Chemosensory proteins (CSPs) are soluble proteins found only in arthropods. Some of them fill the lumen of chemosensilla and are believed to play a role similar to that of odorant-binding proteins in the detection of semiochemicals. Other members of the CSP family have been reported to perform different functions, from delivery of pheromones to development. This report is focused on a member (CSP4) of the family that is highly and almost exclusively present in the proboscis of two sibling noctuid species, Helicoverpa armigera and H. assulta. We expressed the protein in bacteria and measured binding to terpenoids and related compounds. Using specific antibodies, we found that when the moths suck on a sugar solution, CSP4 is partly extruded from the proboscis. A solution of protein can also fill a hydrophobic tube of same length and diameter as the proboscis by capillary action. On this basis, we suggest that CSP4 acts as a wetting agent to reduce the surface tension of aqueous solutions and consequently the pressure involved in sucking.