anically rich mosaic containing both native deciduous and planted coniferous forest. This project involved the collection and identification of several thousands of flower-visiting insects or arachnids in each habitat. Information on flowering and insect phenology and plant breeding systems was collected as well. Again, the site of greatest human disturbance was poorest in species numbers (Kakutani et al. 1990). The total number of arthropod species collected on the plants of the university campus was 37% of the total of the oak forest and 23% of the total mosaic of natural and planted forest (Kakutani et al. 1990).
To look at anthropogenic effects on dipteran species composition, one can compare different sites as in the studies just described, or one can look within a site at how species composition changes over time with changes in habitat. The later type of study involves resampling historical data sets for the continued presence of species. If we attempt to use historical data sets, we need to remember that they may not represent complete lists of the species that were present. Resampling efforts may produce more or fewer species, depending upon the effort and techniques employed. Disney (1986) recounts a survey of insects conducted by the Yorkshire Naturalists’ Union from 1954 to 1958, in which 436 species of Diptera were collected from the Malham Tarn in England (Flint 1963). Renewed attempts to produce a comprehensive list of Diptera in the 1970s resulted in a species list in excess of 1000. Disney attributes the increase in numbers to the use of multiple collecting methods, intensified efforts, and the availability of better keys for recognizing or separating species.
If we recognize the limitations of resampling, large collections from circumscribed locales can still provide useful data on the historical presence of dipteran species; these data can be compared with modern-day presence/absence data. Although presence/absence data are not quantitative, they can provide the same sort of information derived from Bañkowska’s (1980) studies relating to the “weediness” of species that dominate disturbed habitats. Areas that have been extensively sampled in the past could be targeted as sites for renewed collecting activity. Large-scale data compilations of the insect visitors to the flora of a region, such as that compiled for northwestern Europe by Ellis and Ellis-Adams (1993), could also serve as baseline data. I will discuss two large-scale North American studies of historical value.
One collection that lends itself to resampling is the product of an anthophilous insect survey conducted by Charles Robertson at the turn of the last century (see Marlin and LaBerge 2001). Robertson collected and identified insects visiting flowers within 10 miles of Carlinville, Illinois, over the course of about 30 years (Robertson 1929). He examined 441 flowering plant species and identified 446 species of Diptera visiting flowers. Changes in land use have been documented for the area since the original collection.
A major study, initiated by Andrew Moldenke (1975, 1976; see also Moldenke and Neff 1974a, b), documented insect visitors to plants of 17 different habitats in California as part of a hemispheric attempt to compare equivalency of insect communities. Over eight years, 800,000 pollinators were recorded on 2200 plant species. Over 76,000 fly specimens were collected. Pollination studies of several hundred plants confirmed whether or not insects were true pollinators of these species. Moldenke determined the number of plant species used by each major category of pollinator within each habitat. Broad categories of flies included in the study were Bombyliidae, muscoid flies, and Syrphidae. Specimens that were collected were identified to species whenever possible. Resampling sites from this study for the current presence of species would provide significant information relating to changes in fly species diversity. This study also contains semiquantitative data, because all insects visiting flowers were collected during sampling