Siphonaptera (fleas) is a highly-specialized holometabolous insect order with 2,380 described species placed in 15 families and 238 genera (Lewis and Lewis, 1985). Fleas are laterally compressed, wingless insects; the head is shield or helmet shaped, compound eyes are absent, and mouthparts are specialized for piercing and sucking (Dunnet and Mardon, 1991). Fleas are of tremendous medical and economic importance as vectors of several diseases important to human health including bubonic plague, murine typhus, and tularemia (Traub and Starcke, 1980).

From a phylogenetic standpoint, fleas are perhaps the most neglected of all insect orders. While we have a reasonable knowledge about the taxonomy at the species and subspecific level, phylogenetic relationships among fleas at any level have remained virtually unexplored. The major obstacle in flea phylogenetics has been their extreme morphological specializations associated with ectoparasitism, and the inability to adequately homologize morphological characters across taxa. The majority of characters used for species diagnoses are based on the shape and structure of their extraordinarily complex genitalia, and the presence and distribution of setae, spines and ctenidia (Traub and Starcke, 1980; Dunnet and Mardon, 1991).

Fleas are entirely ectoparasitic, and while it has been generally assumed that fleas originally exploited mammals as hosts with a subsequent shift to birds (Holland, 1964), this has never been formally investigated. There are 137 flea species representing 22 genera and 6 families that have birds as hosts, 5 species that are found on both bird and mammal hosts, and the remainder of the species are mammal specific. It is not clear how many times fleas have host shifted, the polarity of the shift (bird to mammal or vice versa), or whether any of these shifts correlate with morphological radiation.

The majority of flea specialists are retired and there exists a dire need to train new specialists in flea systematics. Moreover, many aspects of flea evolution are virtually unexplored and fleas may serve as interesting research organisms for understanding host shifts, coevolution, gross morphological specialization, and molecular evolution.

This page is regulary updated by Katharina Dittmar de la Cruz. For any questions or suggestions please write to katharinad@hotmail.com. This material is based upon work supported by National Science Foundation under grant 9983195. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.