The Ultimate Comparison: Dragonflies and Horse Flies

Dragonflies and horse flies inhabit many of the habitats humans like to visit. They live around water, in sunny meadows, and near forests. As insects, both have exoskeletons, antennae, six legs, and wings. But the similarities pretty much end there.

Dragonflies consume flying insects, many of which bite humans, and are themselves harmless to humans. Horse flies consume either mammalian blood or nectar, depending on the sex. Female horse flies depend on blood to feed their eggs, while males lack the ability to bite.

Many species of both dragonflies and horse flies exist. This post focuses on a few of the species most commonly found in the southeastern United States. Read on for the details of their similarities and differences.

Classification

Dragonflies are classified within class Insecta, order Odonata, suborder Anisoptera. Scientists know of about 5000 species worldwide, including approximately 300 species north of Mexico (Eaton and Kaufman 2007).

In contrast, horse flies are classified within class Insecta, order Diptera, family Tabanidae. Researchers identify around 4400 named species, including approximately 350 found in North America (Eaton and Kaufman 2007).

Larval Stage: similarities and differences

Larval growth, development, and metamorphosis

The vast majority of insects lay eggs and both female dragonflies and female flies do so. The immature individuals that emerge from the eggs are called “larvae” (singular “larva”). Insect larvae change dramatically as they mature from newly hatched to their final adult forms.

This process of development is called “metamorphosis”.

In general, each species of insect metamorphoses in one of two ways: “hemimetabolous” or “holometabolous” metamorphosis. Hemimetabolous metamorphosis is also called “incomplete”, while holometabolous metamorphosis is called “complete”.

Immature insects of both growth types grow physically larger through a series of molts, during which the larvae literally split their skin to accommodate their growing size.

A very few insects change so little in the course of their growth and development that they are said to have “ametabolic” metamorphosis. But this state does not apply to either dragonflies or horseflies.

Dragonflies undergo incomplete, hemimetabolic metamorphosis; horse flies undergo complete, holometabolic metamorphosis.

Dragonfly metamorphosis is incomplete

Dragonflies undergo incomplete or hemimetabolous metamorphosis but their metamorphosis differs slightly even from other hemimetabolous insect species.

In general, hemimetabolous insects are those whose immature forms resemble adults. They develop through three distinct life stages: egg, larva, and adult.

The bodies of adult insects consist of three distinct segments—head, thorax, and abdomen—and a total of six legs arranged in three pairs attached to the thorax.

Young dragonflies hatch from eggs as “larvae” or naiads. Naiads have obvious heads, thoraxes, and abdomens, six jointed legs characteristic of adult insects, large eyes, and wing buds. They grow larger by molting their tough exoskeletons periodically, until eventually gaining their full adult forms.

Naiads and adult dragonflies do not resemble each other perfectly and in fact, look more different from each other than the larvae and adults of other insect species that undergo incomplete metamorphosis.

For example, naiads have gills which help them breathe underwater, which they lose when they develop into adult, terrestrial dragonflies. And adult dragonflies have four large membranous wings, which are only short, stubby buds in nymphs.

But naiads look more and more like adult dragonflies with every successive molt. The two life stages—naiad and adult—share enough physical characteristics in common that their metamorphosis is considered “hemimetabolous” or incomplete.

Horse fly metamorphosis is complete

In contrast, horse flies undergo complete or holometabolous metamorphosis.

Holometabolous insects are those whose immature forms look absolutely nothing like their eventual adult forms. These insects develop through four distinct life stages: egg, larva, pupa, and adult. Larvae transform into adults during the pupal stage.

Horse flies belong to the true fly order Diptera and all Dipterans undergo complete metamorphosis. Horse fly larvae look like caterpillars with pointed ends. Their bodies are ringed with ridges bristling with setae. The ridges are called “creeping welts” and setae are hair-like spines that give the horse fly traction for movement.

Although horse flies are true flies belonging to order Diptera, their larvae are not “maggots”. The term “maggot” is used only for the larvae of true flies in infraorder Muscomorpha (Mullen and Darden, 2002).

Unlike dragonflies and other insects that develop through incomplete metamorphosis, the larvae enter a pupal stage after the final larval molt. During the pupal stage, the insects do not eat but instead transform into their full adult forms. Body tissues actually break down, reform, differentiate, and reorganize into the body segments, antennae, functional wings, legs, and compound eyes of adult horse flies.

After the pupae finish this transformation, they emerge as adult horse flies in a process called “eclosion”.

Newly eclosed adults are vulnerable for a time because their wings are crumpled and usable and their bodies are soft. Eventually, their wings unfurl and stiffen, while their exoskeletons, which cover the outside of their bodies, harden. The horse flies are now free to fly off and live out the rest of their lives as adults.

Larval habitats

Dragonflies and horse flies larvae live in different environments.

Dragonfly larvae, or naiads, are exclusively aquatic. They live underwater in ponds and streams and prey on aquatic invertebrates, tadpoles, and small fish.

Horse fly larvae are considered aquatic also. However, instead of living completely submerged in large bodies of water like ponds and streams, they live often in environments that are simply moist. Horse fly larvae live in damp soil and under wet leaf litter.

Larval attack strategies

Both dragonfly naiads and horse fly larvae are predatory. But they differ as predators in significant ways.

First, dragonfly naiads sport two specialized anatomical structures that support their predatory lifestyle. Neither adult dragonflies nor horse flies in any developmental stage share these structures.

Like many insects, dragonfly naiads have chewing mouthparts made up of a labrum, a pair of toothed, jawlike mandibles, a pair of maxillae, and the labium. The labrum is similar to an upper lip, the sharp mandibles cut prey into small pieces, the maxillae grasp prey, and the labium is similar to a lower lip.

However, the labiums of dragonfly naiads are special in that they are elongated, hinged, tipped with a pair of grasping jaws, and projectile.

At rest, naiads holds their labiums folded under their heads, with only the tips visible in front of their faces. But when the naiads wish to attack prey, they shoot their labiums forward with lightning speed. The terminal jaws clamp around the prey then retract back to the naiads’ heads and the waiting mandibles.

Horse fly larvae prey on other invertebrates also. But instead of projectile mouthparts, horse fly larvae have vertically oriented fang-like mandibles that slash and slice prey.

Larval respiration

Both dragonfly and horse fly larvae extract oxygen from the water that surrounds them but the means by which they do so differs significantly.

Dragonfly larvae breathe underwater through their skin and their anuses. Oxygen passively diffuses across their thin skin but they also breathe actively through their anuses. Dragonfly larvae first expand their abdominal cavities, which depressurizes them and sucks oxygenated water inside. Oxygen transfers from the water into the dragonflies’ bodies, then the larvae contract their abdomens and expel the newly deoxygenated water back out their anuses.

In contrast, horse fly larvae breathe through special tubes that extend from their hind ends (Mullen and Darden, 2002). They do absorb some oxygen through their soft, thin skin like dragonfly larvae but obtain the majority of their oxygen from their breathing tubes.

Larval movement

Dragonfly and horse fly larvae move through their underwater habitats in different ways.

Dragonfly larvae have six, strong, jointed legs on their thoraxes and crawl across the substrate of their underwater habitats. Some species can also use the muscular strength of their abdomens to shoot themselves forward. These larvae expand their abdomens to suck in a quantity of water then clench their abdominal muscles. This expels the water our their anuses under high pressure, which propels the insects forward.

Since they have no legs, horse fly larvae are much more sedentary than dragonfly larvae. They can and do move around their habitats but, when they do, horse fly larvae are much slower than dragonfly larvae and cover less ground.

Horse fly larvae crawl through their environment with the help of “creeping welts”. Creeping welts are raised ridges that ring the larvae’s long, thin, caterpillar-like bodies. Each welt bristles with one to several modified setae or spines” (Merritt et al. 2009). Horse fly larvae drag their bodies forward using a combination of “turgor pressure” and their creeping welts (Thorp and Rogers, 2014).

Turgor pressure refers to the pressure inside the horse fly larvae’s bodies. Horse fly larvae inch themselves along by alternately raising and lowering the pressure under each creeping welt. High pressure expands a creeping welt out while low pressure contracts it. The spines along each welt provide traction.

Adult stage: similarities and differences

Eyes and vision

Both adult dragonflies and adult horse flies have excellent vision, thanks to large eyes consisting of thousands of tiny, individual facets. Their brains combine the sensory information provided by each individual facet into detailed visual pictures.

The adults of some families of dragonflies have eyes that meet on the top of their heads, while other families have separated eyes. Horse flies share this characteristic but for them, whether the eyes meet or not is more related to the sex of an individual insect. The eyes of male horse flies meet on top of their heads, while eyes of females are separated.

Vision is extremely important to both types of insects. They locate prey, potential mates, and predators by sight and both dragonflies and horse flies excel at spotting movement and changes in light and shadow.

While it’s exceedingly difficult to know what insects see, scientists believe that dragonflies can see polarized light. This may explain why some female dragonflies attempt to lay eggs on cars parked in sunny locations; the sunlight reflecting off the cars’ shiny metal tricks the dragonflies into believing the sunlight is actually reflecting off water.

Horse flies—especially the females—may also see polarized light but they may go several steps further. Female horse flies may be sensitive to “black body radiation outside the spectrum visible to humans” (Mullen and Darden, 2002).

They may also “see” temperature differences, much like thermal resolution cameras. They tend to attack dark-colored mammals standing in sunlit areas more often than light-colored, spotted, or striped animals standing in the shade because the warm sunlight increases blood flow to the animals’ skin and also because horse flies can react faster when warmer (Horváth et al. 2020).

Mouth parts and eating

The diets of adult dragonflies and adult horse flies vary, and so does the anatomy of their mouth parts.

All adult dragonflies of both sexes are predators of flying insects.

Adult dragonflies lose the special projectile labiums they possess as larvae but retain hard, sharp mandibles and very powerful jaw muscles. Adapted for chewing, adult dragonfly jaws can snap through the tough exoskeletons of their insect prey easily. Adult dragonflies rely on brute force to kill their prey; they aren’t venomous and don’t manufacture any special chemicals that help them hunt or feed.

The diet of adult horse flies varies by sex; male horse flies feed on nectar and pollen, while female horse flies eat blood. Called “hematophagous” female horse flies require blood meals to feed their eggs and obtain it by biting other animals—often warm-blooded animals like horses, deer, cows, and humans.

Female horse flies have knife-like mandibles that slice through skin easily. They fly silently up to their prey, land gently, then slash a deep slice into the animals’ skin. The blades on female horse flies’ mouths are so sharp—and the resultant wounds so cleanly sliced—that large amounts of blood well from the wounds before the animals even register pain.

Wounds inflicted by female horse flies bleed for several minutes because the flies’ saliva contains an anticoagulant that prevents blood clotting (Milne and Milne, 1980). Prey animals as large as horses and cows can actually die from blood loss if they suffer repeated attacks from female horse flies.

Wings and flight

Both dragonflies and horse flies are strong, speedy, and agile fliers.

Dragonflies have two pairs of long, narrow wings that project out to the side from their thoraxes.

Horse flies, like all dipterans, have only one pair of wings which are held over the abdomen. The second pair evolved into sensory structures called halteres that provide the insects information about air flow and body position.

Both insects’ wings are thin, strong membranes made of cuticle, which is a nonliving composite of chitin and protein. Strong flight muscles housed in the insects’ thoraxes power the wing beats and both dragonflies and horse flies can both hover and change direction in mid-air very quickly.

Dragonflies flap their wings approximately 20-40 times per second (Tennessen, 2009) and buzz when they fly. Stealth is unnecessary as sheer flight speed and agility are more valuable in chasing down fleeing flies in mid-air than sneaking up on them.

In contrast, horse flies also lap their wings many times per second but fly silently. Female horse flies attack large animals like horses, deer, and cows. Silent flight allows the horse flies to sneak up on their prey and buys them precious seconds during which they strike and lap up precious welling blood.

Interaction with humans

Dragonflies are universally beneficial to humans. They do not bite us unless harassed and restrained nor do they damage our agricultural economy. They also serve as important prey for many other animals like birds and fish.

But dragonflies are directly helpful to humans because they eat enormous numbers of flying insects, including horse flies and other biting dipterans.

This is important for two reasons.

First, horse flies cause tremendous economic damage. Their painful bites can distress domestic livestock to the point they cannot eat or devote energy to manufacturing milk or meat. In addition, open wounds caused by female horse fly bites offer easy entry for microbes and parasites to exploit, which result in serious infections and death of the livestock animals.

Additionally, the blood loss from multiple bites alone can weaken animals directly. Female horse flies can suck up to 0.2 cubic centimeters of blood at one feeding (Altunsoy 2015). This amount might sound miniscule compared to the total blood volume of an animal the size of a horse or cow. But blood loss multiplied by hundreds of bites caused from heavy attacks from many horse flies at a time adds up fast.

Second, horse flies can transmit dangerous diseases. These include equine infectious anemia, bovine leukemia, hog cholera, bacterial rickettsia, anthrax (Mullen et al. 2002), and tularemia (Mullen and Darden,

Taken together, dipterans transmit more diseases to humans and other animals than any other group of arthropods (Merritt et al. 2009). Horse flies do not transmit as many diseases as often as other types of flies but they certainly do their share.

Conclusion

Dragonflies and horse flies share many basic characteristics but are very different creatures. It is far easier for most humans to appreciate dragonflies than horse flies as they are beautiful, easy to observe, cause no pain to mammals, and eat tremendous numbers of biting insects, including horse flies. In contrast, while horse flies deserve respect as important parts of many worldwide ecosystems, they are generally loathed as vicious little beasts.

For good reason.

References

Altunsoy, Ferhat. 2015. “Host and Feeding Side Preferences of the Horse Flies (Diptera: Tabanidae.” Journal of the Entomological Research Society 17 (1): 107-115.

Eaton, Eric R., and Kaufman, Kenn. 2007. Kaufman Field Guide to Insects of North America. Mariner Books. HarperCollins.

Horváth, Gábor, Ádám Pereszlényi, Ádám Egri, Tímea Tóth, and Imre Miklós Jánosi. 2020. “Why do Biting Horseflies Prefer Warmer Hosts? Tabanids can Escape Easier from Warmer Targets.” PLoS One 15 (5) (05). doi: https://doi.org/10.1371/journal.pone.0233038.

Mullen, Gary R., Durden, Lance A., and Durden, Lance, eds. 2002. Medical and Veterinary Entomology. Burlington: Elsevier Science & Technology.

Mullens, Bradley A. and Jeffrey V. Freeman. 2017. “Hovering and Swarming Behavior of Male Tabanus Calens (Diptera: Tabanidae) in Tennessee and New Jersey, USA.” Journal of Medical Entomology 54 (5) (09): 1410-1414. doi: https://doi.org/10.1093/jme/tjx070.

Merritt, Richard W., Gregory W. Courtney, and Joe B. Keiper. “Diptera.” In Encyclopedia of Insects, edited by Vincent H. Resh, and Ring T. Carde. 2nd ed. Elsevier Science & Technology, 2009.

Tennessen, K. J. “Odonata.” In Encyclopedia of Insects, edited by Vincent H. Resh, and Ring T. Carde. 2nd ed. Elsevier Science & Technology, 2009.