The Evolutionary Modification of Dispersal in Naiadacarus arboricola Fashing, a Mite Restricted to Water-filled Treeholes (Acarina: Acaridae)

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Abstract
The facultative hypopial instar of acarid mites is a dispersal stage and in most species forms only in response to adverse or declining environmental conditions, thus allowing the species to disperse to more favorable areas. However, field observations and laboratory studies indicate that N. arboricola, an acarid mite restricted to water-filled treeholes, does not form hypopi in response to adverse conditions. It forms hypopi during May and June when conditions in the treehole are optimal, thereby maximizing the colonizing success of the dispersing hypopi. This adaptation is correlated with the fact that the treehole habitat is stable, not transient as are the habitats of most acarids. The hypopial instar of N. arboricola has evolved to serve purely as a dispersal agent for colonizing new treeholes and for outcrossing among populations (treeholes), not to escape adverse conditions. Hypopi are phoretic only on adult syrphid flies of the genus Mallota whose larvae are rat-tailed maggots that also live in water-filled treeholes. Mallota adults, lide N. arboricola hypopi, are present only during May and June. Since male flies normally do not reenter treeholes once they have eclosed from the pupa, hypopi have evolved to use female flies as dispersal agents. Female flies return to treeholes to oviposit. Laboratory rearings indicate that hypopus formation not only lengthens the normal life cycle by the addition of an instar but also affects the duration of instars preceding and following the hypopus.

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