The left panel above is a photograph of a completely melanized stylostome or feeding tube (s) of a larval water mite (b). The feeding tube is collapsed but leads into the mouth region of the water mite, which is an oblong outline (jointed legs are visible just above b) orientated vertically. That mite is dead. The photograph is taken through the cleared damselfly cuticle. The right panel is a photograph of what a healthy stylostome looks like in this species (a long arm with a blind sac at its end). There is some melanization where the mite was attached to the host (marked by an arrow), but this mite engorged and dropped away from the host. Some of my students have been interested in this resistance expression. We know temperature and species associations are important in its expression. Recently, we have discovered marked year to year variation in numbers of Lestes disjunctus individuals showing resistance to a specialist mite, Arrenurus pollictus. We think that when mite loads get heavy enough, those individuals most likely to survive and leave offspring are the ones that respond to the parasite. We suspect it is all about recognition. A recent theoretical model we are working on will hopefully inform us about what patterns should exist between host condition (which is very difficult to define) and resistance expression.
Professor & student