I can't answer all your questions definitively. However, butterflies as adults feed almost exclusively on low mineral materials, and thus benefit greatly by supplementing their mineral intake from damp soils and feces. Dragonflies are carnivores, and would intake more minerals from their diets. But there may be specific materials they seek from the sediments.
So far as identification, dragonfly i.d., at least in North America where I am, is not difficult. Many amateurs and professionals do it routinely without specimens in hand. However, for definitive i.d. learning the wing venation and using technical sources with specimens in hand and magnification is best. The wing venation is not difficult to master, and once you know the species that frequent your area, markers such as body shape and pigmentation are quite enough. A pair of good binoculars helps. Numerous manuals illustrate the wing venation and other features of these fascinating creatures. David McNeely ---- Edwin Cruz-Rivera <edwin.riv...@auw.edu.bd> wrote: > Dear colleagues, > This one is for the entomologists in the crowd. I have been doing some > surveys of intertidal crabs along the Chittagong coast (Bangladesh) and > have been noticing truly impressive abundances of two dragonfly species > (either that or one with strong sexual dimorphism) along the beach. They > seem to regularly land and spend some time on sediments that have been > disturbed by the crabs I am studying. I am familiar with the mud puddling > behavior of butterflies and was curious if dragonflies do the same. Could > they be getting minerals from these sediments? Are there dragonflies > associated preferentially with brackish habitats? Is species > identification of these insects manageable for a non expert or is this a > rather difficult insect group to ID? > Comments and ideas are most appreciated! > Cheers, > > Edwin > ================= > Dr. Edwin Cruz-Rivera > Associate Professor of Biological Sciences > Asian University for Women > 20/A M.M. Ali Road > Chittagong 4000 > Bangladesh > Tel: +880-31-2854980 > Fax: +880-31-2854988 > > "It is not the same to hear the devil as to see him coming your way" > (Puerto Rican proverb) -- David McNeely
