The fly Megaselia scalaris (laboratory fly) is a member of the order Diptera and the family Phoridae, and it is widely distributed in warm regions of the world. The family members are commonly known as the "humpbacked fly", the "coffin fly", and the "scuttle fly". The name "scuttle fly" derives from the jerky, short bursts of running, characteristic to the adult fly. The name "coffin fly" is due to their being found in coffins, digging six feet deep in order to reach buried corpses. It is one of the more common species found within the family Phoridae; more than 370 species have been identified within North America.
Much of the anatomy of this fly is common to the family Phoridae. Megaselia scalaris flies are blackish, brownish, yellowish minute flies with a small, rather flattened head. Ommatrichia or hair-like processes, are located between the facets of the compound eye. The lower facial margin and clypeus are sometimes enlarged or distinctively shaped, especially in females. The antennae are sometimes hidden by first flagellomere.
The thorax is large and frequently described as having a “humpbacked structure.” Major bristles of body are characteristically feathered in this region; this is a characteristic unique to M. scalaris. Legs are attached to the thorax. They are usually well developed with a stout, enlarged, laterally compressed hind femur. A variety of sensory organs are present on the posterior surface. The tibia is frequently composed of short, closely set setae. The wings are also attached to the thoracic region. M. scalaris’ wings are usually large and fringed with short to long setae. Several tiny pore-like sensory organs, not associated with bristles or setae, are present at various places on the anterior veins. The distributional pattern is generally evident.
Egg and larva
The development of Megaselia scalaris fly is holometabolous, consisting of four distinct stages. These stages include: egg, larva, pupa, and adult. There are three distinct larval instars of M. scalaris. The third instar of development usually lasts longer than the first two because there are dramatic changes from a larva into a fly. The development of each life cycle depends on the environmental conditions in which the larva are feeding or being reared. It generally occurs “at 22-24°C, the first instar lasts 1-2 days, the second 1-2 days, and the third 3-4 days before pupation and a further 1-2 days before pupation." The larvae are usually very small, roughly between 1 and 8 mm in length.
Pupa and adult
The male Megaselia scalaris fly matures more quickly than the female pupa, emerging two days prior to the females. Emerging before the females gives the males the advantage to feed allowing their sperm to mature and be ready by the time the females emerge. Adult Megaselia scalaris reproduce by means of oviposition. The females lay relatively large eggs for their size due to the extended incubation period of the eggs.
Many of the flies within the family Phoridae prefer nectar as an energy source; however, Megaselia scalaris is an omnivorous species. It has been recorded feeding on plants, wounds, and corpses. Protein food sources are preferred by the females preceding maturation of their eggs. All meals must be a fluid in order for the flies to access the meal because Megaselia scalaris has sponging mouthparts. This is a characteristic common to the family Phoridae.
Both male and female species have five pairs of sharp teeth. These are not used in retrieval of a food source, like a piercing mouthpart, but are instead used to aid digestion and breakdown of nutrients. Human cases involving skin inflammation are likely due to these teeth. It is important to note the distinction that while Megaselia scalaris can feed on blood meals, the teeth are not used to puncture the host. The blood must be found on the body as an exudate. One theory to the evolution of these teeth is that Megaselia scalaris uses them in order to exit their pupal casings.
Megaselia scalaris' optimal culture temperature is 28 degrees Celsius. They are common in many areas but thrive predominately in moist unsanitary vicinities such as dumpsters, trash containers, rotting meat, vegetable remains, public washrooms, homes, and sewer pipes. Although referred to as scavengers, adults are known to feed primarily on sugars. The larvae, however, depend on moist decaying plant or animal material and feed on a wide range of additional decaying material. "The larvae display a unique behavior of swallowing air when exposed to pools of liquid. This intake of air allows them to float, and may prevent drowning during flood conditions in their normal habitat."
Importance to forensic entomology
Megaselia scalaris are important in the study of forensic entomology because evidence derived from the lifecycle and behavior of these flies is useful in both medicocriminal and abuse/neglect cases and is admissible in court.
Megaselia scalaris are small in size; this allows them to locate carrion buried within the ground and to locate bodies concealed in coffins. They can travel 0.5 m in a four-day period. They lay their eggs on carrion to provide food for the hatched larvae.
Often, Megaselia scalaris may be the only forensic entomological evidence available if the carrion is obstructed or concealed in a place that is hard for other insects to reach. Larger flies are not always able to reach the carrion. Calculations involving M. scalaris can result in an insect colonization time that can be used for a postmortem interval, which may help establish an estimated time of death. M. scalaris are classified in a secondary forensic role because they prefer older decaying carrion.
Evidence collected by forensic entomologists involving Megaselia scalaris has been used to demonstrate in court that caretakers have neglected the care of their elderly patients. Megaselia scalaris is also involved in cases of myiasis. Megaselia scalaris larvae found on a body can be used in court as a tool to show "time of death" or "time of neglect".
Current and future research
Megaselia scalaris is commonly used in research and within the lab because it is easily cultured; this species is used in experiments involving genetic, developmental, and bioassay studies. Research has also been done on the unique neurophysiology and neuromuscular junction within this fly, giving it its characteristic "scuttle" movement. In comparison to Drosophila melanogaster, M. scalaris has decreased excitatory postsynaptic potentials (EPSPs) and facilitation of EPSPs in response to repetitive stimulation. With such a wide range of food sources, the larvae can be considered facultative predators, parasitoids, or parasites.
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