Micron 34 (2003) 345–350
www.elsevier.com/locate/micron
Mouthparts of Megaselia scalaris (Loew) (Diptera: Phoridae)
Kom Sukontasona,*, Kabkaew L. Sukontasona, Somsak Piangjaia, Noppawan Boonchua,
Tarinee Chaiwonga, Roy C. Vogtsbergerb
a
Department of Parasitology, Faculty of Medicine, Chiang Mai University, Chiang Mai 50200, Thailand
b
Department of Biology, Hardin-Simmons University, Abilene, TX 79698, USA
Received 9 July 2003; revised 8 August 2003; accepted 14 August 2003
Abstract
Mouthparts of adult Megaselia scalaris (Loew), a fly species of medical importance, were examined using scanning electron microscopy.
Sexual dimorphism of the labellum was observed in the mouthpart structures of this species. The labella of males were clothed with a dense
covering of microtrichia, but these were found to be entirely absent from the labella of females. Aside from this difference, trichoid and
conical sensilla that are most likely used as taste or contact chemoreceptors appear on the labellum and labrum of both sexes. In addition, five
pairs of sharply pointed teeth at the ventral surface of the labellum is another feature that is shared by the two sexes. A plausible feeding
mechanism for this fly is also advanced.
q 2003 Elsevier Ltd. All rights reserved.
Keywords: Megaselia scalaris; Mouthparts; Scanning electron microscopy; Feeding
1. Introduction
Megaselia scalaris (Loew) is a scuttle fly, or humpbacked fly, whose biology is better documented than most
other species of Phoridae, much being summarized by
Disney (1994). Thus the larvae have been reported feeding
on a wider range of decaying organic materials than any
other species of fly. They are also occasionally reported in
cases of myiasis in man and other vertebrates, in situations
ranging from wounds to the intestines (Zumpt, 1965; Trape
et al., 1982; Singh and Rana, 1989). The catholicity of its
choice of larval pabula means it has proved easy to culture
in the laboratory and is increasing employed as a laboratory
insect in a range of studies.
Increasing the current knowledge of the morphology of
mouthparts of medically important insects should provide
valuable information to researchers, not only for taxonomic
purposes (McKeever et al., 1994; Krenn and Penz, 1998),
but also pertaining to feeding behavior that relates to
humans. As for the minute flies in the family Phoridae, the
adults bear sponging mouthparts. The adults of both sexes
are known to feed on sugars and the females on a protein
* Corresponding author. Tel.: þ 66-53-945342-5; fax: þ66-53-217144.
E-mail address: [email protected] (K. Sukontason).
0968-4328/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.micron.2003.08.003
rich meal prior to maturation of their eggs (Disney, 1994).
Carrion fluids are one such source. Such habits mean they
pose a hygiene risk when they invade premises where food
is being prepared. Due to the lack of information on
mouthpart structures of M. scalaris, we report here the
mouthpart structures of both male and female flies using
scanning electron microscopy (SEM), and how the mouthparts may pertain to their feeding behavior and medical
importance.
2. Materials and methods
Adults of M. scalaris used in this study were obtained
from the F1 generation of flies that were collected from the
wild using fresh pork liver as bait at the Department of
Parasitology, Faculty of Medicine, Chiang Mai University.
Eggs collected from wild-caught females were identified as
M. scalaris in accordance with the morphological features
presented by Greenberg and Wells (1998). Larval stages
were reared on fresh pork liver until the prepupal
stage in transparent boxes placed in rearing cages
(30 £ 30 £ 30 cm3). Upon emergence, adult flies were
removed from the cage using glass tubes and were killed
by placing them in a freezer for 15 min. Thereafter,
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K. Sukontason et al. / Micron 34 (2003) 345–350
Fig. 1. SEM micrograph of the mouthpart structures of male Megaselia scalaris. Oblique lateral view of labrum, labellum, and hypopharynx (asterisk).
Bar ¼ 10 mm.
separation of male and female flies was made under a
dissecting microscope. They were then decapitated and only
the head portion was prepared by rinsing several times with
normal saline solution. Specimens were fixed in a fixative
agent consisting of 2.5% glutaraldehyde at 4 8C for 24 h.
The fixed specimens were then subjected to postfixation in
1% osmium tetroxide and dehydrated in a graded alcohol
series. This was followed by treatment in acetone and
critical-point drying. Finally, the heads were mounted on
stubs, sputter-coated with gold, and viewed with a JEOLJSM840A scanning electron microscope (Tokyo, Japan).
Fig. 2. SEM micrograph of the mouthpart structures of male Megaselia
scalaris. Oblique lateral view of labrum and glossa (dark arrowhead),
trichoid sensilla (white arrowheads), epipharynx (white arrows) and
hypopharynx (asterisk). Bar ¼ 10 mm.
3. Results
The sponging mouthpart structure of a male M. scalaris
is illustrated by SEM photomicrographs in Figs. 1 – 7, with
the main components of the apparatus being the labrum,
labellum, and the hypopharynx, which lies between the
labrum and labellum (Fig. 1). Below the tip of the labrum is
the glossa, a protruding triangular structure situated between
two areas covered with parallel, single-pointed spines
(Figs. 1 and 2). Located underneath this triangular structure
on each side is a tapering panel bearing two trichoid sensilla
with a group of three conical sensilla halfway between them
Fig. 3. SEM micrograph of the mouthpart structures of male Megaselia
scalaris. Glossa bearing two trichoid sensilla with a group of three conical
sensilla halfway between them. Bar ¼ 1 mm.
K. Sukontason et al. / Micron 34 (2003) 345–350
Fig. 4. SEM micrograph of the mouthpart structures of male Megaselia
scalaris. Conical sensilla in shallow depression at higher magnification.
Note small hole in tip of each sensillum. Bar ¼ 1 mm.
(Figs. 2 and 3). At higher magnification, it is clear that these
conical sensilla are situated in a shallow depression and that
each sensillum bears a small hole in its tip (Fig. 4). Proximal
to the tip of the labrum is a pair of epipharyngeal sclerites
(arrows in Fig. 2) that appear as flattened oval plates with a
small central projection (Fig. 5). The hypopharynx is
located ventral to the labrum and is adjacent to the pair of
epipharyngeal sclerites (Fig. 2). It covers a deep groove in
the ventral side of the labrum. As for labellum, its entire
surface is densely covered with long unbranched microtrichia (Figs. 1 and 6). The tip of the labellum bears a
crescent-shaped membranous lobe medially (Fig. 1).
Located in the lateral portions of the labellum are several
long, strong trichoid sensilla, whereas these sensilla are
shorter in more ventral positions closer to the food channel
(Figs. 1 and 6). These sensilla, which emanate from a socket
at their base, have longitudinal grooves and are the most
Fig. 5. SEM micrograph of the mouthpart structures of male Megaselia
scalaris. Epipharynx with small projection at tip. Bar ¼ 1 mm.
347
Fig. 6. SEM micrograph of mouthparts of male Megaselia scalaris. Ventrolateral view of labellum densely covered with microtrichia, but bearing few
trichoid sensilla (arrowheads). Arrows indicate sharply pointed teeth in
labellar groove. Bar ¼ 10 mm.
abundant in the mouthpart structure. The medial edges of
the anterior part of the labellum are each equipped with
a single row of five sharp pointed teeth, with or without
lateral minute spines on them (Fig. 6). Posterior to the last
pair of teeth are three pairs of deep, longitudinal grooves
(pseudotracheae) in the labellar lobes that enclose the food
channel (Fig. 7).
The mouthparts of female M. scalaris share some general
features with those of males (Figs. 8 –14). These include the
trichoid sensilla located on the inner surface of the labrum
(Figs. 9 and 10), long and short trichoid sensilla located
laterally and ventrally, respectively, on the labellum (Figs. 8
and 12) and the membranous lobe at the tip of the labellum
(Figs. 12 and 13). Moreover, five pairs of sharp, pointed
teeth are also observed in females (Fig. 14). However,
Fig. 7. SEM micrograph of mouthparts of male Megaselia scalaris. Three
pairs of pseudotracheae enclosing food channel. Bar ¼ 10 mm.
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K. Sukontason et al. / Micron 34 (2003) 345–350
Fig. 8. SEM micrograph of the mouthpart structures of female Megaselia
scalaris. Dorsal antero-lateral view of labrum and labellum.
Bar ¼ 100 mm.
Fig. 10. SEM micrograph of the mouthpart structures of female Megaselia
scalaris. Trichoid sensillum at higher magnification. Bar ¼ 1 mm.
4. Discussion
the most distinctive feature of sexual dimorphism in the M.
scalaris mouthparts is the surface structure of the labellum.
Females have the surface of the labellum entirely smooth
(Figs. 8 and 12), whereas, the male has the surface of
the labellum entirely covered with microtrichia (Figs. 1
and 6). Three more less distinctive features were found that
differed between the sexes. Females possess a much sharper
glossa with stacked dorsal ridges above (Fig. 9). Secondly,
the conical sensilla on the inner surface of the female
labrum appear as three-minute protrusions on a slightly
curved base bearing two small holes laterally (Fig. 11).
Thirdly, small and large hooks were detected between the
third and fourth pairs of teeth, respectively (Fig. 14), but
hardly recognized in male specimens examined.
Fig. 9. SEM micrograph of the mouthpart structures of female Megaselia
scalaris. Dorsal view of tip of labrum showing sharp protruding glossa and
stacked dorsal ridges above. Arrow indicates trichoid sensillum.
Bar ¼ 10 mm.
From the thorough observations in this study, we confirm
that M. scalaris possesses the sponging type mouthparts
(Schmitz, 1938). The sharp pointed teeth of M. scalaris
correspond to the prestomal teeth in the cyclorrhaphan
proboscis of flies such as house flies or blow flies
(Graham-Smith, 1930; Iwasa, 1983; Broce and Elzinga,
1984; Giangaspero and Broce, 1993). Concerning feeding
mechanisms, these sharp teeth would likely function in a
manner analogous to the prestomal teeth. Medically, the
Phoridae have occasionally been recorded irritating people
by attempting to ‘bite’ them. For example Megaselia abdita
has been reported attacking egg-collectors in poultry
houses, and it was probably this species reported annoying
people in Yorkshire coal mines (Edwards and Schmitz,
Fig. 11. SEM micrograph of the mouthpart structures of female Megaselia
scalaris. Conical sensilla appear as three-minute protrusions on a slightly
curved base bearing two small holes laterally. Bar ¼ 1 mm.
K. Sukontason et al. / Micron 34 (2003) 345–350
349
Fig. 12. SEM micrograph of mouthparts of female Megaselia scalaris.
Ventral view of labellum with smooth surface devoid of microtrichia.
Arrows indicate relatively long trichoid sensilla in a lateral position while
the black arrowhead points to a hook in the food channel. Bar ¼ 10 mm.
Fig. 14. SEM micrograph of mouthparts of female Megaselia scalaris.
Ventro-lateral view of labellum showing small and large hooks (arrowheads) located between the third and fourth pairs of sharply pointed teeth in
the labellar groove, respectively. Bar ¼ 10 mm.
1939; Disney, 1987). Dohrniphora cornuta has also been
reported attacking man (Schmitz, 1951), but this appears to
be atypical behaviour (Barnes, 1990). Large numbers of
Megaselia halterata have been reported causing annoyance
to mushroom pickers (Binns et al., 1979). An office in
Malaysia was invaded by M. scalaris; it was reported that
they ‘bite but don’t settle to suck blood’ (Disney, personal
communication). It seems likely that these various ‘attacks’
involve flies imbibing sweat as a source of salts and other
nutrients. Since these sharp teeth are apparently similar in
both male and female M. scalaris, another function besides
feeding may be to facilitate adult emergence. As previously
reported by Jervis (1998) in a review of mouthpart structures
in parasitoid wasps, similar teeth are used by some
parasitoid wasps to cut an exit-hole in the host’s pupa or
cocoon. Examination of pupal exuviae of M. scalaris in
the laboratory reveals an exit-hole located approximately
just after the intersegmental spine of the fifth segment that
has an extremely smooth margin, thus suggesting the use of
these sharp teeth as an emergence apparatus.
Some exceptional female flies in the family Phoridae
have been reported to feed on host blood, even though
nectar-feeding is the common mode of feeding for most
members of the Phoridae (Disney et al., 1992; Gilbert and
Jervis, 1998). In these few species with blood-feeding
females, the occurrence of teeth on the inner faces of the
labellum was noted. Females of Phalacrotophora spp. were
reported to have an enlarged labrum and a narrow, teethbearing labellum modified for feeding on host blood
(Disney et al., 1992). Females of Megaselia chlumetiae
has been reported to have a narrow labellum, an expanded
and elongated labrum and a heavily sclerotized, piercing tip
of the hypopharynx (Disney et al., 1992). As for M. scalaris,
it has been successfully reared for many generations in the
laboratory without access to blood meals. The fact that a
female may possible feed on fluid oozing from a prior
wound does not contradict the fact that this species does not
suck blood from living people by means of punctures made
by itself.
Our SEM study of the mouthparts of M. scalaris clearly
confirm the sexual dimorphism of male and female flies,
which was demonstrated by Gilbert and Jervis (1998) in
some other phorid species. The fact that the surface of the
labellum is completely covered with microtrichia (cuticular
projections from the epithelial cells) in males but is
completely smooth and devoid of microtrichia in females
is the most distinctive feature of sexual dimorphism in this
species. Other distinctive features of the mouthparts were
found to be shared by both sexes. The laterally located long,
and ventrally located short trichoid sensilla of both sexes
appear in the same region of the labellum. The number (five
Fig. 13. SEM micrograph of mouthparts of female Megaselia scalaris.
Membranous lobe at tip of labellum. Bar ¼ 10 mm.
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K. Sukontason et al. / Micron 34 (2003) 345–350
pairs) and shape (sharp, pointed) of the teeth are also shared
characteristics of both sexes. This similarity in shape of
teeth agrees with other cyclorrhaphan species (Broce and
Elzinga, 1984). The present study shows that trichoid
sensilla are the most abundant receptor type found on the
mouthparts of both sexes of M. scalaris, located primarily
on the lateral and ventral surfaces of the labellum. Only a
few trichoid and conical sensilla were discovered and
confined on the inner surface of the labrum. Trichoid
sensilla such as these have been suggested to function as
taste or contact chemoreceptors (Richards and Davies,
1977; Chapman, 1991), or as mechanosensors (Fernandes
and Linardi, 2002). Hairs with longitudinal grooves in their
cuticular wall that emanate from sockets have been
proposed to be chemosensitive sensilla such as is seen on
the labellar lobes of mosquito Aedes aegypti (Hill and
Smith, 1999). According to Salama (1966), the labellar
receptors in mosquitoes are sensitive to sugars, water and
deterrent substances while the labral receptors trigger a
response to blood. In regard to the conical sensilla,
Chapman (1991) suggested that the cones with their
terminal pores are used to perceive chemicals in response
to contact with sugars, salts, water, and perhaps, deterrent
compounds. The conical sensilla observed in this study are
similar in structure to the contact chemoreceptors of
Vespula germanica (Hymenoptera: Vespidae) (Chapman,
1991).
In conclusion, this study not only enhances the
anatomical information available on the mouthparts of M.
scalaris, but also starts to address the possible feeding
behavior of this medically important fly species.
Acknowledgements
We gratefully acknowledge the valuable suggestion of
Dr Henry Disney, University of Cambridge (UK). Suggestions of two reviewers are also acknowledged. We thank
Budsabong Kuntalue and Natchanart Thijuk for their
assistance. This work received support from the Faculty of
Medicine Endowment Fund for Medical Research, Faculty
of Medicine, and the Chiang Mai University for reprint cost.
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