Libellula

Abstract: Dalpadado, P., Borkner, N., Bogstad, B., and Mehl, S. 2001. Distribution of Themisto (Amphipoda) spp. in the Barents Sea and predator-prey interactions. – ICES Journal of Marine Science, 58: 876–895. Themisto abyssorum and Themisto libellula were the dominant amphipod species observed in the central and the northern Barents Sea during 1984–1996. T. abyssorum was predominant in the subarctic waters, T. libellula in the Arctic waters. A third species, Themisto compressa, was rare and was restricted to the Atlantic waters. Our study showed peak abundances of T. abyssorum and T. libellula in summer and in early autumn. High abundances were usually associated with Polar Front waters. T. libellula has a more near surface distribution than T. abyssorum. Followed by a decrease in the capelin stock from 1985–1987 there was an increase in the abundance of Themisto abyssorum and T. libellula, probably due to the reduced grazing pressure from capelin (Mallotus villosus). In the mid-1980s and 1990s when the capelin stock was at extremely low levels, cod (Gadus morhua) switched from capelin to alternative prey such as amphipods and krill. Detailed analysis of amphipods in the cod stomachs from 1984–1999, showed that cod fed mainly on Themisto spp., especially on T. libellula. Themisto species were consumed by most age groups of cod. With an increase in the capelin stock from 1987–1991, a corresponding decrease in the abundance of Themisto abyssorum and T. libellula was observed. During 1993– 1996 when the capelin stock again was at low levels, the abundance of these two amphipod species increased. The increase in abundance was less pronounced in the mid-1990s for T. libellula probably owing to higher grazing pressure from cod and other predators such as harp seal (Phoca groenlandica) and arctic sea birds. The stock size of cod in the mid-1990s was about twice the amount as in the mid-1980s. These results illustrate strong predator-prey interactions between macrozooplankton species as amphipods and capelin and cod in the Barents Sea. The amphipod populations in the Barents Sea appear to be to a large extent controlled by predation.

Abstract: To determine whether perching dragonflies visually assess the distance to potential prey items, we presented artificial prey, glass beads suspended from fine wires, to perching dragonflies in the field. We videotaped the responses of freely foraging dragonflies (Libellula luctuosa and Sympetrum vicinum—Odonata, suborder Anisoptera) to beads ranging from 0.5 mm to 8 mm in diameter, recording whether or not the dragonflies took off after the beads, and if so, at what distance. Our results indicated that dragonflies were highly selective for bead size. Furthermore, the smaller Sympetrum preferred beads of smaller size and the larger Libellula preferred larger beads. Each species rejected beads as large or larger than their heads, even when the beads subtended the same visual angles as the smaller, attractive beads. Since bead size cannot be determined without reference to distance, we conclude that dragonflies are able to estimate the distance to potential prey items. The range over which they estimate distance is about 1 m for the larger Libellula and 70 cm for the smaller Sympetrum. The mechanism of distance estimation is unknown, but it probably includes both stereopsis and the motion parallax produced by head movements.

Abstract: Comparative data on emergence, adult activity, and larval development were used to interpret patterns of phenology, voltinism, and developmental synchrony in a 14-species assemblage of dragonflies (Odonata:Anisoptera). Interspecific differences in these characteristics provided an opportunity to interrelate life history, population size structure, and the potential for intra- and interspecific interactions. Larval populations of most species comprised an extraordinary size range of individuals at any one time. In many species, this intraspecific size variability was primarily due to developmental asynchrony within generations (e.g., Libellula lydia, Celithemis elisa, Perithemis tenera). Oviposition in these species occurred throughout most of the summer so that the timing of instar development varied considerably among broods. By fall, the largest larvae in these populations were 5 to 10 times the size of smaller conspecifics. In other asynchronous breeders, an even wider size range of co-occurring conspe...