Megaselia scalaris
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Diptera
Family:	Phoridae
Genus:	Megaselia
Species:	M. scalaris
Binomial name
Megaselia scalaris (Loew, 1866)[1]

The fly Megaselia scalaris 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".^[2] The name "scuttle fly" derives from the jerky, short bursts of running, characteristic to the adult fly.^[2] 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.^[2]

Taxonomy

Megaselia scalaris was described by the German entomologist Hermann Loew in 1866.^[1]

Description

Adults of this species are about 2 mm long and yellowish with dark markings.^[3] The labellum and labrum have trichoid and conical sensilla, and the labellum's ventral surface has five pairs of sharp teeth.^[4] The hind femur has hairs below its basal half and these are shorter than hairs in an anteroventral row on the distal half. The hind tibia lacks a clearly differentiated row of spine-like antero-dorsal hairs. There is a pair of translucent wings, in which vein 3 is not or barely broader than the costa.^[3]

In males, the labellum has a dense covering of microtrichia,^[4] the bristles at the tip of the anal tube are longer than the longest hairs of the cerci, and the longest hair of the left side of the epandrium is almost bristle-like.^[3] In females, the tergite of the sixth abdominal segment is short, narrow, shiny, and extends laterally on the segment, unlike tergites of preceding segments.^[5]

Larvae of this species are pale, legless and covered in rows of short spines. The anterior end has the mouthparts, which look like a pair of sharp spines and are darker than the surrounding tissue. The posterior end has a pair of spiracles.^[3][6]

Life cycle

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 larvae 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."^[7] 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 by the time the females emerge.^[8] 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.^[9]

Feeding habits

Many of the flies within the family Phoridae prefer nectar as an energy source; however, Megaselia scalaris is an omnivorous species.^[10] It has been recorded feeding on plants, wounds, and corpses. Protein food sources are preferred by the females preceding maturation of their eggs.^[10] All meals must be a fluid in order for the flies to access the meal because Megaselia scalaris has sponging mouthparts.^[10] This is a characteristic common to the family Phoridae.

The sharp teeth possessed by adults are not used in retrieval of a food source, like a piercing mouthpart, but are instead used to aid digestion and breakdown of nutrients.^[10] Human cases involving skin inflammation are likely due to these teeth.^[10] 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.^[10]

Habitat

Megaselia scalaris' optimal culture temperature is 28 °C. 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.^[5] 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.^[10] "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."^[5]

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.^[11] They can travel 0.5 m in a four-day period.^[11] 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.^[12] 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.^[12] M. scalaris are classified in a secondary forensic role because they prefer older decaying carrion.^[12]

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.^[13] 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".^[13]

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.^[14] Research has also been done on the unique neurophysiology and neuromuscular junction within this fly, giving it its characteristic "scuttle" movement.^[15] In comparison to Drosophila melanogaster, M. scalaris has decreased excitatory postsynaptic potentials (EPSPs) and facilitation of EPSPs in response to repetitive stimulation.^[15] With such a wide range of food sources, the larvae can be considered facultative predators, parasitoids, or parasites.^[14]