Cool season communal roosting of Cratilla lineata in a Taiwanese submontane forest (Odonata: Libellulidae)

We investigated communal nocturnal roosting aggregations of Cratilla lineata in the Chia-Ling Lake area in Tainan, Taiwan. Communal roosting began during the colder months from late October to mid-March. The dragonflies roosted 2–4 m above the ground on slender vines such as Bauhinia championii, Dalbergia benthamii, Hiptage benghalensis, and Pothos chinensis, under open canopies along a forest path. The aggregations exhibited a sex ratio predominantly skewed towards males and their numbers were negatively correlated with daily mean temperature, indicating that more dragonflies roost as temperatures decrease. The dragonflies showed fidelity to specific roosts and vines. Roosting began between 15:30 h and 17:00 h CST (UTC+8) when light intensity was high (252–3 607 lux). They departed between 08:50 h and 12:00 h, 2–6 hours after sunrise, with light levels ranging from 1 103 to 9 567 lux. The mechanisms and adaptive value of this behaviour remain open to speculation.