTERA

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Abstract
Intrapopulation size variation was studied by means of length measurements quoted in taxonomic literature and from material of various groups of Coleoptera collected in the field. The degree of variation was significantly different among groups, and generally the free-living groups had lower intrapopulation size variation than most of the tree-boring and the parasitoid groups. Several tree-boring groups (e.g. Scolytidae, Curculionidae), however, had a comparatively low variation. A high intrapopulation size variation is common in species whose larvae are unable to choose and determine their own nutritional situation. Such larvae have a restricted mobility and the quality of their food is unpredictable. This is characteristic for most of the tree-boring groups. A high size variation increases the niche width of the species and probably has a buffering effect in unpredictable environments. Those tree-borers that are able to reduce the degree of unpredictability (i. e. by parental care, specialization in choice of microhabitat and in larval morphology, as in Scolytidae) have a rather low intrapopulation size variation. There are two possibilities regarding the genetic nature of the high size variation: (1) The population contains a variety of phenotypes with genetically fixed size, and (2) each individual is flexible with the possibility of becoming large or small depending upon the nutritional conditions. Possibility (1) is in agreement with the theories of a correlation between environmental heterogeneity and a high intrapopulation genetic variation. Free-living species and those tree-borers with less intrapopulation size variation are usually subject to more uniform conditions, and if adverse conditions do arise, these species usually fail to complete their development rather than produce smaller adults.