With over eighty-five thousand species worldwide, flies form one of the most
diverse orders of insects, Diptera. Although a number of these
species are reviled as crop pests and carriers of disease, many are beneficial.
In this issue of Wings, we consider fly pollination, how mosquitoes
find mates, the diversity of picture-winged flies, and the conservation of
California's giant flower-loving flies.
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Our cover shows a male horse fly (family Tabanidae) in southern Africa, an area with an extremely diverse plan community but low bee diversity, in which the role of pollinators is likely filled by flies. Photograph by Edward S. Ross. |
Flies and Flowers, an Enduring Partnership The association between flies and flowers has a long history. Flies and beetles have been implicated as the primary pollinators of the earliest flowering plants. Many of these plants were also visited by bees and thrips as secondary pollinators, but pollination by birds, butterflies, moths and bats appeared only in more recently evolved plant families. Fossils dating back to the late Jurassic period, around one hundred and fifty million years ago, indicate the presence of Diptera with elongate mouthparts similar to the mouthparts of modern-day nectar-feeding species. These findings suggest a pre-Cretaceous origin of flowering plants, or possibly fly adaptations for feeding at the reproductive structures of other early seed plants. Studies of fossils show that some of the important flower-visiting fly families-such as flower flies (Syrphidae), bee flies (Bombyliidae), dance flies (Empididae) small-headed flies (Acroceridae), flower-loving flies (Apioceridae), and tangle-vein flies (Nemestrinidae)-were present in the late Jurassic or early Cretaceous. Several underwent extensive diversification during the mid-Cretaceous, simultaneous with the period of explosive radiation of flowering plants. Flies that visit flowers regularly are potential pollinators. The relationship between flies and flowers can be mutually beneficial, although the driving force for the bond differs for the two groups of organisms. Flowers need dispersal and delivery of pollen. It is advantageous for flowers to achieve efficient pollination with a minimal loss of pollen and a minimal expenditure of energy on nectar production. It is advantageous to the fly to maximize its nourishment while still managing to avoid predators, maintain thermal balance, and locate mates. Thus the evolutionary pressures that mold the relationship are different for flies and flowers and the relationship may not be equally beneficial to both parties. (scroll down for complete article, and a list of contents from
this issue) |
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Food is probably the major reward that flies obtain from
visiting flowers. Flower-visiting flies have sensory structures on their
antennae that allow them to detect floral scents and home in on flowers.
They tend to have large eyes with color vision, and the ability to detect
the glint of nectar in an open flower. Upon landing on a flower, taste
receptors on the legs detect the presence of food, and the proboscis,
also loaded with taste receptors, automatically lowers for feeding. Food,
however, is not the only reward that flies can find at flowers. Some flowers
offer warmth and others serve as rendezvous sites for mating, with males
of many species patrolling patches of specific plants, waiting to woo
females that arrive to feed. Female flies also visit flowers to lay eggs. At least seventy-one families of Diptera contain anthophilous (flower loving) species, and flies are pollinators of, or at least regular visitors to (where their pollination effectiveness has not yet been tested), more than eleven hundred species. Flies that are confirmed as pollinators differ widely in their effectiveness. Many have short tongues, sparse hair which holds little pollen, and a tendency to visit flowers of multiple species. Others are highly coevolved with the plants they visit. |
Among the most impressive, specialized fly-pollinators are the long-tongued flies in southern Africa, an area known for an extremely diverse flora concurrent with low bee diversity. Specialized tangle-veined flies (Nemestrinidae), bee flies (Bombyliidae) and horse flies (Tabanidae), may fill some of this void. For example, the showy flowers of Lapeirousia anceps, a plant in the iris family, have extremely long perianth tubes. In the northern and western range of the plant the tubes are between forty-five and seventy-six millimeters (1¾ inch to 3 inches) long and flowers are pollinated exclusively by Moegistorhynchus longirostris, a nemestrinid fly with an even longer tongue - three times the length of the fly's body, and the longest fly tongue on record. In southern parts of the plant's range, M. longirostris is absent. In this area floral tubes are shorter, ranging from twenty to thirty millimeters (¾ inch to 1 3/16 inch) in length, and it is likely that another nemestrinid, with a proboscis length up to thirty-three millimeters (1 5/16 inch), is the pollinator. Despite specialized morphologies, the relationship between the flowers and flies is not equal. Although certain Lapeirousia plants rely on specific long-tongued nemestrinid flies for pollination, the flies do not rely on the flowers for food and will often nectar at other flowers. |
 Horse Flies are like mosquitos in that the females feed on blood but the males chiefly on nectar and pollen. Male Esenbeckia delta, drinking nectar, photographed in Arizona by Edward S. Ross. |
In addition to food, floral heat can be the attractant that draws flies to flowers. Flies are ectotherms. Although some can generate heat by "shivering" their thoracic muscles, most need an external source of heat to warm their flight muscles sufficiently to be active. Heated flowers may encourage or prolong fly visits, which is likely to increase pollination success. There are two ways in which plants can increase the temperature within their flowers, thermogenesis or heliotropism.
Thermogenic plants generate metabolic heat and can maintain floral temperatures substantially above the ambient, sometimes by as much as twenty degrees Centigrade (thirty-six degrees Fahrenheit), irrespective of the weather conditions. They achieve this heat generation via a biochemical path that burns either fat or carbohydrates within cells. In addition to attracting pollinators, this may benefit the plant by stabilizing the temperature for development, protecting flowers during cold periods, increasing pollen tube growth, and assisting the broadcast of floral scent, which can be sweet or quite putrid. Putrid scents attract ovipositing flies that would normally lay their eggs on carrion.
Heliotropic plants move their flowers to track the sun as it moves across the sky. This maintains elevated temperatures two to seven degrees Centigrade (four to thirteen degrees Fahrenheit) above ambient within the flowers without the high energy expenditure associated with thermogenesis. In experiments, flies remained in the warm shelter of these flowers longer than they did in flowers that were tethered to prevent solar tracking.
The remarkable diversity of fly pollinators changes in importance in different regions and between habitats within regions. This diversity is likely to be negatively affected by development and urbanization. Recent pollinator declines, especially among bees, bats, and birds, have created concern among ecologists about a potential pollination crisis, because pollinators are required for seed production in the majority of flowering plants. Pollinator declines are suspected to result from loss of suitable nesting habitat and food resources for pollinators. Such losses may be due to various human activities including development, habitat fragmentation, the practice of monoculture for crop production, and the use of pesticides and herbicides. However, some human activities, such as converting forests into sunny meadows and planting garden flowers, can benefit pollinators (although not necessarily the original inhabitants of these areas). Pollinators with special food, nesting, and habitat requirements may respond differently to anthropogenic change than those with broader or more flexible requirements.
Studies are now underway to determine how development and land use practices affect the diversity and abundance of pollinators. However, anthropogenic changes in the pollinator fauna are difficult to document without adequate baseline data from comparable remote habitats. Insect populations of different species can fluctuate greatly, and independently from each other. With their complex life cycle, factors affecting larval habitats and larval food supplies could be more important in determining fly abundance than the availability of floral resources for adults. Despite these difficulties, clear differences in species composition associated with environmental change have been demonstrated.
For example, in Poland, Regina Bankowska conducted a study of syrphid flies and demonstrated significant declines in urban and agricultural areas compared with species-rich natural habitats. Both the urban and agricultural lands were dominated by syrphid fly species that were generalists with broad distributional ranges. Urban greenspaces were dominated by four species and had a significantly lower diversity than natural areas of oak-hornbeam forests that once characterized the region. In comparison, natural areas had more fly species, and showed distinct faunal changes as the vegetation changed across the landscape. The five-year study concluded that diversity drops significantly with urbanization, and that many syrphid species are unable to cross man-made barriers to enter into green spaces within cities.
Although to some people the mention of flies evokes images of
pesky houseflies buzzing at window screens and landing on picnic foods, flies
are really a diverse group and include many beneficial species. A large number
pollinate flowers ranging from common weeds to delicate orchids, including
flowers of food plants such as apples, peppers, mangoes, and cashews. As the
landscape is degraded and cities become the homes of generalist animals like
pigeons, rats, and squirrels, generalist fly species may come to dominate.
Recent initiatives to conserve open space and to provide wild corridors for
animals will probably benefit flies and other insects as wells as the larger
mammals and birds for which they are designed. Flies may never have the charisma
of birds and mammals, but habitat conservation and urban planning can help
maintain an interesting and diverse group of Diptera and other insects.
About the Author
Carol Ann Kearns is a faculty member in the Baker Academic Program for Natural
Sciences and EPO Biology at the University of Colorado. She is a co-author
of Techniques for Pollination Biologists (with David Inouye), and The Natural
History of Bumblebees: a Sourcebook for Investigation (with James Thomson).
Currently, she is working with Diana Oliveras and a team of students from
the University of Colorado to determine how development and land use practices
affect the diversity and abundance of fly and bee pollinators.
Other articles in this issue include:
--The Mosquito's Buzz, by Thomas Eisner
--Picture Winged Flies: Worth a Thousand Words? by May Berenbaum
--Giant Flower-Loving Flies, by Rick Rogers
--An obituary of Jo Brewer, co-founder of the Xerces Society, by Robert Michael
Pyle
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