Standard morphological terminology, as found in the Manual of Central American Diptera (Cumming and Wood 2009) is used. An exception to this would be use of the term “postpedicel” rather than “1^st flagellomere”, which is an equally acceptable term that the authors deem a more correct description of this segment (Stuckenberg 1999).

Common terms like “hairs” and “bristles” are used to delineate different types of socketed setae. “Hairs” we use to refer to smaller, non-feathered setae, while “bristles” are much larger, more robust and feathered setae. Although both categories can range in length and thickness, the feathering on a true “bristle” can be readily observed on a standard compound microscope at 40×.

The presented character set and states were developed by combining the classic characters outlined by those such as Lundbeck (Lundbeck 1922), Schmitz (Schmitz 1951) and Borgmeier (Borgmeier 1964) with characters that have been shown to be useful more recently, in the authors’ own studies or in descriptions of others working on the group. Additionally, character states for some of the classically utilized characters were refined to remove historical ambiguities and allow them to be used more reliably.

The authors had found certain characters useful to organize large groups of Megaselia, some to help differentiate between similar species, some to organize species into clusters for further examination, and some to most simply allow rapid identification when working through vast amounts of material. Some characters that were commonly included in traditional descriptions are simply too subjective or too variable to be used reliably and efficiently, are therefore taxonomically superfluous, and have been omitted from this refined character matrix. The distillation of traditional systems into a workable, clarified set and addition of more recently introduced characters completed the authors’ description and identification system. With a working system in place, author Hartop further refined her character set and states when visiting and working with R. H. L. Disney, who himself uses a shorthand notation of key characters when working with Megaselia.

As this system had proven utility in their own studies, the authors decided to formally present a new format for published descriptions of this fauna based on ease of use, and in anticipation of the tremendous number of descriptions forthcoming in this genus. The system of description that is proposed organizes key characters into tables supplemented by visual representations of genitalia and any other salient features, as well as clear habitus, hypandrium and wing photographs. While the authors do not intend the tables themselves to be definitively diagnostic in all cases, the combination of the table and visual aids will be so, and will replace unnecessarily verbose descriptions. This will allow any user to efficiently not only compose new descriptions for the fauna, but to more easily scan vast amounts of existing literature composed in this manner.

In addition to tabulated descriptions, all character states have been coded for use in a character state matrix that will be searchable as an online guide (ie. character "x" has states "a", "b", etc.). This online system (found at www.phorid.net/new-megaselia/home.php) allows users to quickly narrow down species matches using the character matrix, and use the visual aids to finish the identification process. This online key will include built-in tolerance to allow for interspecific variation. All species described using this system by the authors will be uploaded into this system using the coded characters. It is at the discretion of the authors to present the coded states here, but not to utilize them in future published descriptions. They will be used in the construction of the online system, and the authors here present them to allow others to code their own descriptions in the same manner.

Rare Megaselia characters are only included in the table if a positive value for such a character state will immediately and definitively place the species into a small group recognizable by that character (such as the lack of wing vein R₂₊₃). Any character that is rare but may be subject to ambiguity and therefore potentially generate confusion has been omitted. An example of such a character is the relative lengths of tarsomeres four and five on species such as M. scutellaris (Wood). Although for the seasoned Megaselia taxonomist the distinction between species like M. scutellaris, which has tarsomere five significantly longer than four, may be obvious, for a novice, species where five appears equal or even slightly longer than four may be misleading. On such potentially ambiguous rare characters, the authors opted not to include another required measurement (which would quantify the character and remove said ambiguity but decrease the ease of use of the table) and instead decided that species descriptions containing such rare characters will do so in the remarks section of the table (either in character-specific or general remarks). Extremely rare characters (such as the presence of M. shadeae’s wing spots) are not included in the table even if definitive, as the presence of such characters is so rare they essentially diagnose the species on their own. Such rare characters will be searchable in the online database with keywords generated from the "remarks" section of the description (see below). This will allow, for example, Megaselia shadeae to be found immediately by searching “wing spot”.

Lastly, characters that are definable but not practically usable (such as absolute colors, which would require the use of color palettes accounting for qualities such as hue, tint, shade, and tone while accommodating variation in digital viewing, printing, and specimen lighting), are omitted. Relative coloration (such as halter to scutum) is much less subjective and easily used for coding. It is important to note that relative coloration will occasionally create a discrepancy between a coded character state and what a user observes. Examples of dark species with dark halters that code as “lighter”, because they happen to be lighter than the scutum, do occur. Similarly, light species with light halters that happen to be slightly darker than the scutum and therefore key as “darker” do exist. The authors therefore include the relative coloration of halter to scutum as an easy to use, unambiguous character that agrees with visual perception in most cases. The habitus photographs included with each description will serve as an excellent way to easily compare qualitative characters, including color, for many parts of the specimen.

In order to utilize this description method, specimens should ideally be available in chemically dried, slide mounted, and ethanol preserved (for dissection) states. Habitus photos of chemically dried specimens are essential for conveying the overall coloration, shape, and gestalt of a species while more detailed characters must be observed on slide mounted specimens with a compound microscope. A photograph of the dissected hypandrium can be an essential tool for identification, and this requires additional specimens available for dissection. Therefore, with this system the ideal number of specimens would be a minimum of three. It is the practice of the authors in cases where limited specimens are available to photograph the specimens for habitus images while they are still in ethanol and then slide mount those specimens for detailed observations and measurements. On occasions where only a single specimen is available, this may mean the omission of a separate, dorsoventrally oriented hypandrial photograph.

Template for Megaselia descriptions (Table 1). Any and all characters may include remarks when described (not included in coded states and thus in a separate column in the tables), and general species remarks may be included at the bottom of the table. The descriptions below are intentionally verbose and thoroughly explanatory to remove potential ambiguity, with the intention that the working table will be usable quickly and easily by those who familiarize themselves with the language of this system. Abbreviations: in text.

Descriptions of characters/states:

Head

Supra-antennal (SA) setae ratio: Rather than use the more subjective visual comparisons used classically, the length of the ventral SA are entered as a percentage (given as a decimal to second digit) of the length of the dorsal SA. Therefore, SA that are equal in length will have a value of “1.00”, ventral SA that are half the length of the dorsal SA will have a value of “0.50”, etc.

Positioning of ventral interfrontal (VIF) setae: Coded as either ”Normal” orientation (a) or “VFO adjacent” (b). Normal orientation is indicated by the ventral interfrontal (VIF) setae being located either roughly midway between the supra-antennal (SA) setae and the ventral fronto-orbitals (VFO), or closer to the SA (Fig. 1 a & b). If VIF setae are skewed significantly closer to (or even directly under) the VFO setae, that is “VFO adjacent” (Fig. 1 c & d). Note, the midrange position includes species that may have the VIF setae slightly closer to the eye margin than the center of the frons, but this is not “VFO adjacent”. “VFO adjacent” is a dramatically skewed position toward the eye margins.

Figure 1.  

Frontal setation patterns showing "normal" (a & b) and "VFO adjacent" (c & d) arrangements. vs = vertical seta, po = postocellar seta, dif = dorsal interfrontal seta, dfo = dorsal fronto-orbital seta, dsa = dorsal supra-antennal seta, vfo = ventral fronto=orbital seta, vif = ventral interfrontal seta, vsa = ventral supra-antennal seta.

Postpedicel Subcuticular Pit Sensillae (SPS) vesicles: absent (a) or present (b) (Pfeil et al. 1994, Disney 2003).

Palpal setae: “long” (a) if longer than the width of the palpi, “short” (b) if shorter than the width of the palpi.

Labellum: “not spinose” (a) if spinose setulae not present, “sparse” (b) if setulae are scattered and present in numbers fewer than 30 per labellum, “dense” (c) if setulae are densely covering labellum (Disney 1999).

Thorax

Anepisternum: An anepisternum without setae is scored as “bare” (a). If setae are present, they can be “hairs only” (b) or they can be “hairs + bristle(s)” (c). If bristles are present, the number and size may be indicated (ie. “hairs + one long bristle”) as a remark, that data will not be coded.

Halter color: halter color is based on the knob of the halter when compared with the color of the scutum. Halteres that are lighter than the scutum are coded as “lighter” (a), halteres that are the same color as the scutum key as "same", and halteres that are darker than the scutum are coded as “darker” (c). It is important to note that in light colored species, halteres may be quite light in color in an absolute sense, but in a relative sense will be scored in the “darker” category, or vice versa. Halter color is best observed on either dried specimens or specimens preserved in ethanol.

Number of notopleural (NP) setae: scored as 2 (a) or 3 (b).

Notopleural (NP) cleft: scored as absent (a) or present (b).

Scutellar setae: scored as “4 =” (a) if anterior and posterior are the same size, “4 /=” [note "/=" is a more computer-friendly version of the mathematical "not equal" symbol], (b) if the anterior and posterior pairs are not the same but the smaller pair are longer than the posterior small setulae on the scutum, and “2+2” (c) if the smaller pair are minute and equal in length to the posterior small setulae on the scutum.

Legs

Foretarsus (ts1) palisade: indicate the segment numbers of the foretarsus on which a setal palisade is present (i.e. Tarsomeres 1–4, etc.).

Midtibial (t2) palisade: indicate portion of tibia on which a setal palisade is present (0.50, 0.75, etc.).

Bifurcated spines in comb of hind tibia (t3): “absent” (a), or “present” (b).

Hind tibial (t3) setulae: a single, posterodorsal, row of setulae on the hind tibia is “PD only” (a); if a second row is present on the anteroventral side of the tibial palisade, that is “PD + AD” (b).

Hind femoral (f3) basal setae: length of basal setae on hind femur indicated relative to setae on the anteroventral surface of the hind femur as “B<AV” (a), “B=AV” (b) or “B>AV” (c).

Hind femoral (f3) basal setae differentiation: “absent” (a) or “present” (b), description in remarks. Examples of differentiation would be setae that are clearly thickened, curved, found in unusual configurations, etc.

Wing

Wing length: given in mm to second decimal, measured as per Schmitz (fig. 45 in Schmitz 1951); from large basal bristle on basicosta to wing tip, taking length parallel to the costa.

Subcosta (Sc): given as “complete” (a) when it reaches vein R₁ or “incomplete” (b) when it fades out before reaching R₁.

Hair at base of vein R: either “absent” (a), “minute” (b) if shorter than the width of the R vein, “short” (c) if longer than the width of the vein but shorter than 2× the width of the vein, or “long” (d) if longer than 2× the width of the vein.

R₂₊₃ vein: either “present” (a) or “absent” (b).

Costal index (CI): given as a decimal to second digit, found by dividing the wing length by the length of the costal vein (fig. 45 in Schmitz 1951). As this is a relative, not absolute, measurement it can most easily be taken on a printed photograph of the wing.

Costal ratios: given as “C1:C2:C3” with C3 at a value of 1 and others to second decimal (fig. 44 in Schmitz 1951). As with the CI, these are a relative measurement and are most easily taken using a printed photograph of the wing.

Costal setae length: measurement in mm of the length of the longest costal setae present in section 3 of the costa.

Number of alular setae: given as a number, coded as “1” (a), “2” (b), “3” (c), “4+” (d) due to observed variation in number of alular setae on species with more than four present.

Alular setae length: given in mm, longest seta present is measured.

Wing color: either “lightly infuscated/clear” (a) or “strongly infuscated” (b); the authors understand that this character may include some ambiguity, so therefore it must be clarified that most wings are state "a", and only wings that are distinctly infuscated should be coded as the second character state. Wings are best viewed on dried specimens (or in a clear mounting medium, ie. not Canada Balsam) against a white background to observe this character.

Genitalia

Anal tube (AT) length relative to length of dorsal face of epandrium (E): length of the anal tube (cerci + hypoproct) is given relative to the dorsal face of the epandrium as “AT<E” (a), “AT=E“ (b), “AT>E” (c). This comparison is most easily done on a slide mounted specimen, where the epandrium can be seen even if it is partially covered by the tergites and must be viewed through them.

Epandrial (E) Setation: “hairs only” (a) or “hairs + bristle(s)” (b).

Relative lengths of posterior setation: indicate the relative lengths (not thickness) of setae found on Tergite 6 (T6), Epandrium (E), Cerci (C) and Hypoproct (H) with operators < [less than], = [equal], or ~ [subequal: approximately equal] (ex. T6<C=H<E).