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.
Excellent post! Mite phoresy on insects is a fascinating yet understudied topic. There are untold volumes about co-evolution waiting to be studied in these interactions.
ReplyDeleteMy favorite example is of the burying beetles (Nicrophorus, Silphidae) and their phoretic mites in the genus Poecilochirus. Unfortunately, the taxonomic difficulty of mites is a barrier to work on this relationship. I suspect the same is true for the Hydrachnida.
Thanks, Kai! Yes, certainly. I'm no mite expert, but my understanding is that larval water mites are not identifiable to species, so you can't just look at what's attached to a host to figure out the species involved. That would take some real dedication to rear those things out to adults.
DeleteGreat post. I've come across these sometimes as well, but I usually only notice them when I have the photograph up on the monitor. Perhaps not so pleasant for the bug in question, but finding phoretic critters after the fact is one of the little pleasures of being a bug photographer!
ReplyDeleteThanks, Adrian. I agree--it's always fun to find little "surprises" in your photos later on.
DeleteThanks for this post Jim. I just took some photos of dragonflies and noticed many red "orbs" on a couple of them and was wondering what they were.
ReplyDelete