Function of the dendritic setae in Aedes aegypti mosquito pupae: float hairs don't float

Correspondence: William S Romoser Department of Biomedical Sciences, College of Osteopathic Medicine, Ohio University, Athens, Oh 45701, USA Tel +1 740 592 3417 Fax +1 740 597 2778 email wromoser@gmail.com Purpose: Pupal behavior varies with buoyancy, which changes spontaneously at the water’s surface and during diving. This behavioral regulation is energy conserving, which is a critical need in the nonfeeding but highly motile pupa. Although adult structures are apparent, there are uniquely pupal structures, including setae (most or all mechanoreceptors), ranging from hair-like to the complex, moveable, bilateral “dendritic setae” on the first abdominal tergum. Our aim has been to elucidate the function of the dendritic setae (“float hairs” in earlier studies). Based on the position and shape of dendritic setae plus ultrastructural evidence of mechanoreception, we hypothesized a buoyancy-sensing function related to bending of the setae as water currents flow over them during descent and ascent. Methods: Using Aedes aegypti, we checked to see whether the dendritic setae are hydrophilic and tested the behavioral effects of their removal. In a preliminary closed-system experiment, a pupa was placed in water in a test tube and a syringe attached in continuity with the air space above the water. When pressure was increased by depressing the plunger, a pupa responded by swimming toward the surface. When negative pressure was applied by lifting the plunger, a pupa actively dove. Using a more sophisticated apparatus with a pressure transducer, we tested the effects of dendritic setae removal on behavior in response to pressure changes. Results: The cuticular surface of the dendritic setae is hydrophilic. No significant differences were found in pupae with or without the dendritic setae relative to dive duration, applied pressure duration, or maximum pressure applied, but response time to pressure change in pupae (males and females) without the setae was significantly increased. Conclusion: Hydrophilic dendritic seta cuticle is consistent with our hypothesis but not with a floatation function. Ablation experiments supported our hypothesis that the dendritic setae are involved with buoyancy sensing by bending in response to directional water currents.