Male Glow-Worms Combine Flying With Walking and Impede One Another in Their Scramble to Reach Females

For male glow-worms to achieve a mating, they must detect the bioluminescent glow emitted by females at night and subsequently reach them. Several aspects of male behaviour suggest they engage in scramble competition, with many males striving simultaneously to reach a female first and acquire a mating opportunity. Although male glow-worms fly during the initial stages of their search, little is known about the final stages of their approach, despite this potentially involving the most intense competition. In a laboratory arena, males combine walking with flying short distances to reach a dummy female (DF; green LED), though walking predominates. To determine if this was representative of behaviour in the field, we used infrared videography coupled with DF traps. Most males landed near a DF, thereafter walking or making short bounding flights to reach it. The time taken by males to reach the DF after their initial landing increased with increasing numbers of males in the vicinity. Combining a DF attached to a wire stand with infrared videography showed that during the final approach, males engage in frequent, typically brief interactions. Greater numbers of males in the vicinity reduced the rate at which they reached the DF. After reaching the DF, males frequently fell in clusters, ending their mating opportunity. Our results show that the final stage of the males' approach to a DF (and we infer to females) is dominated by walking and influenced by interactions with other males, consistent with features of scramble competition found in other polygynous insects, including fireflies. Our findings offer novel insights into the life history of male glow-worms and, more generally, features of scramble competition in insects.