An insect's acute sense of smell enables it to find mates, locate food, avoid predators, and even gather in groups. Some insects rely on chemical cues to find their way to and from a nest, or to space themselves appropriately in a habitat with limited resources. Insects, you may have noticed, don't have noses. So how are they able to sense the faintest of scents in the wind?
Insects produce semiochemicals, or odor signals, to interact with one another. Plants also emit pheromone cues which dictate insect behaviors. In order to navigate such a scent-filled environment, insects require a fairly sophisticated system of odor detection.
Insects possess several types of olfactory sensilla, sense organs which collect the chemical signals. Most of these smell-gathering organs occupy the insect's antennae. In some species, additional sensilla may be located on the mouthparts or even the genitalia. Scent molecules arrive at the sensilla and enter through a pore.
Simply collecting the chemical cues is not enough to direct an insect's behavior, however. Once those odor molecules enter the sensilla, the chemical energy of the pheromones must be converted to electrical energy, which can then travel through the insect nervous system.
Special cells within the structure of the sensilla produce proteins, referred to as odor-binding proteins. These proteins capture the chemical molecules and transport them through the lymph to a dendrite, an extension of the neuron cell body. Odor molecules would dissolve within the lymph cavity of the sensilla without the protection of these protein binders.
The odor-binding protein now hands off its companion smell to the receptor molecule on the dendrite's membrane. This is where the magic happens. The interaction between the chemical molecule and its receptor causes a depolarization of the nerve cell's membrane.
This change of polarity triggers a neural impulse that travels through the nervous system to the insect brain, informing its next move. The insect has smelled the odor, and will pursue a mate, find a source of food, or make its way home, accordingly.