An Introduction to Medical Entomology
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Also See: <Key To Simuliidae Genera>
A family of blood-sucking flies, known as the Simuliidae, consists of small flies with a hump-backed appearance and with broad wings. The spindle-shaped larvae live in running water and are characterized by the possession of thoracic prolegs and an anal pad provided with setae by means of which they adhere to rocks, etc., in the rapidly flowing water of their environment. Still included in the suborder Orthorrhapha are the flies with short antennae, the Brachycera. Though included in this system with the Orthorrhapha, characters on their wing vein characters indicate a close relation with the Cyclorrhapha. In general, the basal joints of the antennae are larger than the terminal ones, these being reduced in number as compared with the nematocerous condition. The maxillary palpi are porrect (not pendulous). Their larvae are hemicephalous (head capsule incomplete posteriorly), with vertically biting mandibles, and the pupae are free and spiny (Borradaile & Potts, 1958).
There are four genera among over 2000 species that are of principal importance to humans: Simulium, Prosimulium, Austrosimulium and Cnephia. However, Simulium is the most important as many are vectors of disease. Service (2008) reports that in Africa the main concern is with the Simulium damnosum complex and the S. neavei group. Central and South America have Simulium ochraceum (S. ochraceum is similar), S. metallicum and S. exiguum complexes that transmit parasitic nematodes that can cause onchocerciasis. In Brazil Simulium amazonicum is a vector of Mansonella ozzardi, which is a filarial parasite.
The common name for these flies varies, with "Black Flies" being used in some countries or "Sand Flies" in others. In New York State blackflies are so pestiferous that a special early summer festival is held in their honor.
All species are tiny and are recognized by a humped thorax. Further details of their morphology are given in Service (2008).
The dark colored eggs are coated with a substance that allows them to adhere to rock in flowing streams. Their habitats are usually in rapidly moving streams but sometimes only slow moving water can suffice. Female flies lay their eggs over the water, but sometimes dive into it to oviposit. In cold areas the eggs may enter diapause.
Larvae pass through 6-11 instars, with the final instar being about eight times larger than the adult. The larvae are sedentary for a long time on submerged rocks and vegetation, adhering by a secreted glue-like substance. However, if disturbed they may dislodge and be carried downstream but still attached to some substrate by a long silk-like thread. They eventually reattach to some object downstream. When matue the larvae make a cocoon into which they pupate. The time they remain as pupae varies with temperature. Service (2008) noted that in Africa and Asia blackflies show variable behaviors during their larval and pupal development. Adults emerge and crawl onto vegetation from where flight begins.
Adult blackflies feed on plant juices and other sugary substances, but only the females take blood meals. Biting is outdoors during daytime. Depending on the species there is considerable variation in host preference and where they prefer to bite on the body. Activity is enhanced during cloudy, stormy weather. Many species of blackfly feed preferably on birds while others show a preference for mammals. The distance that adult blackflies will range varies with the species and windy weather. In colder climates biting activity is seasonal, whereas in the tropics activity is all year with heightened activity during rainy seasons.
Service (2008) reported that in Africa and Asia blackflies have a curious aquatic association where East African larvae and pupae of S. neavei are not found on submerged rocks or vegetation but on other aquatic arthropods like the nymphs of mayflies and some crustaceans.
MEDICAL IMPORTANCE OF BLACKFLIES
Allergic responses to the bites of blackflies can be very serious, but differ among persons and animals. Side effects can produce swollen lymph glands, aching joints, fevers and headaches. Onchocerciasis or "River Blindness" that is caused by the filarial parasite Onchocerca volvulus is serious in some parts of the world. A positive attribute of blackflies is their use in medical research because of the very large polytene chromosomes found in their salivary gland cells. Some studies have already shown that there are 55 cytoforms in the Simulium damnosum complex, which then facilitated the designation of distinct species.
Repellents and clothing treated with pyrethrum compounds provide some protection against blackflies. The application of insecticides to breeding areas is widely practiced with World Health Organization involvement, but resistance is a universal problem that makes this approach ineffective after prolonged treatment.
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