What Is The Morphology Of Black Flies

The life stages of a typical muscid fly consist of egg, larva, pupa, and adult (Fig. 14.1). Eggs are similar to those of closely related families. They may occur singly or in groups. They are generally creamy in color, 0.8-2.0 mm long, elongate/ovate in shape, and concave dorsally where two ribs form hatching pleats (Fig. 14.2).

Larvae of muscid flies and related families are known as maggots, and there are three instars in all species. The body is tapered, with the head and mouth hooks at the pointed end and the anus and spiracles at the blunt end (Fig. 14.1). The head is greatly reduced; it lacks eyes and has minute antennae that resemble papillae. The thorax is legless and has a pair of lateral prothoraeic spiracles. There are eight segments of the abdomen, and each is marked ventrally with transverse rows of spines forming creeping welts.

Although the head lacks a sclerotized capsule, it is supported internally by a sclerotized cephalopharyngeal skeleton (Fig. 14.3). This complex structure is partially visible through the integument of live larvae and is best visualized in cleared, slide-mounted specimens. The size, shape, and arrangement of elements of the cephalopharyngeal skeleton are useful in the identification of larvae. Paired mouth hooks, reduced from ancestral mandibles, can be extended and retracted from the oral cavity. They help in crawling, burrowing, and tearing into food and other substrates. Internal dental sclerites, accessory sclerites, and pharyngeal sclerites make up the rest

Morphology Black FliesStable Fly MorphologyStable FlyHorn Fly Sting
FIGURE 14.12 Stable fly (Stomoxys calcitrans) feeding on human. (A) Fully fed female, abdomen distended with blood; (B) resultant bite reaction in form of welts. (Photos by Elton Hansens)

sources such as nectar from plants and honeydew from sap-sucking aphids and scale insects. The species that feed on vertebrates obtain the bulk of their protein from blood, serum, saliva, mucus, and lachrymal secretions. Both sexes of the biting horn fly and stable fly obtain nearly all their nutrients from blood.

The feeding behavior of a nonbiting fly differs from that of biting species. A nonbiting fly opens its labella and presses it against the substrate. If the food is not liquid, the fly releases enzyme-laden saliva and repetitively opens and closes the labella to scarify solid food into the saliva. The suspension is then drawn along the pseudo-tracheae into the food canal. Feeding by nonbiting flies can be characterized as a process of salivating, scrubbing, and sucking; they cannot physically penetrate skin. When a biting species feeds (Fig. 14.12), it presses its proboscis against skin and rapidly opens and closes the labella, directing the prestomal teeth in a downward and ourward rasping motion. Once the skin is penetrated, the teeth anchor the proboscis while blood flows into the subsurface lesion and up the food canal.

All of the important muscid flies are anautogenous meaning that females require protein to complete their first gonotrophic cycle. Protein stimulates yolk synthesis and allows maturation of eggs. In the nonbiting species, eggs in the ovarioles mature in synchrony 2—5 days after protein is first obtained. Development of a subsequent batch is arrested hormonally until the preceding batch has been laid. There are corresponding cycles of attraction to different substrates, first to sources of protein and then to oviposition sites as eggs mature. In the horn fly and stable fly, eggs develop asynchronously, and feeding and oviposition are distributed more evenly in time.

Behavior associated with mating differs among species, Males of Fannia species and some of the sweat flies and garbage flies hover in swarms, usually in locations shaded by trees or the roofs and eves of buildings. These males are attracted to females that fly into the swarm. Once coupled, a mating pair will fail to the ground and complete copulation. Males of the other muscid flies do not form swarms. Instead, they generally perch or rest in sunny locations on substrates such as tree trunks, fence posts, and rocks, and the males intercept passing females. Females of all the important muscid flies typically store enough sperm from a single mating to fertilize all the eggs they can produce during the remainder of their lives.

Activities and locations of adult muscid flies vary markedly with time of day. All the important species are active during daylight hours, and almost all are inactive at night. Activities include flying, host location, feeding, mating, and ovipositing. Sight and olfaction are used to locate hosts and oviposition substrates. Most muscid flies are exophilic, being reluctant to enter buildings, A few species are more endophilic and will enter buildings. Species that feed on animals may be on a host as briefly as a few minutes, just long enough to obtain available foods. The flies leave their hosts when replete and rest in the surrounding environment while digestion proceeds. Because feeding times are much shorter than digestion times, the adults on a host at any instant are likely to be only a small fraction of all the adults present in the host's environment,

Muscid flies apparently choose daytime resting sites, in part, according to their needs for thermoregulation. They rest in sunny sites when the air temperature is below about 20°C and in shady sites when temperatures exceed about 30°C.

An exception to the generalized pattern of daytime activity and host-visiting behavior occurs in the horn fly. Once a host is located, the adult remains on its host almost continuously, except when disturbed or laying eggs. Horn flies feed and oviposit at all hours of the day and night.

The flight range of muscid flies is extensive. Detectable numbers of all the important North American species have been collected more than 5 km from known or presumed points of origin. Large numbers of stable flies can appear on beaches 10 or more miles downwind from the nearest likely breeding sites.

The seasonalpatterns in abundance and age structure of adult subpopulations vary among species, years, and locations. In localities with cold winters, population growth outside buildings is restricted to a distinct breeding season—the warmer, wetter months of spring, summer, and autumn. In these cases, populations grow to a single peak of abundance, normally in early autumn. A notable exception is the house fly, which can breed continuously in heated buildings. In warmer climates, the breeding season for most species is longer and may be continuous year-round. For example, adults of the house fly, Fan-nia species, and horn fly occur throughout the year in the southeastern United States and southern California. Densities of adults have two seasonal peaks, with growth phases in spring and autumn separated by periods of decline during summer and winter.

Most muscid flies of medical/veterinary importance are multivoltine, developing through two or more generations per breeding season (Table II). These generations usually overlap, so recruitment of new adults is continuous; eggs, larvae, pupae, and adults of all ages are present simultaneously throughout most of the breeding season. Population growth within the breeding season is influenced by availability of breeding media, by weather and its effects on survival of immature stages, and by the fly reproductive rate. Survival of larvae is enhanced if their breeding habitat remains wet enough to support filter feeding, yet dry enough to allow aerobic respiration. Suitable moisture levels are about 30—75%. Substrate moisture is critical because the saprophagous larvae feed by filtering particles suspended in their medium.

Muscid flies overwinter in different ways (Table II). The house fly and the stable fly breed continuously in frost-free southern regions of North America. Breeding by these flies is restricted to the warmer months in more northern latitudes, because they lack a stage that can endure temperatures below freezing for much more than a day. It was once thought that these two filth flies, lacking a freeze-tolerant life stage, died out each winter in temperate latitudes and were repopulated each spring from milder regions. However, it is now known that local populations persist through winter in protected, semi-heated substrates associated with humans and livestock. Regional repopulation does occur, however, with the bush fly in Australia, where immigrants disperse southward from more northerly latitudes that remain warm during winter (Hughes, 1977).

Some other muscid flies of medical/veterinary importance overwinter in diapause, a state of developmental arrest typically associated with a tolerance for freezing. The face fly and Muscina species overwinter as adults. In autumn, these flies enter hibernacula, such as occur under bark of dead trees and siding on buildings, and emerge the next spring to begin reproduction. The horn fly, in contrast, overwinters in temperate regions as a diapausing pupa. Garbage flies, Fannia species, and sweat flies are thought to overwinter as larvae, but further study is needed to determine if they exhibit a true diapause or are in a simpler state of cold-tolerant quiescence.

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  • duenna
    What is the morphology of black flies?
    2 years ago
  • lalia
    Does a black fly have spiracle?
    2 years ago

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