An intensive survey of the hymenopteran fauna of Costa Rica provides a
n opportunity to explore the size and composition of a species-rich in
sect order in a large tropical region. A working estimate of the richn
ess of the fauna is 20,000 species. This is consistent with some publi
shed estimates of the numbers of species of Hymenoptera worldwide, but
at odds with suggestions that all insect species number several tens
of millions. Sixty-one families of Hymenoptera have been collected in
Costa Rica, and the numbers of species per unit area is more than twic
e that of most of the other regions for which data are available. The
Costa Rican fauna has higher proportions of egg parasitoids and of eus
ocial species, and a lower proportion of primary phytophages than that
of the British Isles.