Manitoulin’s blood feeders: Part I

Horse fly ready to bite into an arm. Photograph captured from the internet.

Wherein we examine the various summer insects who are out to drink our blood

by Joe D. Shorthouse

MANITOULIN—Insects are the most abundant animals on Manitoulin Island. There are more kinds of insects here than all the other kinds of animals combined and our natural ecosystems would collapse without them.

Most kinds of Manitoulin insects are not even noticed as they spend their lives providing ecosystem services such as breaking down organic matter, eating other insects, or pollinating plants.

However, a few kinds cause us no end of grief because they eat our blood. These insects, and a few other kinds of small animals that also feed on blood, are the subject of this two-part series.

In Part 1, mosquitoes, black flies, horse and deer flies are discussed. Centuries ago, all were feeding on the blood of deer, moose, snowshoe hares and birds, but once humans arrived, we quickly expanded their banquet choices. 

In Part II, lice, fleas, bed bugs and ticks are discussed. Ticks are not insects, but since they are related arthropods, and of concern to residents of Manitoulin Island, they are included.

Although most of us become squeamish at the thought of animals eating our blood, a few hours at the exhibit gave a new perspective as to the role blood feeders play in our environment and the amazing ways in which they are adapted to their way of life.

For the animals discussed in these two articles, blood is an energy and nutrient-rich source of food used to produce eggs. However, a problem for all feeders of vertebrate blood is that blood easily clogs in tiny mouthparts. To avoid this, blood feeders have evolved anticoagulants in their saliva.

Although anticoagulants solve a problem for the blood feeders, it is the saliva that is responsible for much of our discomfort when fed upon, and in many cases, it is the saliva that spreads disease.

There are two types of blood feeders—the so-called capillary feeders and pool feeders. Capillary feeders such as mosquitoes, bedbugs, lice and ticks have mouthparts modified to form two flexible tubes in which they seek out a blood meal from a vein below the surface of the skin.

In one tube they inject anticoagulants to stop the blood from coagulating, while in the other they suck up the blood meal. They can usually feed without alerting the host to their presence.

Pool feeders like horse flies, deer flies and blackflies cut a hole in the skin and wait for blood to flow freely and pool. They add anticoagulants to the droplet and lap up the blood that pools on the surface. The bite of most pool feeders is painful.


Adult mosquitoes are the flying insect people dislike the most. They buzz in our ears as we try to sleep, and their bites leave us itchy, spotty and swollen.

There are about 82 species of mosquitoes in Canada, and 67 in Ontario and likely about 50 species on Manitoulin Island. The appearance of the adults of each species is scattered over the season so there are mosquitoes to bother us during all the warm months.

Although delicate little flies, mosquitoes are the deadliest animal for humans on the planet because of the infectious diseases they spread. Mosquitoes kill hundreds of thousands more people in the world each year than all other deadly animals combined.

Mosquitoes are both aquatic and terrestrial; however, they spend most of their lives in the aquatic stage. Eggs are laid in standing water and they hatch into mobile larvae called wrigglers which feed on algae and microscopic detritus floating in the water.

Larvae cannot breathe under water and must come to the surface where they use a breathing tube on their dorsal surface to get oxygen. The larval stage lasts from six days to two weeks depending on the species and water temperature.

Unlike most other insects, the pupal stage of mosquitoes actively swims, but it does not eat. It has a comma-shaped body and must also come to the surface to get oxygen.

Even though mosquitoes are delicate, they can fly long distances and have an uncanny ability to locate human hosts. Adults are about 6-8 mm in length, with long gangly legs.

Their heads have large eyes and a pair of long antennae. Antennae of the males are bushier than those of the females and contain auditory receptors to detect the characteristic whine made by wings of the females. Mosquito wings beat 300 to 600 times per second.

Their large eyes allow them to see people several metres away and sensilla on their antennae to detect levels of CO2 and other compounds expelled as we breathe.

They can detect sweat and other body odours, along with body heat, and as a result, some people are more attractive than others. Mosquitoes can smell body odour and CO2 up to 20 metres away.

Adult mosquitoes can feed at any time of the day, depending on the species. Most species feed actively at dusk or dawn. Both females and males feed on nectar which they use for energy; however, only the females feed on blood which is used to produce eggs.

Most mosquitoes feed on warm-blooded mammals or birds. Mosquitoes that feed on birds transmit West Nile Virus.

Development from egg to adult takes about 40 days. Adults live one to two weeks. Most mosquitoes rest in a cool place during heat of the day and wait for the evenings before foraging for a meal, although they may still bite if disturbed.

Mosquitoes are some of the most adaptable and successful insects on Earth and are found in some extraordinary places. Virtually any natural or man-made collection of water can support mosquito larvae.

Mosquitoes have been discovered in mines nearly a kilometre below the surface, and on mountain peaks at 420 metres. Mosquitoes readily fly through open windows of vehicles and houses and seek human inhabitants upon which to feed.

Black Flies

Black flies are robust little flies (most are 5-8 mm in length), black or gray that are a scourge to people and livestock across Canada. They are most common beginning in about the middle of May, and can continue making us miserable well into June, or even July. They can bite at any time during the daylight hours, but do not feed at night. In contrast to mosquitoes, they are not delicate and are shaped more like small prairie buffalo, thus their other common name ‘buffalo gnats.’

Black flies have a habit of swarming and are most annoying when they get under our clothing and feed on our blood without being noticed. They do not spread disease among humans, though their bites are painful, itchy, and slow to heal. Some people have an allergic reaction to their bites.

Black flies have short legs and antennae. There are 162 species in Canada and about 63 in Ontario and at least 40 on Manitoulin Island.

Like mosquitoes, they are both aquatic and terrestrial with most of their lives spent as larvae in fresh flowing waters of streams and rivers.

They have one generation per year. Their eggs are laid at the edge of streams and rivers in mid-summer. When the larvae hatch, they sink to the bottom where they attach themselves to rocks using hooks on the posterior end.

Larvae are shaped like little urns and bend in the direction of the current with the head downstream. Their mouthparts consist of foldable fans that expand when feeding to catch passing debris such as organic particles, algae and bacteria. The larva scrapes captured food into its mouth every few seconds.

The larvae develop into pupae, an inactive phase of development during which time they do not feed. The pupae develop into adults which float to the surface inside a bubble of air.

Adults are ready to fly when they emerge from the water and they immediately search for a blood meal. They can lay eggs soon after a blood meal. The average life span of an adult black fly is three weeks.

Mature adults can disperse tens or hundreds of kilometers from their breeding sites under their own power or assisted by prevailing winds, complicating control efforts.

Both males and females depend on nectar from plants for their flight energy whereas females require blood for egg development. Males are rarely seen and they do not feed on blood.

Females have specialized mouths with toothed “stylets” which they use for slicing into the skin to reveal blood vessels. Anticoagulants in the saliva also partially numbs the feeding site, reducing the host’s awareness of being bitten and thereby extending the flies’ feeding time.

Blood continues to pool at their feeding sites after the flies have left. Feeding often causes localized swelling and inflammation and the site becomes itchy. Intense feeding can cause “black fly fever,” likely caused by the anticoagulant, resulting in headache, nausea, fever, swollen lymph nodes, and aching joints.

Black flies feed only during daylight hours and tend to zero in on areas of thinner skin, such as the nape of the neck or ears and ankles. Of interest, they will not feed on humans inside vehicles or houses. Black flies of Manitoulin Island do not spread disease.

Horse Flies and Deer Flies

Horse flies and deer flies are large, robust flies with prominent compound eyes. As their names imply, their main sources of food are the blood of large animals such as horses and deer. However, they will feed on any large mammal they can find such as cows, dogs and humans.

Deer flies will readily seek out human blood, whereas horse flies less commonly feed on people. Both flies are often found buzzing around stables and fields. Depending on the species, we can be bothered by horse flies and deer flies from May to September.

There are about 3,700 species of horse flies in the world with 144 found in Canada. There are about 44 species of deer flies found in Canada, with most likely found on Manitoulin Island.

Horse flies and deer flies can be distinguished by their size. Horse flies are generally larger, black or dark brown, and can range in size from about 10 to 25 mm in length. Most deer flies are about 15 mm in length, or the size of a housefly.

The mouthparts of both types of flies are large and prominent, projecting downward and forward in front of the head. They have large, fan-shaped wings and are capable of rapid flight and flying long distances. The wings of horse flies are usually clear or uniformly cloudy whereas the wings of deer flies have dark bands or patches.

A stunning feature of horse and deer flies is their enormous compound eyes. Horse flies sport a psychedelic pattern of rainbow stripes whereas those of deer flies have spots or blotches. Eyes of horse flies are often green or purple with horizontal stripes.

Anyone out walking who has been attacked by these flies knows that they are persistent feeders. That is, they will quickly return to bite again if they are interrupted before they take a complete blood meal. They will even chase your car and can fly at speeds of up to 15 mph. However, unlike mosquitoes, female horse and deer flies will not enter buildings and will not feed on people indoors. 

Female horse flies primarily feed on stationary hosts, and they typically bite the legs and body, rarely on the head. Deer flies prefer to attack moving animals and usually bite on the shoulders and head.

They commonly fly several kilometres away from their larval sites, and as fishermen will attest, they are often attack people in boats who are hundreds of metres from shore. Horse flies make a low-pitched humming noise when they fly. They aren’t as noisy as mosquitoes, so you don’t always hear them coming.

Only the females feed on blood. Males have similar mouthparts but they are weaker and incapable of slicing into skin. They feed only on nectar.

Bites by both types of flies are painful as their mouthparts consist of two pairs of cutting ‘blades’ that lacerate the skin and cause the flow of blood from severed capillaries.

They then lap up the blood with the sponge-like region of their mouthparts. Bites from both types of flies take longer to heal than mosquito bites as they cut into the skin, rather than pierce it. There is often an allergic reaction to the salivary secretions which causes irritation and swelling.

The larvae of both kinds of flies are worm-like and live in muddy soils at the edge of ponds and streams. They are predators and feed on soft-bodied animals such as insect larvae and worms.

Larvae of some large species of horse flies feed on small vertebrates such as tadpoles, frogs, and toads. They lurk below the surface then impale their prey using fang-like mandibles dragging them underground to consume, like a scene from a horror movie.

Unfortunately for sun seekers, these flies are not inhibited by insect repellent, so the best way to prevent your day being ruined by painful bites is to cover up with layers of thick clothing. Deer flies are attracted to blue so wear clothing of a different colour.

Horse and deer flies are not known to be vectors of disease or capable of transmitting harmful disease-causing bacteria. Controlling them is nearly impossible. The use of insecticides to kill larvae is not an option because the vast majority of species develop in natural habitats where pesticides would cause environmental damage.