Wounding as an index of aggressive interactions in larval Zygoptera (Odonata)

We investigated the use of the frequency of wounds to legs and lamellae of larval Zygoptera as an index of aggressive interactions. Criteria for recognizing recent wounds were based on laboratory studies of Ischnura verticalis and Enallagma ebrium larvae with autotomized legs and lamellae; legs and lamellae showing less than 75 and 66% regeneration, respectively, were likely wounded in the preceding instar. Laboratory studies indicated that the frequency of wounds was strongly related to the number of aggressive interactions and that, when smaller larvae were paired with larger larvae, the smaller larvae were more likely to be wounded. Under field conditions, the frequency of wounds of larval I. verticalis and E. ebrium was not correlated with population density but was correlated with dispersal rates. Also, wounded larvae were significantly smaller than nonwounded larvae. The seasonal pattern of wound frequency also suggested that wounds were the result of aggressive interactions. Wounds were more frequent when scarcity of vegetation should have increased the number of encounters between larvae. Frequency of wounding may be a more accurate method of indexing aggressive interactions than density since density estimates are confounded by the quality and quantity of available substrate. The index should prove useful in analysing the effects of behavioural interactions on larval odonate populations.