Mite-y Dragons: Odonata and Water Mites

If you look at odonates often enough, closely enough, you’re bound to notice some that are accessorized with tiny orbs—often orangish or reddish in color, clinging to various parts of the body like jewelry. But it isn’t “bling” or the latest fad in body art. When I first started catching odonates and looking at them in-hand, I thought they might be eggs which got stuck to the body, but that isn’t right either. What are these sanguine spheroids, these rusty rondures, these blushing bulbous globules?

Male Tule Bluet (Enallagma carunculatum) with a string of mites under the abdomen.

 

These are ectoparasites—meaning that the host–parasite interaction occurs on the outer surface of the host rather than internally. More specifically, these are larval water mites or Hydrachnida, which are in the same class of arthropods as spiders, scorpions, ticks, and their colleagues. Water mites that use odonates as hosts are predominantly species of Arrenurus in the family Arrenuridae—of which at least 55 species have been described as ectoparasites of Odonata so far.

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
middle leg.

Water mites that are attached to an adult odonate actually initiated their relationship underwater during the odonate’s nymph stage. The free-swimming larval mites find final instar odonate nymphs and cling to them, but they are not parasitic at this point—they just try to find a place on the body where they can hang on and avoid being knocked off when the nymph grooms itself (under the wing pads, for example). It would be interesting to know how the mites determine which odonates are nearing emergence and which are not. When the odonate emerges from the water to become an adult, the mites crawl from the exuvia (the shed nymphal skin) onto the just emerged adult (which is probably still going through the process of expanding its abdomen and wings), and attaches somewhere on the body while the exoskeleton is still soft and easy to pierce. The mites most often attach to the ventral areas of the thorax or abdomen. Rarely you see them higher on a dorsal surface or on the head, and they sometimes even attach to the major wing veins although I’ve never seen this myself. Apparently attachment site preference varies by mite species.

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.

 

Eventually the odonate host returns to wetland to reproduce (this may be the place where it emerged or it may be someplace else). When this occurs, their hydrachnid passengers—fully engorged and significantly bigger than when they first found their host, have their opportunity to unplug and take a triple gainer into the water where they can continue their life cycle. So the parasitic use of adult odonates by water mites is temporary.

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.

 

It’s interesting to consider which odonates are frequent water mite hosts and which are not. Water mites occur primarily in non-flowing (lentic) or slow-flowing waters, so naturally, odonates which prefer those habitats are good candidates. In general damselflies (Zygoptera) seem to host water mites more often than dragonflies (Anisoptera); however certain dragonflies in the skimmer family (Libellulidae) are parasitized regularly and these include the pondhawks (Erythemis), whitefaces (Leucorrhinia), king skimmers (Libellula), and meadowhawks (Sympetrum). On the other hand, some other groups of dragonflies—for example darners (Aeshnidae), occur in lentic habitats too, but they rarely host water mites. Are the mites discriminating or are these dragonflies somehow mite-resistant? I don’t know. Maybe the nymphs of these dragonflies are more skilled at evicting stowaways before emergence.

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.

 

So keep an eye out for odonates carrying these little hitchhikers and think about how far they may have traveled and how far they still have to go before they dive into a pond. At least they get a meal with their flight which is more than I can say for my flying experience lately!

 

Further Reading

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.