The life cycle of Tanytarsus sylvaticus (van der Wulp, 1859) has been analysed in two quite different sites in Belgium(Mirwart, Ardennes and Diksmuide, West-Flanders). T. sylvaticus is strictly univoltine ; its single emergence period occurs in early spring. A summer diapause in the 3rd instar is typical of this species. The synchronization of the emergenceperiod, however, seems to depend, on a second diapause, this time in the 4th instar, during winter.