Like every insect, a fly is built in three sections, with six legs and a hard outer shell. Unlike most other flying insects, it has only one pair of wings. Where a bee or dragonfly has a second pair, a fly has two tiny knobs that work as built-in gyroscopes. Much of what makes a fly hard to swat comes from how its eyes, nerves and flight parts are tuned for speed.
The Fly Body Diagram at a Glance
- Head: two large compound eyes, each made of many small light-sensing units side by side, plus three small simple eyes on top.
- Head: two short antennae, each with a feathery bristle.
- Head: sponge-like mouthparts called the proboscis.
- Thorax (the middle section): three pairs of jointed legs ending in claws and sticky pads.
- Thorax: one pair of clear, veined wings.
- Thorax: two halteres, small knobbed stalks behind the wings.
- Abdomen: the segmented rear section holding the gut, waste organs and reproductive organs.
- Whole body: covered with bristles, with breathing holes running along the sides.
If you want to draw your own labelled diagram, start with three body sections and make the thorax the largest and broadest. Attach all six legs and both wings to the thorax, never to the abdomen. Put the halteres just behind and below the wing bases. Then let the eyes cover most of the head.
It also helps to know what separates a fly from a bee or wasp on paper. Bees and wasps have four wings, flies two. Fly antennae are short and stubby, not long and elbowed, and fly eyes are much bigger relative to the head.
Eyes Built for Speed
Each unit of the compound eye works like one pixel, so a fly sees the world as a mosaic of dots. In some tiny predatory flies, the light sensors are shrunken, which makes each pixel smaller and stops neighbouring sensors from blurring together.1PubMed Central. Compound eyes and retinal information processing in miniature dipteran species match their specific ecological demands Those sensors also turn light into nerve signals unusually fast.2PubMed Central. Compound eyes and retinal information processing in miniature dipteran species match their specific ecological demands
Compared with ours, a fly’s view has lower detail but a much wider field around it. Movement registers faster for a fly than for us, and the mosaic makes sudden shadows very noticeable. That is why a slow hand often gets closer than a fast swat.
The eye surface is also packed with tiny bumps far smaller than a hair’s width.3PubMed. Bio-inspired fabrication of antireflection nanostructures by replicating fly eyes These bumps cut reflection, so more light reaches the light sensors.4PubMed Central. Laser-based 3D printing and optical characterization of optical micro-nanostructures inspired by nocturnal insects compound eyes Engineers copy fly eyes to make anti-glare optical surfaces.5PubMed. Replication of fly eyes by the conformal-evaporated-film-by-rotation technique
Antennae and the Feathery Bristle
Each antenna carries a feather-like bristle.6PubMed Central. Smelling on the fly: sensory cues and strategies for olfactory navigation in Drosophila The feathery bristle acts like a sail, catching even very gentle air movement.7PubMed Central. Smelling on the fly: sensory cues and strategies for olfactory navigation in Drosophila A hearing-and-touch organ at the antenna’s base senses that movement.8PubMed Central. Smelling on the fly: sensory cues and strategies for olfactory navigation in Drosophila Because the bristle responds best to air moving across it, the fly can tell which way the wind is blowing.9PubMed Central. Smelling on the fly: sensory cues and strategies for olfactory navigation in Drosophila In blowflies, the bristle may also help sense humidity and airborne chemicals, though this isn’t fully understood.10eCommons. The Role of the Arista on Lucilia sericata in Sensing Wind and Airflow, Relative Humidity and Volatile Compounds
Smell and wind work as a team when a fly hunts for food. Smell tells the fly that food is nearby, and wind direction tells it which way the smell came from. The fly then flies upwind in zigzags to follow the odour trail. On a diagram, the antennae sit between the eyes, short and three-segmented in most common flies, with the bristle sticking out sideways from the last segment.
Mouthparts That Sponge Up Food
The tip of the proboscis is a soft double pad lined with tiny grooved channels. In blowflies, liquid food is drawn through gaps between ridges into these channels and then to the mouth.11Epidemiology & Infection. Some Observations on the Anatomy and Function of the Oral Sucker of the Blow-fly (CalliPhora erythrocephala) Particles wider than about 6 thousandths of a millimetre are usually rejected.12Epidemiology & Infection. Some Observations on the Anatomy and Function of the Oral Sucker of the Blow-fly (CalliPhora erythrocephala) In fruit-pest flies, the gaps are only about half a thousandth of a millimetre, and only particles smaller than that reached the gut.13PubMed Central. Functional Morphology of the Mouthparts of the Adult Mediterranean Fruit Fly, Ceratitis capitata These flies drink filtered liquid rather than chewing solid food.14PubMed Central. Functional Morphology of the Mouthparts of the Adult Mediterranean Fruit Fly, Ceratitis capitata
Because they can only drink liquid, flies release saliva onto dry food like sugar. The saliva dissolves the food so the sponge pad can mop it up.
Not all flies sponge. Horse flies and stable flies have cutting or piercing mouthparts and bite. Mosquitoes are flies too, with a needle-like proboscis. House flies cannot bite, so a fly bite comes from a different kind of fly, such as a stable fly, which looks very similar.
Tasting With Their Feet
Fruit flies have two main taste sites: the proboscis and the legs.15PubMed. Sex-specific non-pheromonal taste receptors in Drosophila The taste hairs at both sites are built the same way, each usually holding four taste nerve cells.16PubMed. Sex-specific non-pheromonal taste receptors in Drosophila Males and females have different numbers of taste hairs on their feet.17PubMed. Sex-specific non-pheromonal taste receptors in Drosophila So a fly can taste food just by landing on it, before it ever opens its mouth.
In practice, a fly lands, tastes sugar with its feet, then lowers its proboscis. That’s why flies walk around on food before eating.
Wings and the Engine in the Thorax
Flight muscles don’t attach to the wings. They squeeze the thorax shell, and the shell moves the wings. Two sets of muscles run at right angles, one front-to-back and one top-to-bottom.18PubMed Central. Evolution of Flight Muscle Contractility and Energetic Efficiency When one set contracts it stretches the other, which triggers that set to contract, and the cycle repeats on its own.19PubMed Central. Evolution of Flight Muscle Contractility and Energetic Efficiency This avoids firing a nerve signal for every beat, so the wings can beat far faster than nerves fire.20PubMed Central. Evolution of Flight Muscle Contractility and Energetic Efficiency The muscles shorten and lengthen by only about 3 to 4 percent each beat.21PubMed. In vivo length oscillations of indirect flight muscles in the fruit fly Drosophila virilis
The buzz you hear is the sound of the wings beating hundreds of times a second. Smaller flies tend to beat faster and buzz higher.
The wings themselves are stiff in one direction and bendy in the other. Insect wings are about 10 to 100 times stiffer along their length than across their width.22American Entomologist. Flexural Stiffness in Insect Wings: Effects of Wing Venation and Stiffness Distribution on Passive Bending Thick or clustered veins along the front edge stiffen the wing lengthwise.23American Entomologist. Flexural Stiffness in Insect Wings: Effects of Wing Venation and Stiffness Distribution on Passive Bending The exact pattern of veins doesn’t change overall stiffness much; wing size matters more.24American Entomologist. Flexural Stiffness in Insect Wings: Effects of Wing Venation and Stiffness Distribution on Passive Bending So the wing bends across its width as it flaps, which helps it push air. On a diagram, the veins are the dark lines running from base to tip, and they are thicker near the front edge.
Halteres, the Fly’s Balance Sensors
Halteres are drumstick-shaped remnants of the second pair of wings.25Journal of Zoology. Scaling of sense organs that control flight: Size and sensory cell number of dipteran fly halteres They beat up and down in time with the wings.26Journal of Zoology. Scaling of sense organs that control flight: Size and sensory cell number of dipteran fly halteres When the fly turns, rolls or tilts, twisting forces act on the beating halteres.27Journal of Zoology. Scaling of sense organs that control flight: Size and sensory cell number of dipteran fly halteres Dome-shaped sensors in the shell at the haltere base detect these strains.28Journal of Zoology. Scaling of sense organs that control flight: Size and sensory cell number of dipteran fly halteres The haltere bends in the direction it senses but stays rigid in the direction it beats, so the sensors register turning.29PubMed Central. Modelling of soldier fly halteres for gyroscopic oscillations Flies also appear to actively adjust the signals coming from their halteres during flight.30PubMed Central. Timing precision in fly flight control: integrating mechanosensory input with muscle physiology Halteres give feedback far faster than eyes alone, which lets flies make sharp turns and hover.
To spot them on a real fly, look just behind the wing base for a small pale knob. In crane flies they are easy to see; in house flies they are partly hidden.
Legs That Stick to Glass
Each foot ends in a pair of claws and pads covered in fine hairs. Claws grip rough surfaces, while the pads hold on to smooth ones like glass. Flies move their foot segments in specific ways to attach and peel off on smooth surfaces.31PubMed. Tarsal movements in flies during leg attachment and detachment on a smooth substrate The pads also release a thin fluid that helps them stick.
Landing on a ceiling shows this at work. The fly flips and reaches its front legs up to touch first. It peels each foot off at an angle rather than pulling straight. Dirty or dusty surfaces make sticking harder.
How a Fly Breathes
Flies breathe through small breathing holes along the thorax and abdomen that feed a network of air tubes. In flying blowflies, air flows one way, entering a front breathing hole and leaving a rear one.32Journal of Experimental Biology. Structure of the thoracic spiracular valves and their contribution to the unidirectional gas exchange in flying blowflies Calliphora vicina A flap on the rear hole is pushed open on the wing upstroke and sucked shut on the downstroke.33Journal of Experimental Biology. Structure of the thoracic spiracular valves and their contribution to the unidirectional gas exchange in flying blowflies Calliphora vicina So the flight muscles themselves pump fresh air in.34Journal of Experimental Biology. Structure of the thoracic spiracular valves and their contribution to the unidirectional gas exchange in flying blowflies Calliphora vicina
Flying fruit flies open their breathing holes only as wide as needed for the oxygen they use.35PubMed. Matching spiracle opening to metabolic need during flight in Drosophila This cuts water loss by up to 23 percent compared with keeping them wide open.36PubMed. Matching spiracle opening to metabolic need during flight in Drosophila Not opening fully also keeps air pressure inside the tubes low, which draws fresh air in.37Journal of Experimental Biology. Structure of the thoracic spiracular valves and their contribution to the unidirectional gas exchange in flying blowflies Calliphora vicina Tube breathing works well over short distances, and oxygen reaches each cell directly, without blood carrying it.
On a diagram, the breathing holes show as small dots along the sides of the thorax and abdomen. The front thorax pair is the largest, and they often have filters that keep dust out.
Heart, Blood and Waste
A fly’s heart is a long tube running along its back, pumping blood that bathes the organs rather than flowing through veins. The heart regularly reverses, pumping forward and then backward.38International Journal of Insect Morphology and Embryology. Functional morphology of the heart and of a new cephalic pulsatile organ in the blowfly Calliphora vicina Diptera Calliphoridae and their roles in hemolymph transport and tracheal ventilation In blowflies it beats about 3 times a second going forward and about 4.6 times going backward, at room temperature.39International Journal of Insect Morphology and Embryology. Functional morphology of the heart and of a new cephalic pulsatile organ in the blowfly Calliphora vicina Diptera Calliphoridae and their roles in hemolymph transport and tracheal ventilation Forward pulses push blood toward the head; backward pulses suck it from the front and push it out the rear.40Journal of Experimental Biology. Drosophila flies combine periodic heartbeat reversal with a circulation in the anterior body mediated by a newly discovered anterior pair of ostial valves and `venous’ channels
Fly blood is usually clear or pale, not red. It carries food and waste but very little oxygen, which comes through the air tubes instead.
For waste, thin tubes float in the abdomen.41Open Access Insect Physiology. Comparative physiology of Malpighian tubules: form and function The tubes filter blood into a fluid that carries waste and toxins into the gut for removal.42Open Access Insect Physiology. Comparative physiology of Malpighian tubules: form and function
The Nervous System and the Escape Reflex
Flies have a brain in the head and a nerve cord running into the thorax. Two types of eye nerve cells that respond to looming objects supply 98.5 percent of the visual input to a giant escape nerve.43PubMed Central. The Drosophila escape motor circuit shows differential vulnerability to aging linked to functional decay So an approaching hand triggers a near-instant jump and takeoff. This escape circuit weakens as flies age.44PubMed Central. The Drosophila escape motor circuit shows differential vulnerability to aging linked to functional decay
Bristles, Male and Female Differences
The arrangement of bristles, especially on the head, is one of the main ways experts identify fly species.45Wikipedia. Morphology of Diptera Bristles and other features also help tell males from females. The tip of the abdomen holds the reproductive parts, which differ between males and females.46PubMed Central. Morphological Characteristics of Terminalia of the Wasp-Mimicking Fly, Stomorhina discolor (Fabricius) Females have an egg-laying tip whose shape fits where they lay eggs.47PubMed Central. A standardized nomenclature and atlas of the female terminalia of Drosophila melanogaster Species that lay in firm fruit or bark tend to have a long tip with tooth-like bristles, while those laying in rotting fruit have a short rounded one.48PubMed Central. A standardized nomenclature and atlas of the female terminalia of Drosophila melanogaster
With house flies, males often have eyes almost touching at the top of the head, while females have a wider gap between the eyes. Females also usually have a fuller, more pointed abdomen.
From Maggot to Adult
A fly goes through egg, maggot, pupa and adult stages. Maggots carry small folded sheets of cells for future wings, legs, eyes and other adult parts.49PubMed Central. The legacy of Drosophila imaginal discs During the pupa stage, a hormone signal makes these sheets turn inside out and unfold.50PubMed Central. The legacy of Drosophila imaginal discs They then fold, join the outer skin, and form the adult’s outer parts.51PubMed Central. The legacy of Drosophila imaginal discs Adults emerge at full size and don’t grow afterward, so a small fly is not a young fly; it is usually a different species or one that was poorly fed as a maggot.