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2 Maggots

On Flies and Maggots

The Greek philosopher Aristotle (384-322 BC) named the fly “Diptera”. The Greek word “dipteron” means “two-winged”, referring to the single pair of functional wings that distinguish the fly from virtually all other insects. When working on his system of taxonomic classification of living organisms, or Systema naturae, published in the 18th century, Carl von Linné (Linnaeus, 1707-1778) adopted Diptera as the taxonomic name for the order of insects to which all true flies belong.

The origin of the dipterans is unknown. The oldest known fossils date back to the Triassic period and are some 210 to 220 million years old. These relicts mainly consist of the wings of adult flies. Signs of other stages of early fly development are practically non-existent.

The coexistence of flies with humans and domestic animals (synanthropy) has left notable marks in the history of humankind. Flies gained a reputation as pests, parasites, and carriers of harmful diseases. Written records and cult objects surviving from various periods testify to the explosive multiplication of fly populations during wars, famines, and other catastrophes. In all of these periods, people's attention was most strongly drawn to the seemingly apocalyptic plagues of flies that occurred throughout history.

The historical narratives cited below underline the timelessness and the global impact of the fly problem. The best known reference to plagues of flies is probably that in the Old Testament book of Exodus:

“This is what the Lord says: Let my people go so that they may worship me. If you do not let my people go, I will send swarms of flies on you and your officials, on your people and into the houses. The houses of the Egyptians will be full of flies, even the ground where they are. And the Lord did this. Dense swarms of flies poured into Pharaoh's palace and into the houses of his officials, and throughout Egypt the land was ruined by the flies.” Exodus 8:20-21 and 8:24, NIV Version, 1984

Fly populations multiply rapidly in warm weather and on corpses. The military physician Ambroise Pare (1510-1590), who reported on the Battle of Saint Quentin (1557), described this phenomenon as follows:

“We saw more than half a league round us the earth all covered with the dead; and hardly stopped there, because of the stench of the dead men and their horses; and so many blue and green flies rose from them, bred of the moisture of the bodies and the heat of the sun, that when they were up in the air they hid the sun. It was wonderful to hear them buzzing; and where they settled, there they infected the air, and brought the plague with them.”

(Quoted from: Ambroise Pare, Journeys in Diverse Places. The Harvard Classics. 1909-14)

Development Cycle of the Fly

There are over 100 000 species of flies, representing a variety of shapes and sizes (morphology), of habitats, and of behaviors. Yet they all have in common a 4-stage “complete” (holometabolous) metamorphosis, by which they develop through the stages of egg, larva, pupa, and finally adult (Fig. 1). As an example, the blowfly life cycle will be described in more detail. Female flies lay masses of up to 200 eggs, usually on dead bodies and decaying meat, but also on open wounds. Flies have special sensory organs that enable them to immediately recognize decayed flesh that is suitable for feeding and egg laying. The adult female unfurls its ovipositor (Fig. 2) and lays (“blows”) hundreds of its eggs on the meat. Hence the name blowfly.

Fig. 1 Immature fly stages. A: Eggs of Phoenicia (= Lucilia) sericata. B: Hatching first instar larvae

A female fly can lay up to 3 000 eggs in her lifetime. The number of eggs laid is determined by the size of the female and by the quality and quantity of food she consumes. On a protein-rich diet, a female fly may start to lay eggs as soon as five days after emerging from the pupal case. The time required for egg and larval development is mainly determined by ecological factors, such as environmental temperature and humidity. Fly eggs generally hatch into maggots in 12 to 24 hours, the maggots mature to pupae approximately one week later. Normally pupae transform into adult flies within one to three weeks; but under unfavorable conditions, it can take weeks or even months for this process to occur.

Fig. 2 Adult female Phoenicia (= Lucilia) sericata with projecting ovipositor.

The adult fly is whitish-gray in color when it initially emerges from the pupal case. The cuticle (external chitinous shell) then stretches, hardens, and dries, resulting in the typical metallic appearance of the adult blowfly. The life span of a fly is roughly one to two months. Accordingly, four to eight generations of flies can develop during the major breeding months of May through October.

Fig. 3 Blowfly Iifecycle

Transmission of Disease by Flies

Flies have a great impact on human and animal health. Most of the known fly species are harmless to humans, but around 11 000 species can cause disease in one or more ways: 1) as vectors, carrying (and sometimes breeding) parasites within their own body, which they then inject into their host as they suck mammalian blood; 2) as fomites, mechanically transporting infectious bacteria or viruses from one site to another as they visit dumpsters, excrement, and prepared food; or 3) as parasitic larvae (myiasis), living and feeding on the tissues of live vertebrate hosts. The bloodsucking flies are responsible for the transmission of a number of harmful diseases, such as malaria, filariasis, onchocerciasis, leishmaniasis, and African trypanosomiasis. They transmit microbial pathogens or parasites that enter the host skin or circulation when the flies bite.

Phaenicia (Lucilia) sericata (Green Blowfly)

Blowflies (Calliphoridae) “blow” (lay eggs) on rotting organic material, especially animal tissue. The female fly has a keen sense of smell that helps her find a suitable host, such as an infected wound or decaying meat, located even very long distances away.

Members of the genera lucilia and Phaenicia are commonly known as green blowflies (greenbottle flies) because of their iridescent golden-green color. The eyes of the female fly are wide-spaced and are separated by the forehead (Fig. 4), whereas those of the male are close-set. There are many different species within these two genera, each with their own distributions and habits. There may even be multiple strains or subspecies within each species, although it has not been possible to identify strain differences morphologically. Lucilia and Phaenicia are synanthropes, living in close association with humans.

Fig. 4 Phoenicia sericata (green blowfly, greenbottle).

The earliest blowflies (Calliphoridae) probably fed exclusively on decaying flesh from the bodies of dead vertebrates, as described hundreds of years ago in one of the first European medical textbooks, Hortus Sanitatis (Mainz, Germany, 1491).

Larvae of Phaenicia sericata

The larvae of Phaenicia sericata (classified by some as Lucilia sericata) have a typical maggot shape, i. e., they have a sleek, tapering front end (head) and a blunt, flattened back end (tail). The body of the maggot consists of 12 segments without any clear division between the head and the other body segments. A furrow divides the head into a left and a right lobe; the mouth is situated inferiorly, at the base of the furrow. The complex cephalopharyngeal skeleton, the mouth hooks of which are visible externally, is operated by a strong muscular apparatus. The cephalopharyngeal skeleton helps the maggots move about. Annular spicules on each segment of the body keep the maggot from sliding backward.

Maggots breathe through apertures called spiracles, which are located at the anterior and posterior ends of their body. The posterior respiratory spiracles of growing maggots are often mistaken for eyes.

The head of the blowfly maggot contains primitive sensory organs that only allow the maggot to distinguish between light and darkness. Unlike the adult fly, maggots always move away from light (negative phototaxis). Several maggots unite to form feeding communities. The maggots feed by dipping their front end into the liquid nutritive substrate while breathing through their posterior respiratory apertures.

Fig. 5 Front end of a blowfly maggot as seen under a scanning electron microscope.

Fig. 6 Maggot anatomy

The fact that maggots require air to breathe should always be borne in mind when dressing wounds with live maggots.

Digestive enzymes are continuously produced by two labial glands (salivary glands) and secreted into the surroundings. A powerful pharyngeal pump sucks in the liquefied, bacteria-laden food, which is then passed through a filtering system that concentrates the nutrients roughly five-fold. This feeding strategy allows the blowfly maggot to ingest a quantity of food equivalent to half its body weight within five minutes.

The larvae of P. sericata have a prominent crop that can hold and store large a quantity of food until it is later needed. The glands that produce the digestive enzymes, the powerful suction apparatus, and the extremely distensible crop are evolutionary adaptations that reflect the main purpose of the larval stage: namely, to ingest a large quantity of food as quickly as possible, storing some of it for later use during the migratory and pupal stages. In its short life span of nonstop feeding, a single maggot can process as much as 0.3 g of nutrient substrate, or necrotic tissue, or pus and wound fluid.

Fig. 7 Average increase in weight of fly larva from the time of hatching until pupation. Under optimal conditions the weight can increase to 90 milligrams.

Fig. 8 External anatomy of a blowfly maggot. A: Head segment with protruding mouth-hooks and sensory organs; each larval segment is divided by a ring of annular spicules B: Anterior breathing apperture (anterior spiracle). C: Posterior spiracles.

The maggot's intestinal tract is ideal for the optimal resorption and utilization of nutrients. In the case of P. sericata, the larval intestine is five times the body length of the maggot. Ingested nutrients pass through the intestine at a rate of several millimeters per minute. This high metabolic capacity is reflected by the maggot's enormous growth rate. Under optimal conditions, a blowfly maggot can increase its weight one hundred-fold within a few days.

Fig. 9 Internal anatomy of a blowfly maggot. (a) Cephalopharyngeal skeleton with mouth hooks (sclerite); (b) crop; (c) ganglion (central nervous system); (d) salivary glands; (e) tracheae (respiratory system); (f) cardia (proventriculus); (g) midgut (intestine); (h) adipose body.

The maggot's energy stores are essential for fueling the process of metamorphosis into an adult fly, which occurs during the inactive pupal phase. The maggot also has another remarkable capability that is useful during metamorphosis: auto-disinfection, that is, the ability to rid itself of bacteria and other harmful micro-organisms. Bacteria ingested during the growth phase must be eliminated before pupation because they could otherwise multiply, infect, and kill the pupa.