Effects of a mite, Limnochares americana (Hydrachnida: Limnocharidae), on the life-history traits and grooming behaviour of its damselfly host, Enallagma ebrium (Odonata: Coenagrionidae)

We examined whether experimental parasitism by a mite Limnochares americana (Lundblad) affected survivorship and maturation of adult damselflies Enallagma ebrium (Hagen). We then tested whether differences in grooming activity between control and exposed individuals (within different age or sex categories of host) mirrored reductions in fitness that resulted from experimental parasitism. We based our choice of experimental numbers of mites on our finding that adult damselflies had between 0 and 12 mites (71% had 0 mites), and mature adults had a higher prevalence and intensity of parasitism than did prereproductive damselflies in two of three field surveys. Low numbers of mites did not affect survivorship of teneral or mature males and females; however, high numbers of mites significantly depressed survivorship of teneral males and females and mature males, and were associated with a delay in maturation of females. Of teneral individuals, only females groomed more than controls when challenged with low numbers of mites; mature individuals of both sexes groomed more than controls in response to high numbers of mites but not in response to low numbers. Our results suggest that variation in grooming behaviour partially reflects variation in fitness costs, due to mite parasitism, across age and sex categories of hosts.

Negative covariance between larval Arrenurus sp. and Limnochares americana (Acari: Hydrachnidia) on male Leucorrhinia frigida (Odonata: Libellulidae) and its relationship to the host's age

Numbers of larval Arrenurus sp. and Limnochares americana Lundblad mites, parasitic on adult males in a population of Leucorrhinia frigida Hagen, were found to be negatively correlated. The two mite species did differ in their seasonality, but a strong negative covariance remained even when analyses were run separately on data grouped by day. When dragonflies were separated into tenerals and sexually mature males, almost all newly emerged males were heavily parasitized by Arrenurus sp. and rarely parasitized by L. americana, whereas males holding reproductive territories were almost always heavily parasitized by L. americana but carried reduced loads of Arrenurus sp. When territorial males were subdivided dependent on presence or absence of Arrenurus sp., the dragonflies with Arrenurus were significantly less heavily parasitized than those without Arrenurus sp. Through the above partitioning of data, plus recapture of previously marked dragonflies, it was established that the negative covariance between mite taxa reflects the differences in host discovery: Arrenurus sp. attaches to dragonflies during the host's emergence from the water, whereas L. americana attaches to males while they defend reproductive territories at the water's margin. We conclude that the age-correlated differences in infestation of dragonflies by mites could be a valuable method for determining relative age within odonate populations.

Stable swimming by diagonal phase synchrony in arthropods

The aquatic mite, Limnochares americana Ldbld., in common with several types of insects which are also secondary invaders of fresh water, can swim only by stroking the legs on opposite sides of successive segments in phase (diagonal phase synchrony). A planar model developed to analyse the distribution of forces generated during this type of swimming demonstrates that there is a substantial potential for lateral instability (yaw) inherent in the locomotory pattern. The model also suggests morphological adaptations that could minimize the instability. Substitution of measured variables for L. americana and for the beetle Hydrophilus piceus in the model indicate that, consistent with observations, the mite has been considerably more successful than the beetle in minimizing lateral instability. It is concluded that diagonal phase synchrony represents an ancestral coordination pattern which is less efficient than the more derived method of coordination by transverse phase synchrony.

Swimming by the water mite Limnochares americana Lundblad (Acari, Parasitengona, Limnocharidae)

Locomotory patterns of Limnochares americana were analyzed from visual observations and cinema and still photography. In free swimming it uses only the last two pairs of legs such that the third leg on the left side and the fourth leg on the right side are in phase and the third leg on the right side and fourth leg on the left side are in phase. Legs on opposite sides and successive legs on the same side show a phase difference of 0.5 cycle (180°). Orientation appears to be cued primarily by direction of illumination and water movement. The swimming hairs form collapsible blades that improve the efficiency of each leg in swimming. Manoeuvering and control of roll, pitch, and yaw are accomplished primarily by altering the direction of the power stroke of various legs as the distal segments are rotated at the trochantofemoral joint.