For human beings, sight, hearing, and touch are the primary means of perceiving the world. However, many animals rely heavily on the sense of smell to detect food sources, predators, nesting sites, etc. Animals can smell a scent chemical emanating from a source only when that chemical comes in contact with receptors associated with its nervous system (hence the term chemoreception). In dampwood termites, there is a water-based fluid layer between the surface of antennae sensors and the olfactory nerve beneath. Chemical signals, however, are almost invariably insoluble in water and therefore unable to cross the thin aqueous layer unaided. That help comes from odor-binding proteins that encapsulate the scent chemicals in a water-soluble coating giving them a free pass through the layer. Negatively charged receptors on the surface of the olfactory nerves cause a structural change in the surface coating which causes the encapsulated scent chemical to be ejected onto the nerve receptor.
Artist: Emily Harrington. Copyright: All rights reserved. See gallery for details.
“Insects perceive the world through small molecules that carry essential information about nestmates, food sources, intruders, and many other environmental cues. These hydrophobic semiochemicals are solubilized by odorant-binding proteins (OBPs) and transported through an aqueous environment (sensillar lymph) to the olfactory receptors, where the signal transduction starts…[OBPs] all have a hallmark of six conserved cysteine residues forming three disulfide bridges, which are essential for the rigidity of their three-dimensional structures” (Ishida 2002:1888)
“[O]dorant receptors are surrounded by an aqueous environment – the sensillar lymph. Although thin (1µm), this aqueous layer is impenetrable for hydrophobic compounds per se. Thus, the transport through this barrier is assisted by odorant-binding proteins (OBPs).” (Leal 2005:7)
“Pheromones (and other semiochemicals) enter the sensillar lymph through pore tubules in the cuticle (sensillar wall), are solubilized upon being encapsulated by odorant-binding proteins, and transported to the olfactory receptors. Bound pheromone molecules are protected from odorant-degrading enzymes. Upon interaction with negatively-charged sites at the dendritic membrane, the OBP-ligand complex undergoes a conformational change that leads to the ejection of pheromone. ” (Leal 2005:8)