Effects of food availability on larval development and inter-instar predation among larvae of Libellula lydia and Libellula luctuosa (Odonata: Anisoptera)

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Abstract
I conducted two types of laboratory experiments with larvae of the dragonflies Libellula lydia and L. luctuosa. In one experiment I varied the number of Daphnia magna fed daily to these larvae to determine the effect of food availability on growth and survivorship. The time spent in each instar decreased dramatically with increased food availability, but the number of molts did not vary and the size at each molt was only slightly affected. Mortality was low in all but the lowest feeding treatment, despite 2- to 5-fold differences in instar duration. These results suggest that the number and size of instars are determinate in these species, and that starvation is an unlikely cause of larval mortality in nature. In a second experiment I used naturally co-occurring size combinations of L. lydia and L. luctuosa to determine how inter-odonate predation varies as a function of larval size difference. For both intra- and inter-specific combinations, (i) little or no predation occurred between larvae similar in size, (ii) some predation always occurred when larvae differed by more than two instars, and (iii) the number of larvae consumed increased dramatically as a function of instar difference. The proportional difference between the labial (gape) width of the larger instar and the head width of the smaller instar was a good estimator of inter-odonate predation rates across all instar and species combinations. Together these results suggest that the effects of inter-odonate competition and predation can be disentangled in the field by manipulating the instar structure of experimental populations.