The vast majority of flies are upstanding citizens in the insect community. They feed on nectar – not scat, otherwise known as poo – and hence, they share the space on the flower heads with bees. They are also pollinators.

Since flies, at least according to many birds and lizards, are a tasty meal, they need to protect themselves. Some do this by using mimicry to look like a bee, as well as having superb vision, at least to pick up motion. The challenge is to identify which of the insects (Fig. 1 and Fig. 2) shown here is a fly and which is a bee.

Bees and flies belong to two different orders in the overall class of insects (phylum arthropoda, class insecta), all of which have an external skeleton, three pairs of jointed legs (six total) and three distinct body segments.

Bees belong to the order hymenoptera, which also includes wasps, hornets and ants. The key common factor between all of these insects is that they have a narrow waist. For the larger bees, the narrow waist might not always be apparent.

Fig. 1. Note the large eyes that meet in the middle and the total lack of a distinct waist. Also, the antennae are virtually invisible. Arrows point to each antenna. They are hair-thin and short.

Like insects that have wings, they will have four wings, two on each side, but commonly they overlap such that the two wings on each side can appear as one, particularly when on a flower. When flying, they move too fast to easily count the wings.

Flies, on the other hand, belong to the order diptera. This means they have only two functional wings, one on each side.

Actually, if one looks closely, they have a second pair of wings that are very tiny (Fig. 3). These are called halteres. They provide balance and stability while flying, similar to the tail rotor on a helicopter. Flies have a thick waist with no obvious narrowing between the thorax and the abdomen.

Fig. 2. Note the large eyes that meet in the middle; the antennae are also tiny, and there is no distinct narrow waist. This is a Mexican hover fly, a mimic of the carpenter bee. It is large, shiny and black. In neither photo can one see the halters, or the small hindwings that help with stabilization during flight.

As noted, it can be difficult to see the waist on a large bee, depending on the angle in which it is viewed. It can also be difficult to determine if there is a total of four wings (bees) or only two (flies).

Something that is readily visible is the antenna, particularly if one uses close-focus binoculars or a good camera. Both orders have a pair of antenna originating on their head, but flies have tiny, thin antennae, while bees and its cousins have relatively long antennae, and these are relatively easy to see.

Another difference is the eyes. These large eyes are made of of thousands of smaller, individual eyes, the result being called a compound eye. A fly’s eyes are so large that they come together in the midline, providing for superb motion detection, whereas a bee’s compound eyes are smaller and clearly separated from one another.

Fig. 3. This is also a fly that is mimicking a wasp, but it was photographed in Ecuador. The two eyes do not come together in the midline, but the antennae are small and it has only a single pair of wings for flying. The halteres are readily visible, placing this insect in the order diptera.

If you’re able to look closely, bees also have three small simple eyes (ocelli) on the top of their head between the two large compound eyes (Fig. 4).

Both Fig. 1 and Fig. 2 are photos of flies. Fig. 1 is a mimic for wasps such as the yellow jacket, while Fig. 2 is a mimic for carpenter bees.

Remember, there are no absolutes. One cannot totally rely on a single characteristic, and the classification is often more complex than presented here. But these guidelines will usually hold.

Paul A. Levine is a docent-naturalist at Placerita Canyon Nature Center and an avid butterflier.

Fig. 4. This bee, with large antennae, is a little smaller than the wasp-mimic in Fig. 3. The eyes are widely separate, and the three ocelli are readily visible. There is also a distinct waist between the thorax and the abdomen.

Fig. 5. A carpenter bee. The hover fly in Fig. 2 is about the same size. One gets an impression of a waist, but it is not clearly seen here. The antennae are prominent, and the eyes do not meet in the midline. Bees tend to have “hairier” legs compared to flies to enable them to hold onto the pollen.