Smoke is a dynamic aerosol mixture of fine particulate matter and gaseous compounds, including various volatile organic compounds (VOCs). The composition of this mixture changes constantly depending on the fuel source and temperature of combustion. A fly’s behavioral response is determined by how its sensory system interprets this chemical cocktail, often linking it to a threat or a potential resource. The prevailing reaction, however, is one of avoidance, driven by survival instincts.
The Immediate Response to General Smoke
For most common sources, such as a campfire, the immediate reaction of a fly is repulsion. This defensive behavior is driven by the fly interpreting the smoke as a sign of imminent danger, such as fire or excessive heat. Smoke from rapid combustion contains elevated levels of carbon dioxide (\(\text{CO}_2\)), a gas flies are highly sensitive to. A sudden spike in \(\text{CO}_2\) concentration signals a potential threat, such as an overheated environment.
Flies, like the common fruit fly (Drosophila), avoid \(\text{CO}_2\) concentrations above a low threshold, prompting an immediate escape. Furthermore, the physical components of dense smoke act as a deterrent by interfering with the fly’s ability to navigate and locate food sources. The fine particulate matter can physically clog or reduce the sensitivity of the insect’s antennae, making it difficult to process host odors.
How Flies Detect Smoke Compounds
Flies possess highly specialized chemosensory systems, primarily located on their antennae, for detecting airborne chemicals. Smoke is a complex blend of hundreds of specific VOCs, including phenols, aldehydes, and aromatic hydrocarbons. These compounds are detected by olfactory receptor neurons (ORNs) tuned to certain molecular structures. The detection of these smoke-related molecules actively engages the fly’s odorant receptors (ORs).
This engagement of multiple receptors by the smoke’s chemical output effectively “corrupts” the specific odor code the fly uses for navigation. The sensory system is overloaded with a confusing mix of chemicals, rather than receiving a clear signal for a food source or a mate. This phenomenon is chemical masking, where the smoke’s volatile compounds overwhelm the fly’s ability to track essential cues. The fly leaves the chaotic environment to restore clarity in its olfactory perception.
When Smoke Signals Opportunity
While high-heat smoke is a strong repellent, flies may sometimes appear attracted to the localized area of a low-heat, smoldering fire. This attraction is usually not to the smoke itself but to the specific odors released by the material that is incompletely burning. Smoldering organic matter, such as decaying wood or animal fat, releases a different profile of VOCs than a clean, high-temperature flame. This low-heat combustion can produce chemical signatures that mimic the smell of fermentation, carrion, or other food or breeding sites.
For instance, the slow burning of fat drippings can release organic acids and esters that a fly might mistake for a desirable food source. In these cases, the fly moves toward the misidentified food scent, ignoring the low-level smoke signal because the attractant cue is stronger.
The smoke from certain plant materials, like those used in commercial mosquito coils, contains concentrated repellent VOCs such as geraniol and cineole. These compounds actively interfere with the fly’s olfactory system. The attraction is a rare exception driven by a misinterpretation of a food-like odor associated with the smoldering process, rather than the fly seeking the smoke as a primary attractant.