Table 2.

Examples of parasites that are “lineage-specific” in particular host groups during at least part of their life cycles.

Parasites	Species (n)	Hosts
Digenetic trematodes (as larvae)	18,000	Mollusks (usually gastropods)
Unionid bivalves	1,000	Fish
Rhizocephalans	260	Decapod crustaceans
Poecilostome copepods	400	Cnidarians
Sisyridid sponge flies	50	FW sponges and some bryozoans
Hydracarina water mites	5,000	Aquatic insects
Tantulocaridans	30	Crustaceans
Acroceridae	>500	Spiders
Pipunculids	1,388	Leafhoppers and planthoppers
Tetracneminae chalcidoid wasps	815	Pseudococcid insects
Banchinid ichneumonid wasps	1,500	Lepidopteran insects
Ichneumoninae ichneumonid wasps	350 genera	Lepidopteran insects
Aphidiinae braconid wasps	400	Aphids
Conopidae	800	Mostly wasps and bees

This list is not exhaustive and merely serves to illustrate the concept that particular host lineages acquire unique parasites that are likely to have distinctive methods of infectivity that could influence how their host’s immune systems are shaped by selection. Although cases where members of the parasite groups identified colonize hosts outside the indicated host lineage certainly occur, they do not negate the idea that the host groups indicated above have been far more affected in aggregate than a host lineage containing an isolated member harboring a peculiar outlying parasite. Also, for some huge groups, such as the ichneumonid wasps, although when viewed more inclusively they infect much broader groups of hosts (such as insects or terrestrial arthropods), the point remains they have had relatively little impact on other major host lineages beyond the insects.