Egg production of migrating dragonfly, Pantala flavescens, in relation to the daily food intake (Odonata: Libellulidae) (#835)
The migrating dragonfly, Pantala flavescens, arrives in Japan from tropical regions every spring, and then the population increases toward autumn, but followed by death in the winter cold. They often form mass flights above open grasslands for feeding on small insects such as Diptera, while the oviposition activity is observed at various open waters. Despite that egg production requires the nutrition from the prey insects, little relationship between the daily food intake and the number of eggs laid has been considered due to the difficulty of the observation on foraging behavior. In the early morning, females of reproductive stage were captured from mass flights at the grassland in Tsukuba, Japan. Immediately after capture, the artificial oviposition technique was applied for each female to release all mature eggs loaded. Then, they were reared with water in the lab, and the dry weight of faeces excreted and the number of eggs produced were measured in each 24 hr after capture. Faeces excreted within 24 hr after capture were oval pellet, dark brown including a lot of cuticles of the prey. The total dry weight of faeces was 8.00 mg. Subsequent faeces size decreased, and the color changed to reddish brown without detectable cuticle fragments, suggesting that most indigestible food must be excreted within 24 hr after the feeding. When a female fed on a sheep blowfly by using the hand feeding methods, 4.51 mg faeces was excreted within 24 hr after feeding, while starved females did 2.23 mg. The daily food intake was estimated about 14 mg, corresponding to about 200 small prey insects. Females reared with water 24 hr released about 800 mature eggs, while the daily oviposition number was about 1000 in the fields. Therefore, the daily food intake must be enough amount for the daily egg production.