A note on the behaviour of larval Cordulegaster boltoni (Don.) in captivity (Anisoptera: Cordulegasteridae)

Larvae of C. boltoni inhabit well-oxygenated current water. They are usually regarded as mud-dwellers par excellence (P. ROBERT, 1958. Die LiheUen (Odonalen). Kümmerley & Frey, Bern), burrowing in layers of humus overlying sand (P.S. CORBET, 1962, A biology of dragonflies, Whiterby, London). This species is quite commonly found in shaded rivulets in the hilly area between Wassenberg and Roermond on both sides of the Dutch-German border, immediately east of the river Maas. The larvae lie buried in the fine white sand, covering the bottom of these mostly swift streams. In order to study the emergence, I transferred in June, 1977 one ultimate instar and three younger larvae into a large tank, the bottom of which was covered with a thick layer of sand, taken from the original habitat. When put into the water, the mature larva did not sink down, but drifted motionless at the water surface. I embedded the larva into the sand outside the water, so that only head and thorax remained uncovered. In this position it remained motionless for five days. One day, touched by direct sunlight, it very slowly, with almost unperceivable movements, retracted a little deeper into the sand. Though a certain inertness may be regarded as characteristic of burrowing larvae (like those of Cordulegasler and gomphids), the inactivity of this specimen, which did not move at all since its capture, was almost certainly due to metamorphosis and near ecdysis. It has been reported (J.G. NEEDHAM & C. BETTEN, 1901, Bull. N. Y. Si. Mus. 47: 383-612) that a Cordulegasler larva remained buried in the same position for weeks.