TRANSVERSE FLOWER FLY
Eristalis transversa
FLOWER FLY FAMILY (Syrphidae)

Description
This flower fly is a bee mimic and is approximately 3/8 inch in length. It has
large reddish-brown eyes, clear wings and yellow legs. The thorax is black with
wide gray bands. The abdomen is black with bright yellow bands. The antennae
are short. The flies in the genus Eristalis are very similar to each other, but
the distinguishing characteristics of transversa are the gray color on the front
half of the thorax and the bright yellow scutellum.

Occurrence
Adults can be seen nectaring on flowers in late summer.

Comments
There are 23 species in North America. The larvae of the genus are known as
"rat-tailed maggots" for the long breathing tube they can extend to the surface
of the water. The larvae live in polluted water or wet carcasses.

http://fnanaturesearch.org/index.php?option=com_naturesearch&task=view&id=1055&cid=123 

Eristalis transversa - Transverse Flower Fly

Female http://bugguide.net/node/view/17282/bgimage
The combination of yellow scutellum (盾片) and bicolored thorax (gray front half, black rear half) is diagnostic. The scutellum is usually bright yellow (at least in males; it may be duller yellow in some females such as this one).

Male http://bugguide.net/node/view/20570
The combination of yellow scutellum and bicolored thorax is diagnostic. Compare this male's bright scutellum with the dull one on the female above.

There are roughly 200,000 varieties of animal pollinators in the wild, most of
which are insects.[2] Entomophily, pollination by insects, often occurs on
plants that have developed colored petals and a strong scent to attract insects
such as, bees, wasps and occasionally ants (Hymenoptera), beetles (Coleoptera),
moths and butterflies (Lepidoptera), and flies (Diptera). In zoophily,
pollination is performed by vertebrates such as birds and bats, particularly,
hummingbirds, sunbirds, spiderhunters, honeyeaters, and fruit bats. Plants
adapted to using bats or moths as pollinators typically have white petals and a
strong scent, while plants that use birds as pollinators tend to develop red
petals and rarely develop a scent (few birds have a sense of smell).

Insect pollinators such as honeybees (Apis mellifera),[3] bumblebees (Bombus
terrestris),[4][5] and butterflies (Thymelicus flavus) [6] have been observed to
engage in flower constancy, which means they are more likely to transfer pollen
to other conspecific plants.[7] This can be beneficial for the pollenisers, as
flower constancy prevents the loss of pollen during interspecific flights and
pollinators from clogging stigmas with pollen of other flower species.[8]

http://en.wikipedia.org/wiki/Pollination 

Bumble Bee Mimic -

Volucella bombylans is a large species of hoverfly. It is larger than most hoverflies at 20mm long, and looks something like a Bumble bee with a furry black, yellow and/or white body, but is given away by its head and eyes which show it is a true fly, like a Blow-fly. It occurs in several forms each of which mimics a species of Bumble bee. It can be found even in urban gardens, sunning itself on a leaf. It is a fast flier.  wiki

http://www.microscopy-uk.org.uk/mag/artmay07/cd-hoverflies.html

Hoverflies (Syrphidae)

It is always easy to tell males and females apart. Like all other flies the males have much bigger eyes. These eyes almost touch each other in the middle. Females have much smaller eyes, placed further apart.

To the left the Striped Hoverfly (Helophilus pendulus), to the right the almost identical Helophilus trivittatus.

http://www.gardensafari.net/english/hoverflies.htm

myinsectspiderlist.htm