Aggregation of ovipositing tandems in Zygoptera: are tandem pairs able to distinguish whether they are approached by single males or tandems?

In several zygopteran species apparently there is no inter-tandem competition foroviposition sites, so tandems may aggregate and oviposit in shared sites (e.g. E. SCHMIDT, 1926, Konowia 5: 134- 144; W.J. CRUMPTON, 1975, Odonalologica 4: 149-168; – E.LOIBL, 1958, Z. Tierpsychol. 15: 54-81; – W. DREYER, 1978, Odonatologica 7: 309-321; – C. UTZERI. E, PALCHETTI & G. CARCHINl, 1983, Odonatohgica 12; 259-278; – A. MARTENS, 1989, Zool. Anz. 228: 124- 128; – G. REHFELDT, 1989, Abstr. Pap. 10th Int. Symp. Odonalol., p. 29; — G. RÜPPELL, R. RUDOLPH & H. HADRYS, 1987, Pubi. wiss. Filmen (Biol.) 19(20): 1-9; – A. MARTENS & G. REHFELDT, 1989, Anim. Behav. 38: 369- 374). Since in the Zygoplcra tandem pairs display their wings to closely approaching individuals, thus inducing them to retreat (C. UTZERI, 1988, Odonatologica 17:45-54) and, if persistently dis- turbed, may shift to another site, oviposition in communal sites suggests that tandem pairs must better tolerate to be approached by other tandems than by unpaired individuals. To test this, in 39 Lestes virens tandems in the field (Castel Porziano, Roma, Italy, August and September, 1989) we have recorded frequencies of the wing display, which were elicited by approaching (from 20 cm up to physical contact) unpaired or in-tandem conspecific indiviuals. In the observation site, this damselfly (1) is very abundant; (2) often oviposits in tandem in communal sites; (3) shows a male-skewed sex ratio at the reproductive sites, so there are usually many unpaired males besides several tandems on each day; and (4) single males frequently attempt to seize either the females or the males of tandem pairs. Reactions of the approached/disturbed tandems were mainly wing fluttering by one or both partners (C. UTZERI, E, FALCHETTI & R. RAFFI, 1987. Fragm. em. 20: 1-22), which was sometimes joint to a short interruption of oviposition or followed by shifting of the pairs to other sites.