|Male Tule Bluet (Enallagma carunculatum) with a string of mites under the abdomen.|
|Top: Male Alkali Bluet (Enallagma|
clausum) with a couple of mites tucked
under the thorax between the middle
and hind legs. Do you see them?
Bottom: Male Pacific Forktail (Ischnura
cervula) with a single mite on his
"shoulder" above the base of the
Finding a meal isn’t the only objective for the mites. They want to travel too. The technical term for this is phoresy—the use of one animal by another for transportation. Lots of mites are phoretic, at least during some stage of their life cycle, and they use a variety of arthropods and other animals for transport. Why do mites want to travel? There are a number of potential benefits of dispersal among which are finding food (or hosts), finding habitat, expanding the species’ range, and increasing genetic variation by reproducing with individuals from other populations.
|Male Eight-spotted Skimmer (Libellula forensis) with a number of mites on the thorax and one on his head below the eye.|
Although odonates carrying water mites typically appear to be healthy and energetic, studies indicate that their longevity, flying endurance, and reproductive success can be negatively impacted by the stowaways. This seems to be especially true when lots of mites cluster together and cause significant damage to the cuticle of the exoskeleton, perhaps leading to desiccation. I assume that clusters of mites at particular locations on the odonate body—until they drop off anyway—can also interfere with reproduction by impeding copulation or by blocking sperm transference to the male’s secondary genitalia. It seems that just a few mites attached to unobtrusive areas of the body have negligible impact, and it’s more of a commensal relationship in that case.
|Left: Female Lyre-tipped Spreadwing (Lestes unguiculatus) with several mites on the thorax and another about halfway down the abdomen. Right: Male Spotted Spreadwing (L. congener) with a cluster of mites near the tip of the abdomen and another single about halfway up. I can't see how the male can transfer sperm to his secondary genitalia with all those mites there.|
|Left: Male Striped Meadowhawk (Sympetrum pallipes) with a number of mites on the thorax. Right: Female Band-winged Meadowhawk (S. semicinctum) very heavily infested with mites. This is one of the worst cases that I've ever seen and it can't be good for the host.|
Some of the information I present here was gleaned from Philip Corbet’s monumental Dragonflies: Behavior and Ecology of Odonata published in 1999. I recommend that you start here if you’d like to read more about this topic (or pretty much anything having to do with odonates). Every time I crack open this tome I learn something new.
I also found an online PDF of a 2006 paper by Andrzej Zawal on the use of odonates by Arrenurus larvae at a lake in Poland, and this indicates that odonate nymphs are sometimes parasitized by mites too. You can find that PDF here. Perhaps some species of mites are parasitic on odonate nymphs and others are strictly phoretic until the odonate emerges.