Parasite on Common Darter at Priory Water

• By Steve Houghton

The dark winter nights are perfect for sorting through the vast amount of photographs that have accumulated on the computer's hard drive during the dragonfly season. It always amazes me just how many I take. Whilst sorting through the 3751 images of odonata taken in 2003 I came across this image of an immature Common Darter taken at Priory Water in early August.

I must have over-looked it at the time, but the darter seemed to have a strange looking red/brown appendage on the underside of its upper abdomen. The photo is a 4 million pixel high resolution image so I was able to enlarge the area concerned to get a better look at the appendage. It looked very much like a tick, a parasite that I am very familiar with having been host to several myself this year, one of the bonuses of creeping around in reeds and long grass whilst taking photos!

I must admit that the thought of dragonflies having parasites had never really crossed my mind so I set about trying to find out more on the subject. None of the books in my collection contained any mention of parasites on odonata so the next stop was the Internet. After several refined searches this proved successful. There are several species of parasite that use odonata as their host, the most prolific of which is the Water Mite.

The water mite’s life cycle has four stages: egg, larva, nymph and adult. It is only in the larval stage that it is parasitic; in the two final stages it is predacious. The mite larvae start to look for a host as soon as they hatch from the egg. They target a fully-grown odonate larva and attach themselves to it beneath the wing rudiment. At this stage they are not true parasites as they are just hitching a lift with the larva until it emerges, a process known as forensic parasitism.

Once emergence has taken place the mite larva detaches itself from the exuvia and moves onto the newly emerged dragonfly or damselfly and attaches itself , the preferred sites for attachment being the thorax, abdomen or the wing veins. They have the appearance of small red/brown balls. Once attached to the host the mite larva will feed readily and grow many times its original size. Once it has achieved its maximum size the larva very cleverly waits for the host to fly over the water before detaching itself and falling into the water where it metamorphosizes into the next stage of its life cycle, an aquatic, predacious nymph. The mite nymph then hunts and feeds underwater before eventually entering the final adult egg-laying stage of its life.

There are several species of water mite, each species having its own preferred attachment site on the host. The species Arrenurus papillator prefers to attach itself to the wings of dragonflies but will only attach to the thorax or abdomen of a damselfly, presumably the blood supply within a damselfly's wing is not sufficient to provide the mite larva with enough food to sustain its growth rate. Some dragonflies have been found hosting huge amounts of larva. Large numbers of parasites can cause the host to weaken and in extreme cases can even kill it.

I have no idea just how common this parasite is. Having reviewed all of my photos again including the 2002 season, I can find no other with any sign of infestation. So finally, here’s a challenge to all you dragonfly photographers out there: have another look through all those photos and see if you too have managed to photograph a water mite without knowing it!