Vertical stratification during emergence in odonates

Emergence is a crucial event in the life of larval dragonflies. This is the only moment during their development in which larvae are immobile, and therefore are susceptible of damage or predation. Careful site selection should be expected. In his comprehensive study of the life history of Anax imperator, PS. CORBET (1957, J. Anim. Ecol. 26: 1-69) showed that larvae prefer certain parts of the shore for emergence. This produces great concentrations of emerging individuals in some spots, to thp extent that some emerging larvae can be damaged by other emerging individuals climbing on them (see pl. 2B in CORBET, 1957, loc. cit.). S.D. GRIBBIN & D.J. THOMPSON (1990, Freshw. Biol. 24: 295-302) and S. BENNETT & P.J. MILL (1993, Odonatologica 22: 133-145) studied the emergence of Pyrrhosoma nymphula and found that exuviae could be very close, but never so close as to produce competition for emergence sites or damage. To my knowledge, Corbet’s data are the only to indicate intraspecific competition for emergence sites in Odonata. During field work on a man-made pond at Salcedo (Pontevedra, NW Spain) during 1985 and 1986, I noticed that the exuviae of several species seemed to be located at a characteristic height on their substrate.