The Sympatric Origin of Phytophagous Insects

Abstract

Global estimates of the number of insect species now range from 10 to 30 million and the tally keeps growing. This means that roughly 75–95% of all living eukaryotic organisms are insects. No matter which figure you care to choose, the numbers are impressively large. What is it about insects that accounts for this inordinately large number of species? An assessment of their biological attributes provides at least three important clues. The most important concerns their relatively high degree of resource specialization. Approximately 70% of British insects, which are probably representative of the world’s insect fauna, are parasitoids or parasites on animals and plants (Price 1980). Of these about half feed on plants, with the majority infesting one or a few closely related hosts (Strong et al. 1984). A second important clue is that when sister species of these host specialists are found coexisting sympatrically or parapatrically they are almost always feeding on different host plant species. Finally, a third important characteristic shared by many of these host specialists is that they use their host plant or their host plant’s habitat as a rendezvous site for locating a mate (Bush 1975b; Zwölfer 1975).