MANY ectoparasitic insects are characterized by one or more regular rows of closely set, large stout spines. These rows of spines are termed combs, or ctenidia, and have evolved independently in fleas (Siphonaptera), bat flies (Nycteribiidae), bat bugs (Polyctenidae), and in the beaver beetle Platypsyllus. Hitherto there has been little evidence about their function. It has recently been shown1 that the spacing of spines in the combs of fleas bears a close relationship to the diameter of the hairs of the appropriate host. The gap between the tips of the spines is usually about 1.75 times greater than the average diameter of the host's hairs, but the gap narrows almost to zero towards their base. The same relationship occurs between the combs of Nycteribiids and the fur of their Chiropteran hosts. The general applicability of this principle among ectoparasites was confirmed by measuring the comb of Platypsyllus castoris and the fur of the beaver. The results are summarized in Fig. 1.