Phlebotomine sand flies are minute, biting insects that transmit several significant diseases, including leishmaniasis. These small, hairy flies measure only 1.5 to 3.5 millimeters in length. Understanding the sensory mechanisms that drive the female sand fly’s search for a blood meal is necessary for developing effective strategies to monitor and control their populations. Host location transitions from long-range chemical detection to short-range thermal and visual cues.
Chemical Cues Emitted by Hosts
The primary means by which a sand fly detects a potential blood meal is through chemical signals known as kairomones. The most significant long-range attractant is carbon dioxide (CO2). This gas serves as an initial signal of a host’s presence, prompting the sand fly to begin its host-seeking flight.
The insect tracks the CO2 plume using specialized olfactory receptors, flying up the concentration gradient toward the source. As the sand fly gets closer, it is guided by a more complex blend of volatile organic compounds (VOCs) that create a unique host odor profile.
Specific components found in human sweat and breath, such as lactic acid, ammonia, and certain volatile fatty acids, become important attractants. For instance, 1-octanol and nonanol, associated with human skin odor, attract species like Lutzomyia longipalpis. The precise combination of attractive chemicals can vary among different sand fly species, reflecting their host preferences.
Caproic acid, lactic acid, and ammonia are integral to the close-range chemical signature of a host. This complex detection system allows the sand fly to narrow its search from the broad CO2 signal to the immediate vicinity of the host.
Thermal and Visual Tracking
When in close proximity to a host, the sand fly switches its focus to non-chemical sensory inputs. Thermal cues pinpoint the exact location of exposed skin or blood flow. Heat detection is a short-range sensory input.
Body heat helps the sand fly differentiate a host from the ambient environment and locate suitable feeding sites, such as areas with thin skin or high vascularity. Experiments show that heat alone can act as an attractive feature in traps.
Visual cues also play a role, particularly as sand flies are crepuscular or nocturnal and exhibit positive phototaxis. Ultraviolet (UV) light is one of the most attractive features for sand flies after CO2. Their visual system is sensitive to both UV light and the blue-green-yellow range of the spectrum.
Visual attraction relies on contrast and movement. Sand flies are attracted to the high contrast created by a host’s silhouette against a background. Their visual system detects movement, which aids in the pursuit of a moving host. The combined input of heat and visual contrast directs the sand fly to the precise point of feeding.
Environmental Conditions Influencing Activity
Host-seeking mechanisms depend on favorable environmental conditions. Sand flies are vulnerable to desiccation and require high levels of relative humidity (RH). Optimal activity often corresponds with a rise in RH.
The peak biting activity of most sand fly species is observed during the crepuscular hours of dusk and continues throughout the night. High daytime temperatures, particularly above \(32^\circ\text{C}\), and direct sunlight tend to inhibit their flight activity.
Activity is strongly linked to temperature, with optimal ranges for many species between \(21^\circ\text{C}\) and \(30^\circ\text{C}\). When temperatures drop below \(10^\circ\text{C}\), sand flies may enter a dormant state.
Female sand flies are more likely to seek a blood meal when a host is near their shelter, such as animal burrows, stone walls, and poorly ventilated indoor dwellings. This interplay between host cues and environmental timing determines the overall risk of sand fly bites.