Sandflies and mosquitoes are both blood-feeding insects, but they differ significantly in size, how they bite, the diseases they carry, and how you need to protect against them. Sandflies measure just 2 to 3 millimeters long, roughly half the size of a mosquito at about 5 millimeters. That size difference has practical consequences for everything from how painful the bite feels to what kind of bed net will actually keep them out.
Size and Appearance
Sandflies are tiny, often appearing as small black specks on your skin. At 2 to 3 millimeters, they’re easy to miss entirely. Mosquitoes, at around 5 millimeters, are more visible and recognizable with their long legs and narrow wings. Both belong to the same order of flies (Diptera), and in both species, only the female bites.
The easiest way to tell them apart in the wild is movement. Sandflies travel in a distinctive short, hopping flight pattern and are weak fliers. Even a light breeze can ground them. Mosquitoes fly more deliberately and can cover much greater distances. Sandflies rarely travel more than a few hundred meters from where they breed, while some mosquito species range over several kilometers.
How They Bite
The two insects feed on blood in fundamentally different ways, which explains why their bites look and feel different. Mosquitoes are “vessel feeders.” They insert needle-like mouthparts directly into a blood vessel beneath your skin and draw blood through it like a straw. You often don’t feel the bite until afterward.
Sandflies are “pool feeders.” They slash the surface of your skin to create a small wound, then lap up the blood that pools at the surface. This is a rougher process, and sandfly bites are typically more painful at the moment they happen. Despite that initial pain, sandfly bites often go unnoticed because the insects are so small that people don’t always see what bit them.
Sandfly bites produce small red bumps and blisters that can become intensely itchy, sometimes lasting days or weeks. They’re also more prone to becoming infected or causing prolonged skin inflammation. Mosquito bites usually form a raised, round welt that itches for a few days and resolves on its own.
When and Where They’re Active
Both insects are most active in warm, humid conditions, but their preferred habitats differ. Mosquitoes breed in standing water: ponds, marshes, rain-filled containers, tree holes, even a bottle cap holding a few milliliters of water. Some species prefer clean water, others nutrient-rich sources. The common thread is still or slow-moving water where larvae can develop.
Sandflies breed in moist soil and organic debris rather than open water. Cracks in walls, animal burrows, leaf litter, and rubble piles all serve as breeding sites. This makes them harder to control because you can’t simply dump out standing water the way you can with mosquitoes.
Most sandfly species feed at dusk and through the night, when temperatures drop and humidity rises. Mosquitoes vary more by species. Some bite primarily at dawn and dusk, while others (particularly the species that carry dengue and Zika) are aggressive daytime biters. If you’re being bitten during broad daylight in a breezy area, it’s almost certainly not sandflies.
Diseases They Carry
Mosquitoes are the more prolific disease vector of the two. Different mosquito species transmit malaria, dengue, Zika, yellow fever, chikungunya, West Nile virus, Japanese encephalitis, and several other infections. In the United States, West Nile virus and eastern equine encephalitis are the primary mosquito-borne concerns.
Sandflies carry a narrower but still serious set of diseases. The most significant is leishmaniasis, a parasitic infection found in roughly 90 countries, mostly in tropical and subtropical regions. Leishmaniasis takes several forms. Cutaneous leishmaniasis, the most common, causes skin sores. Mucosal leishmaniasis can produce sores in the nose, mouth, or throat and can be life-threatening. Visceral leishmaniasis affects internal organs and can be fatal without treatment. In the U.S., locally acquired cases of cutaneous leishmaniasis have occurred in Texas and occasionally in Oklahoma and Arizona. Sandflies also transmit sandfly fever (a viral illness common in Mediterranean and Middle Eastern regions) and bartonellosis in parts of South America.
For travelers, the practical difference is geographic. In sub-Saharan Africa and South Asia, mosquito-borne diseases like malaria are the dominant concern. In parts of the Middle East, Central Asia, and Latin America, sandfly-borne leishmaniasis becomes a real risk alongside mosquito-transmitted infections.
Repellent Protection
The same EPA-registered repellents work against both insects. DEET, picaridin, and oil of lemon eucalyptus all reduce biting by mosquitoes and sandflies. Higher concentrations provide longer protection: products with less than 10% active ingredient typically last only 1 to 2 hours. For DEET specifically, protection peaks at around 50% concentration, and going higher doesn’t meaningfully extend it.
Because sandflies are weak fliers, fans and even moderate air movement add an extra layer of protection that doesn’t work as well against mosquitoes. If you’re sitting outdoors in sandfly territory, a portable fan pointed at your lower legs can make a noticeable difference.
Bed Nets and Physical Barriers
This is where the size difference between the two insects matters most. A standard mosquito net with a typical polyester weave will stop mosquitoes but may not stop sandflies. Sandflies are small enough to walk through some synthetic mesh.
Standard polyester bed nets are woven at around 360 holes per square inch, tight enough for mosquitoes but not necessarily for sandflies. Cotton nets, woven at around 256 holes per square inch, have a structural advantage: tiny fibrous hairs along each thread physically deter sandflies and midges from probing through the weave. Synthetic fibers lack these surface hairs, so even a tighter hole count doesn’t provide the same barrier against the smallest biting insects.
If you’re traveling to an area where both sandflies and mosquitoes are present, a cotton net is the better choice. In areas where mosquitoes are the only concern, polyester works well and is easier to maintain. Insecticide-treated nets add chemical protection regardless of material, which helps compensate for any gaps in mesh size.
Quick Comparison
- Size: Sandflies 2 to 3 mm; mosquitoes about 5 mm
- Bite method: Sandflies slash skin and pool-feed; mosquitoes pierce a blood vessel directly
- Bite feel: Sandfly bites are more painful initially; mosquito bites are often painless until they itch
- Breeding sites: Sandflies use moist soil and organic debris; mosquitoes need standing water
- Flight: Sandflies hop weakly and stay within a few hundred meters of breeding sites; mosquitoes fly farther and tolerate wind
- Peak activity: Sandflies feed at dusk and night; mosquitoes vary by species, some bite during the day
- Key diseases: Sandflies transmit leishmaniasis and sandfly fever; mosquitoes transmit malaria, dengue, Zika, West Nile, and others
- Bed nets: Standard polyester nets block mosquitoes; cotton nets with fibrous threads are needed to reliably block sandflies