by: Eric Georgeson
SPRING 1995
"But bugs are good liars. In fact, lying is one of the things they do best--it's a way they keep from being killed".
Over 10,000 different kinds of insects are known to occur in Nova Scotia. But it wasn't until one summer day when I saw my daughter Melissa catching what appeared to be honey bees (with her hands), that I took an interest in flower or syrphid flies. Syrphid flies are members of the insect order diptera, or true flies. True flies have one pair of wings, while other flying insects have two pairs. In Canada, there are over 7,000 different types of flies in the diptera order. For easy identification, they are divided into 113 different families.
One family is the Syrphidae or flower fly. Each fly in this family has a fold in the wing that looks like an extra vein (Figure 1). It's possible this extra vein strengthens their wings, making them masters in hovering. One observes these flies near flowers with their wings beating so quickly (over 1000 beats/second) that they are invisible, as the body remains motionless in midair. When you get too close, they quickly dart off. Their ability to hover in midair is amazing, especially when one considered the interference they encounter from wind and air currents. It is thought that a syrphid fly remains in one midair location by visually fixing their position to one or two other objects near it.
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Figure 1: Syrphid flies have an extra vein in their wing that helps them hover in one spot in mid-air. |
Harold Oldroyd, in The Natural History of Flies, writes the following about syrphids and their hovering:
"If it were not to risk being branded as anthropomorphic and sentimental, it would be pleasant to think that syrphids enjoyed their hovering as much as we who watch them. After all, flies have to do something all their waking hours. Carnivores have to hunt, and their prey is hard to find, hunting may take up moat of their time. Flower feeders have an easier time. They can feed at any time, and unlike bees, they do not have to collect food for a never-ending succession of larvae in a hive or, worse still, to make honey for an insatiable bee-keeper. Many flower-feeding flies bask in the sunshine, while others strut about in an absurd way. Syrphidae spend much of their time, weather permitting, hovering in the air, with every appearance of enjoying it: or, if you prefer to express it differently, the state of hovering seems to be one of content, from which they are disinclined to depart."
In Nova Scotia, there are approximately 103 different kinds of syrphid flies. There are not well-studied, even though they are the most common visitors to flowers. Their life cycle is simple: they overwinter as larvae, in spring or early summer the larvae pupate for two weeks, and the new adults feed on flower pollen.
Depending on the type of syrphid, females lay their egg near aphids, plant bulbs, dung, or decaying flesh. Larvae feed during the summer to gain protein requirements and then usually bury into the soil or leaf litter to overwinter.
The feeding habits of larvae are a story themselves. The syrphid fly Microdone crawls into any hills to deposit its eggs. Ants don't take kindly to the female syrphid fly invading their nest so they drive her out.
Larvae that hatch from the eggs don't look like a fly larvae at all. They have been mistakenly described as a new type of slug. They look like an upturned pot, rounded on top and flat on the bottom. A fly larva in an ant's nest is something like a castaway living with a head-hunting tribe: as long as you don't draw attention to yourself, you get to keep your head. The Microdone has the same dilemma. Because it resembles a small pebble, ants pay no attention as it crawls over the floor of the any chambers, feeding on pellets dropped by ants. From ant hills to manure piles, and especially in the putrid puddles that form around them, the fly larva lives. It is called a rat-tailed maggot (Figure 2) because, in order to breathe under water, it has a tube on its rear end that sticks just above the water surface. This tube extends itself like a telescope up to 10 cm and then retracts, depending on the water depth. It feeds on abundant organic matter found in these ponds.
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Figure 2: The rat-tailed maggot has a long tube that helps it breathe when under water. |
A large number of syrphid larvae feed on aphids, which is of great interest to those who develop biological control methods for aphids. During an aphid outbreak in balsam fir about three years ago in Nova Scotia, a large number of syrphid flies were seen flying around aphid colonies. The most common species found was the American syrphid fly. This insect was observed laying eggs in aphid colonies. Then the eggs hatched, the larvae hunted for aphids. One sryphid larva can consume up to 1,000 aphids. Their potential as a biological control of would be worth the effort to do further research.
But why would one call these flies liars? What Melissa was catching that summer day was the common drone fly, a syrphid that looks so much like a honey bee that a bird would not try to eat it. How did Melissa distinguish the difference between it and real bees? When beeds try to hover they tend to bob up and down, while the drone fly remains fixed in midair. With the patience of a six-year-old, she simply observed the insects and grabbed the non-bobbers.
A number of other sryphids are flying bits of yellow and black. These are warning colours in the insect world. They mean: Beware! I taste bad, I am a poisonous or I sting! Most birds, as well as toads and frogs, seem to err on the side of caution. This serves syrphids well, although swallows are able to tell the difference between syrphid flies and wasps (perhaps they look for non-bobbers). Although some predators can be fooled some of the time, they can't be folled all the time.
Syrphid flies vary in size and shape (Figure 3). The adults live on the nectar of flowers. They have no piercing or crushing mouth parts and, apart from sending a false message of being dangerous, they have no way to defend themselves. But these liars are a joy to watch as they hover around flower beds.
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Figure 3: There are 103 different types of syrphid flies in Nova Scotia. These six are a sample of the different types. |