When comparing host sister clades we tend to find taxonomically richer parasite fauna on the taxonomically richer group of hosts.<unk>  is one of several coevolutionary rules which states that parasites tend to be highly specific to their hosts, and thus it seems reasonable to expect a positive co-variation between the taxonomic richness of hosts and that of their parasites.
Eichler's rule
Köllner's rule
Clark's rule
Javal's rule
Goldwater rule
Arndt–Schulz rule
Reed's rules
Westgard rules