Observations on the Occurrence of Knotting Behavior in the Horsehair Worm, Paragordius varius

abstract: Horsehair worms, such as Paragordius varius, exhibit knotting behavior that may serve various functions such as water regulation, mate facilitation, and predator avoidance. Previous studies have found knots consisting only of males, suggesting that this behavior may be mediated by an aggregation pheromone. We conducted a quantitative field study to determine the relative number and size of adult P. varius that occur singly and in knots in a creek known to have a large number of adults in September of 2018. We collected 96 horsehair worms; all were male and associated with vegetation. We found significantly more adult male P. varius occurring singly than in knots and found no significant difference in their length or dry weight. Behavioral assays in the lab can help determine the possible presence and function of hairworm aggregation pheromone.