This
week we break the EPOW mold for the first time,
presenting photos and an essay by a special guest ... wildlife biologist and
master photographer, Mr. Tom Kogut.
(See more from Tom
Kogut here.)
Explanation: What are the strange spheres on the thorax of the
dragonfly above?
At first sight they appear to be
eggs or evidence of some unsightly disease. Instead, they are Hydrachnida
water mites that are using the dragonfly for transportation in an
ecological process called phoresy (using another organism to move
about).
The water mites attach to the
dragonfly larva during the dragonfly's aquatic stage in ponds, lakes, and
other slow-moving waters. The tiny mites then transfer to the adult
dragonfly or damselfly when it exits the water and emerges
from its larval casing.
Like miniature hobos "riding
the rails," the mites travel with the dragonfly, eventually detaching
from it as it becomes sexually mature and returns to water to breed. At
this point, the water mites drop off and begin their life cycle all over
again, perhaps in a new location some distance from their native waters.
_Tom_Kogut_s.jpg)
This Northern Spreadwing (Lestes disjunctus)
-- a damselfly, not a
dragonfly -- also is host to several water mites on its thorax.
Some species of water mites prefer attaching to
the abdomen, and
some prefer the thorax, of various hosts, as favored
attachment sites.
Although the mites use their host for transportation, they also obtain
moisture and nutrients from their host, and thus the mites grow considerably
larger over time. However, even dragonflies and damselflies with large
mite loads can forage and reproduce successfully, so the parasitism* of
the mites does not appear to adversely hamper the health and breeding of their
dragonfly hosts in most but not all cases.
Although both damselflies
(suborder Zygoptera)
of many families and species are common hosts for water mites, the occurrence
of mites on dragonflies (suborder Anisoptera)
appears to be much more limited. Some dragonfly families rarely, if
ever, serve as hosts for water mite hitchhikers (Dennis Paulson, personal
communication).
However, dragonflies of family Libellulidae
(skimmers) are particularly vulnerable to water mite attachment, as our
Striped Meadowhawk specimen above can attest!
* A quick word
about words:
Formally, "parasitism" refers to the
condition whereby one organism attaches externally or internally to another
organism and benefits in some way such as by ingesting nutrients from its
host ... and the host suffers with lesser health, or a lower
reproductive or survival rate.
In some references, "phoresis" is not
strictly the same as parasitism, in that the former is viewed as a type of
"commensalism" whereby one species (the mite) benefits (by being
transported) and the host (the dragonfly or damselfly) suffers no
ills.
It might be the case that with some water mite
species, or at some lower load levels of attached mites, would not adversely
affect their host, so in this case "phoresis" is also
"commensalism" ... but with other mite species or at higher load
levels, the host does suffer, so in this case "phoresis" is also
"parasitism."
The lesson here is that, the type of relationship
-- commensalism or parasitism -- that a mite forms with its host is
dependent upon the effect on that host ... and also that phoresis
is the general overall context and condition.
- note by Bruce G. Marcot
Reference:
Corbet, Philip S. 1999. Dragonflies: Behavior and Ecology
of Odonata. Ithaca: Cornell University Press. 829 pp.
Dragonfly Sites of Interest:
OdonataCentral (website of
the Dragonfly Society of the Americas).
University
of Puget Sound, Slater Museum dragonfly site.
Acknowledgment:
Thanks to Dr. Dennis Paulson, Director Emeritus, Slater
Museum, University of Puget Sound, for his review of the column and for
generously sharing his vast knowledge of dragonfly biology.
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