Flies, order Diptera, are the topic of the fourth segment of the Year of Pollinators scheme (see the segments on Bees, Wasps, and Lepidoptera). Flies are related to the Hymenoptera (bees, wasps, ants), Lepidoptera (moths and butterflies), and the Coleoptera (beetles), but are distinctive by having only two wings. Flies also possess a pair of special organs called halteres which evolved from hind wings into small knobby protuberances used for balance and for making quick aerial maneuvers (males in the parasitic order Strepsiptera are the only other insects which possess halteres, yet they lack functional mouthparts).
Compared to bees and wasps, they can be told apart by their large eyes which are usually oriented forwards while hymenopteran eyes are located on the sides. Flies also typically have short antennae, often between their eyes or on the front of their heads. There are quite a few bee mimics that are actually flies, and they can be identified by the stated characteristics and often by their flight behavior (i.e. hovering). Compared to bees they are sparsely hairy, the hairs being unbranched.
Flies are incredibly adaptable, and have found their way into nearly every environment on Earth. The larvae of many flies feed on rotting matter, be it soil, plant, or dead animals. Others feed on other insects or floral rewards. The focus of this blog is plant-pollinator interactions, so my emphasis will be on the flower visiting flies. The larvae of some feed on pest insects like aphids. Flies are also the most reliable pollinators in high altitude and tundra landscapes, even though bees are often found in those ecotypes, yet flies are able to withstand harsher conditions (low temps, inclement weather, etc.) and are important to the long-term survival and fitness of many alpine, tundra, and early blooming plants. Most of the flowers flies visit are shallow with easily accessible rewards, though there are long-tongued flies in many places in the world which pollinate flowers with deep corolla tubes. For the home gardener, most plants in the carrot family (Apiaceae) are great for flies. Below I will address other plants that have attracted flies.
Flies certainly aren't as glamorous or beautiful as butterflies or bees (except perhaps the syrphids), and their life cycles can send shivers down ones spine (figuratively speaking, I hope). Their ecological role, however, is indispensable, and they do many good things for us. This list is not conclusive, as there are surely twice or thrice the number of flies just in my back yard that I did not photograph or even see. Nonetheless, I hope you enjoy this chapter: Flies!
Genus EmpisThe genus Empis, aka dance flies, dagger flies, or balloon flies (the latter a tip of the hat to their odd mating ritual), are a widespread group with over 400 species worldwide and nearly 90 in the United States. They are often found near wooded areas in Spring, their larvae believed to require moist rotting detritus in the ground or under tree bark. As one would guess, they use their large rigid proboscis to eat prey, often smaller flies, but they also feed on nectar and are quite common on flowers. To attract a mate, a male dagger fly of many of the species will capture and kill a small insect and encase it in a small silk-like balloon to entice females to mate with them (see images of the balloons here). The flowers that I have seen them visiting are, in no particular order: Crocus spp., Prunus hybrids, Arctostaphylos viscida, Fragaria vesca, Calochortus tolmiei, and Lomatium spp.
|Empis on Crocus vernus, coated in pollen|
|Empis feeding on Prunus flowers|
|Empis, haunting your dreams|
|Empis on Lomatium|
Family CalliphoridaeBlow flies, or maybe better known as bluebottle flies (some), carrion flies, or cluster flies, are a familiar sight, and likely to be the image brought to mind when one thinks of a fly. Many of the species are very similar to the common house fly, Musca domestica, though are in a different family (Musca are in the family Muscidae). As the common names suggest, the flies in this family feed on rotting matter including carrion and feces, though they also visit flowers. They are short tongued flies, so they are restricted to flowers with easily accessible nectar. The larvae, maggots, are useful in the ecological spectrum by feeding on decaying matter, thus cleaning up the landscape.
The genus Pollenia feed mostly on plant products (fruit) and their secretions (nectar, sap), as well as feces and meat. The larvae, once hatched on the ground under dense vegetation where humidity is high, proceed to seek out earthworms by following the natural pores in the soil. After finding one, they burrow inside and feed on its insides before pupating.
|Bellardia (or perhaps the genus Melinda) on Laurus nobilis|
The genus is sold as pupae for pollination services. Flies are habitually used for production of carrot seed as bees don't like it unless there is nothing else in bloom. Along with houseflies (Musca domestica), bluebottles are often bagged within a single plant or placed in huge tents or greenhouses to keep them on the target crop, both to keep the flies from leaving and to prevent contamination from other plants which might produce unwanted hybrid seed. They are successful pollinators of a variety of crops including but not limited to vegetables and fruits in the following families: Amaryllidaceae (Alliums), Apiaceae (Daucus, Petroselinum, Apium), Asparagaceae (Asparagus officinalis), Asteraceae (Cichorium, Lactuca, Helianthus), Brassicaceae (Brassica napus, B. oleracea, B. rapa), and Solanaceae (Capsicum, Solanum) among others.
Lucilia has a somewhat disturbing, but useful life cycle. They can be agricultural pests due to their habit of laying eggs in wool. They are also known to be the first flies to lay eggs in a dead animal (including humans). The timing of their life cycle has been studied heavily, and their presence or absence as well as their state of growth can tell forensic specialists a lot about the condition and history about the corpse.
Genus ScathophagaScatophaga (the original, intended name for the genus) is comprised of dung flies, so-called due to their habit of laying eggs in- and pupating in dung. Nearly all genera in the family are predators of other insects, and probably do not contribute much in the way of pollination (like spiders, their occurrence on flowers is circumstantial). They have been reported as feeding on dung and nectar on occasion, but my observations of the insects on a Prunus tree represented otherwise. They were still, waiting for prey to come within reach, upon which time they would strike by pulling the small prey (other Dipterans, possibly small wasps or bees as well) to their mandibles.
|Scathophaga stercoraria on Prunus hybrid|
Scathophaga stercoraria stalking Apis mellifera
Family SyrphidaeThe most appreciated and cherished of the order Diptera must be the syrphids, affectionately known as the flower flies. This is because they spend much of their adult lives feeding on flowers, or seeking plants to lay their eggs on. The larvae of many genera seek and kill aphids and similar pests (some feed on scale insects), so the adults seek plants with aphids. This is why they are well known among gardeners as beneficial insects. The larvae of a few feed on plant matter, either living or dead, though most are predators. As adults they visit flowers for sustenance, and unlike wasps and butterflies they feed on pollen as well as nectar. In general they are bee/wasp mimics, perhaps to keep predators at bay or some other reason, though they do not bite or sting. They are fast fliers, and if approached (even by a photographer) will flee with haste.
|Tribe Bacchini (Platycheirus or Melanostoma) on Prunus hybrid|
|Toxomerus marginatus on Rosa elegantula|
|Sphaerophoria ♀ on Madia elegans|
|Pipiza on Hylotelephium spectabile|
I have only encountered this syrphid once, and it was mid September on the leaves of Hylotelephium spectabile, the flowers of which were covered in honeybees at the time. The larvae apparently feed on gall forming aphids.
This family is notable because of the large size of some of it's members, as well as many of them being large hairy bee mimics. The adults are generally considered to be beneficial flower-visiting insects. They are endoparasitoids of the larvae of many lepidopterans, dipterans, coleopterans, and enen antlions (neuropterans), meaning they lay eggs on or near a host (perhaps a caterpillar) and the larvae then feeds on the host from the inside until exiting horrifically at some point and pupating elsewhere, likely the soil or in rotting detritus. This can be beneficial if the host is a destructive pest.
|Villa sp. on Hypochaeris|
These flies are known as root-maggot flies since many of the larvae of the species feed on plant roots. Maggots also feed on a variety of others parts of the plants such as the leaves or seeds, or feed on other things like mushrooms, feces, or as kleptoparasites of hymenoptera. Most adults feed on nectar and a few species feed on pollen, making them valuable pollinators. They are similar to muscid flies, but are generally more bland in color and have more slender bodies.
|Family Anthomyiidae fly on Lomatium|
|Family Anthomyiidae fly on Lomatium|
One of the largest fly families around, the tachinids are comprised of nearly 1400 species in around 300 genera. Most are generally large and bristly, sometimes larger than honeybees but not as long. In general they are parasitoids of other insects and arthropods such as millipedes, spiders, and scorpions, though the most common hosts are caterpillars. In most cases, the tachinids lay their eggs on the outside of the body of the host. Upon hatching, the larvae burrows inside and eats the host from within. When fully grown, the larvae exits the host (think John Hurt from Alien) and pupates nearby. This can be beneficial if the host happens to be a destructive crop pest, however even if the host is not a pest the presence of the parasites in the region suggest healthy host numbers.
Adult tachinids feed on nectar, and their large hairy bodies probably enable efficient pollination. Being larger flies, I have seen them on a number of plants not typically associated with flies including: Agastache, Daucus, Marah, Allium (particularly A. amplectens), Thymus, and others.
|Archytas sp. on Thymus|
|Archytas sp. on Daucus|
|Gymnosoma, a very small tachinid|