SIR — Your Research Highlight 'Identity crisis' (Nature 446, 834; 2007) notes that DNA barcoding data revealed that most commercially available medicinal leeches marketed as Hirudo medicinalis are actually H. verbana and that these annelids are distinct species. During past decades, they have been regarded as 'colour variants' of H. medicinalis, a widely distributed European species.

However, linnaean taxonomists distinguished between the two species in the eighteenth century. On Plate 10 of Georges Cuvier's book Iconographie Du Règne Animal (Baillière, Paris, 1829), shown here, the medicinal leech H. verbana Carena, 1820 is depicted in dorsal and ventral views, and is juxtaposed to its sister taxon, H. medicinalis Linnaeus, 1758.

These drawings show the characteristic, species-specific pigment patterns of both 'leech varieties' in remarkable detail. Cuvier's original figures are of the same quality as recently published photographs of these blood-sucking annelids, which do not interbreed in captivity.

This case study illustrates the significance of the work of traditional systematists, as highlighted in your Editorial 'The legacy of Linnaeus' (Nature 446 231–232; 2007). Linnaean taxonomy is the science of classification of organisms based on diverse character sets. DNA barcoding can supplement this, but cannot replace it.