Harrison's rule

Launcelot Harrison, an Australian authority in zoology and parasitology, published a study in 1915 concluding that host and parasite body sizes tend to covary positively,[1] a covariation later dubbed as 'Harrison's rule'.

Harrison himself originally proposed it to interpret the variability of congeneric louse species.

However, subsequent authors verified it for a wide variety of parasitic organisms including nematodes,[2][3][4][5] rhizocephalan barnacles,[6] fleas, lice, ticks, parasitic flies and mites, as well as herbivorous insects associated with specific host plants.

Recently, Harnos et al. applied phylogenetically controlled statistical methods to test Harrison's rule and Poulin's s Increasing Variance Hypothesis in avian lice.

The allometry between host and parasite body sizes constitutes an evident aspect of host–parasite coevolution.

Launcelot Harrison 1880-1928
Schematic diagram of Harrison's rule: small host species harbor small, large host species harbor large parasite species
Schematic diagram of Harrison's rule with Poulin's supplement: small host species harbor small, large host species harbor both small and large (on average: larger) parasite species