George Dimmock.

The anatomy of the mouth-parts and of the sucking apparatus of some Diptera online

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in Ctde.r, but are the continuations of the two lateral halves of the labium. The
tip of the labium projects between them, however, as can be seen in section fif
where the cross-section of the tip of the labium is shown, at its firat point of
connection with, and between the two labellae. Each labella contains a flexor
and extensor muscle, the flexor a little above the extensor in position. The
labellae are usually pressed closely together, but can be separated, as seen in
fig. 2, or even wider, so as to be at right angles to the axis of the proboscis.
On the inner side of each labella are three longitudinal grooves or channels, held
open by semi-rings of chitin at right angles to their axes, and toothed on each
side. These, on account of their general resemblance to tracheae, were termed
by Suffolk*^ pseudotracheae, in his description of them as they appear in Musca
romitoria. These pseudotracheae are seen in section in fig. 1, a. Fig. 5, a, is
a perspective view of the pseudotracheae of Bomhylim major, when at rest, with
their teeth turned inward (fig. 6, « is cross-section of the same) ; fig. 5, 6, shows



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them in perspective, when Bombylim is feeding, with their teeth turned outward
(fig. 6, h, is a cross-section of the same).

The functions and finer structure of these pseudotracheae, together -with other
points in the anatomy of the mouth-parts of Bovibylius, will be further discussed
in the part of this paper devoted to a comparative study of the mouth-parts of
different diptera.

ANATOMY OP THE MOUTH-PARTS OP ERISTALIS.

Little has been done in the study ofj the anatomy of the. mouth-parts of
St/rphidae, of which I have chosen Eristalis Iwrticola for anatomical investigation.
Gerstfeldt* (p. 28-29) gives a general description of the mouth-parts of the
Syrphidae, which can be condensed as follows : labrum blunt, with the tip separ-
ated into several points; hypopharynx always present; mandibles more or less
rudimentary and coalesced with the sheath of the proboscis; maxillae fairly well
developed and setiform; labium with well-developed terminal lobes. Menzbier*
(p. 57-60) describes the proboscis of Syrphus taematus. His results, as regards
the typical structure of the mouth-parts of Syrphtis, may be sunmaarized thus :
labrum and epipharynx united ; hypopharynx with a channel for salivary secretion;
mandibles present; only remnants of the maxillae are the two maxillary palpi;
labium with well developed terminal lobes. Menzbier, in describing the proboscis of
Syrphidae, divides it into three portions, a basal, a middle, and a terminal portion.
Leaving all reasons for my own views to be given in detail later, I will only say
here, that this division of the proboscis of the Syrphidae, Muscidae, and other
families of diptera, into basal, middle, and terminal portions, has served to render
the study of the mouth-parts of these insects more complex and more difficult than
it would otherwise be, and that, while retaining the term basal portion, for reasons
to be given later, as a convenient designation for a part of the proboscis, I lay
little value on such a division of parts. The so-called middle and terminal portions
of the proboscis of the Syrphidae and Muscidae are really a single portion.

The proboscis of Eristalis hordeola (side view in PI. 3, fig. 1) is hung upon
the under side of the head, and, unlike the proboscis of Culex or of Bombylim,
is extensible and retractible. When extended the proboscis of E. hoHicola is as
long as the head, and points nearly directly perpendicular toward the surface on
which the insect is resting. Its retraction is accomplished by means of joints,
near the points indicated on the figures by d and g. If the retraction is begun,
by a partial revolution toward the head, of the portion of the proboscis between
d and gy around the extremity d as axis, then the end g will follow the course



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represented by the dotted segment of a cirde gn ; and if, during the time that
the above mentioned revolution is progressing, a revolution of the portion of the
proboscis gk around the axis g takes place, then the tip of the proboscis, k, will
follow the direction indicated by the paraboloid curve kd. When these two partial
revolutions, around the axes of the joints at d and ^, are completed, as far as
possible, the points g and n, and k and d^ will have been closely approximated,
and the proboscis will have been folded upon itself by a fold near its middle
point, and, folded thus, the whole proboscis will have been retracted into a groove'
on the under side of the head. When fully retracted the dorsal * portion of the
basal half of the proboscis is closely pressed against the dorsal portion of its
distal half. Fig. 2 represents a longitudinal section of the head of Eristalis
hordeola with the basal half of the proboscis wholly retracted, and its distal half
partly retracted. A section through the distal half of the proboscis of Eristalis
(Fig. 1, fi') shows all its mouth-parts in their normal positions. They consist
of a labrum-epipharynx (/r-e), closed beneath by a hypopharynx (A). To each
side of the above named parts lay the only paired mouth-parts, the maxillae {mx)
and their palpi {mp). Beneath these parts the labium forms a channel for all
the other mouth-parts, a channel into which they fit when the proboscis is retracted.
No mandibles are present. A section through the proximal half of the proboscis
(fig. 1, 6') shows none of the true mouth-parts. In the middle of this section
is the channel for the passage of food, the pharynx (p), surrounded by the
chitinous distal end of the so-called fulcrum, and its muscles (pw). At each side
of the pharynx is a chitinous rod {x) which supports at its distal extremity the
maxilla. Around these parts is a thin elastic membrane (c /') which is. continuous,
proximally, with the chitinous covering of the head ; and, distally, on the upper side,
with the labrum, and on the under side with the lower walls of the labium.
This elastic membrane, surrounding the forward extremity of the fulcrum, folds
itself together, on the under side, between n and g (fig. 2), when the proboscis
is retracted ; on the upper side, when the proboscis is retracted, it receives in its
folds the dorsal surface of the labrum.

The labrum-epipharynx is, in section (fig. 1, /J' and y', fr-e), nearly the shape
of a horse-shoe, convex above, and is composed of the labrum, the continuation
of the flexible walls of the upper side of the basal portion of the' proboscis, and
of the epipharynx, the continuation of the upper walls of the pharynx; but, in

* I shall use dorsal and ventral, upper and under, distal and proximal, and anterior
and posterior, of the parts of the proboscis, in the same way and with the same meaning
as if the proboscis were extended out from the front of the head.



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. — 28 — •

Eristalis, the labrum and epipharynx are not connected, as in Bomhylim, by
infolding delicate membranes, to mark, in section, their line of union. The basal
half of the labrum-epipharynx contains muscles passing obliquely upward and
backwards from the epipharynx to the labrum ; these muscles are only a forward
continuation of the system of pharyngeal muscles in the fulcrum, which the base
of the labrum-epipharynx joins. At its tip the labrum-epipharynx is divided into
six parts, as seen in tig. 5, which represents the tip unrolled, from within, and
these are hairy, the rest of the labrum-epipharynx being naked. The hairs
on the inner side of the outer parts of the tip have the appearance of being
sense-hairs, but I have only studied them superficially, and cannot therefore speak
with certainty of their nature. The division of the tip of the labrum into several
parts is regarded by Meigen** (Theil 8, p. 381) as a characteristic of the
Syrphidaey and the researches of Gerstfeldt, as well as my own examination of
the labrum of about a dozen species, tends to confirm Meigen's statement.

The hypopharynx (fig. 6; in cross-section, fig. 1, /J' and /, A) is lanceolate,
naked, and strongly chitinized. It is a rigid tube, opening apically on the upper
side, and is the outlet for the saliva. Its proximal end is slightly broadened,
and unites, with a true joint, to the under walls of the distal end of the fulcrum,
or pharynx. The margins toward the tip and the tip itself of the hypopharynx
are transparent; the remainder of the hypopharynx is opake.

The maxillae (fig. 4, r/w, and, in section, fig. 1, /?', rnx) are thin naked
blades of chitin, concave on their inner sides, convex on their outer sides, and
uniting at their bases with the maxillary palpi. The bases of the maxillae lay
directly outside of the labrum-epipharynx, when the proboscis is extended, but the
maxillae curve downward and inward, distally, so that their tips are between the
hypopharynx and labium. The margins and tip of the maxillae are very trans-
parent, the middle a little thickened. The maxiUary palpi (fig. 4 mp-, fig. 1,
/?', mp) are cylindrical, one-jointed, hairy, a trifle longer than the maxillae, and
lay along the sides of the labrum-epipharynx, when the proboscis is extended
(fig. 1). The chitinous supports which bear the maxillae at their tips, extend
back along each side of the pharynx, where they can be seen, in section, at x
in fig. 1, d* and e\

In Enstalis horticola I was unable to find the least traces of the mandibles,
either as free rudimentary structures, or as portions united to the labium. What
Menzbier considered to be mandibles in Syrphm taeniatuji, are, undoubtedly,
maxillae. Menzbier* writes (p. 60) "At the sides of the basal portion of the
labium and united with it, lay two thick chitinous structures, which project right



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and left of the opening of the mouth in the fonn of three-cornered blades, or
sharp prongs. These blades or prongs resemble so much the well-developed
mandibles of chewing insects that we can rightly regard them as such. SyrpJms
has, then, besides labrum, epipharjnx, hypopharynx and labium, a pair of man-
dibles. Near the mandibles, united with the basal portion of the proboscis, are
two palpi. Since neither maxillae nor anything similar to them, are to be found
on the mouth-parts of Syrphm, we must consider the aforesaid palpi to be the
only renmants of the maxillae, sinde the latter always have palpi." * The above-
quoted remarks give no valid reason for regarding the parts under consideration
as mandibles, for the form which a part assumes in an animal has little value
in determining its homology in comparison with the worth, in such determ-
inations, of the position and attachments of the organ, the homology of which is
in doubt ; and, in this case, the situation of the maxillary palpi so near the base
of the organs in question, together with the fact that the maxillae are present,
and attached at the base to their palpi in Eristalis (fig. 4), leads me to suppose
that the parts, which Menzbier regarded as mandibles in Syrphm^ were really
maxillae. This view is further supported by the figure ** which Menzbier gives,
and by Gerstfeldt's statement that the mandibles are more or less rudimentary
and united to the sheath of the proboscis, in the Syrphidae.

The labium of Eristalis horticola (side view in fig. 1; section in fig. 1, /J'
and /, /) is larger than the other mouth-parts, muscular, pubescent on the outer
or under side, and throughout its length run two tracheal stems and numerous
longitudinal muscles. Its under surface and the middle of its upper side are
strongly chitinized, but between these two portions the walls of the labium
are very flexible, as can be seen in the sections /J' and y* of fig. 1. At its
base the upper or inner surface of the labium joins the under side of the
hypopharynx, at the point where the latter is jointed to the fulcioun; the under
or outer surface of the labium passes (with a fold or two when the proboscis

• "An den Seiten des Basalkegels der Unterlippe und mit ihm verwachsen liegen zwei
dicke Chitingebilde, die rechts und links von der Mundoffiiung in Geatalt dreieckiger Schnei-
den Oder scharfen Hacken vorspringen. Dieae Klingen oder Haoken gleichen so sehr den
wohlentwickelten Mandibeln der kauenden Insekten, dass wir mit voUem Eechte sie als
solche deuten konnen. Syrphus besitzt also ausser Labrum, Epi- und Hypopharynx und
Labium noch ein Paar Mandibeln. Neben den Mandibeln, mit dem Basalkegel verwachsen,
sitzen zwei Palpen. Da weder Unterkiefer noeh etwas ihnen ahnlich^s an den Mundtheilen
von Syrphus zu finden ist, so miissen wir die erwahnten Palpen als einzige Reste der Unter-
kiefer betrachten, da Letztere immer Palpen besitzen.'*
♦* See Menzbier's pi. 3, fig. 3.



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is not fully extended) directly into the under surface of the basal portion of the
proboscis. On the upper side the labium is hollowed out to form a channel into
which the labrum-epipharynx, hypopharynx and maxillae fit, when the proboscis
is retracted, and usually when it is extended, To each side of the tip of the .
labium is attached a labella (in s'ection in fig. 1, or'). The two labellae are
short, fleshy, and hang below the level of the lower wall of the labium, often,
when they are in use, seeming to be at an angle to it; thus they have more
the appearance of distinct organs than they have in Bonibylius. They are,
however, nothing more than two hanging lateral wings of the labium, joined to
it only by flexible chitinous walls, not by time joints. The labellae fold together
so that the interior face of the one presses upon the interior face of the other,
when they are not in use, and when the proboscis is folded in its channel beneath
the insect's head. This seems to be the normal, or resting position, of the
labellae, from which they are brought into action by a pair of extensor muscles,
one in each labella (as seen in fig 1, a' in section); their further action will be
described more fully later. Each labella has on its inner side pseudotracheae, all
branching from a main pseudotracheal stem, which extends anteriorly and
posteriorly along the upper side of each labella. The anterior portion of this
pseudotracheal stem has about twenty-four branches, the posterior portion about
eighteen branches. The branches of these pseudotracheal stems extend, approxim-
ately parallel to one another, from the upper to the under margin of each
labella, and are much finer than, but of similar structure to. the pseudotracheae
of Bombylius.

The structure of the so-called fulcrum, which occupies the central portion of
the basal part of the. proboscis, the use of the pseudotracheae, the mode of
expanding the labellae, and other points pertaining to the structure and use of
the proboscis in Eristalis will be discussed later, and more appropriately, in that
part of this paper which will be devoted to a comparison of some of the cor-
responding organs in Cule^v, Bombylius and Muscck



ANATOMY OF THE MOUTH-PAETS OP MUSCA.

The proboscis of Mmca domestica, the common house-fly, and that of M.
vomitoriay the blow-fly, have attracted the attention of naturalists, from the time
when Aristotle wrote, down to the present day, and to attempt anything like a
complete notice of the diflferent papers which, wholly or in part, treat, more or
less fully, more or less scientifically, of this subject, would be an extravagant



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expenditure of space and a waste of time. I select, therefore, for notice a few
of the papers which treat upon the subject of the proboscis of Mmca^ or of its
mouth-parts, such papers as seem to me to have furnished the more important
contributions to our knowledge of the subject, or such papers as have so fully
dealt with the subject that they ought not to be overlooked.

The earliest paper which I have thought worthy of notice here, that
deals with the anatomy of the proboscis of Musca, is Raaumub»s memoir**
upon the trunks, with swollen and fleshy lips, which belong to two-winged
flies.* This paper by Reaumur, — altho published in 1740, over thirty years
before Fabricius had reduced the terminology of the oral organs of insects to
anything like a system, and over seventy years before Savigny had settled the
question that the mouth-parts of insects of all the diflFerent orders were homo-
logous, — is a very complete and accurate presentation of many anatomical facts
in regard to the proboscis of Musca, Reaumur, whose observations in regard to
Musca were made, for the most part, on M. vomitoria^ studied the proboscis, as
was his usual method of study, with especial reference to its functions, and to
the functions of each of its parts. Reaumur believed the proboscis of Musca to
be essentially an organ for sucking, altho he thought that the food was aided,
perhaps, by the undulatory motion which he observed in the proboscis, in its
passage upward from the mouth, which was, according to him, between the jiwo
labellae. By watching a fly rolling a bit of dry sugar between the labellae,
gradually moistening it, and thus slowly dissolving it away, Reaumur came to the
conclusion that the fly had a salivary secretion, but, altho he describes the
hypopharynx, by him termed the spur {aigmllon), as channeled above, he does
not seem to have discovered that the channel of the hypopharynx was the
outlet for the saliva. Reaumur correctly recognized the labrum-epipharynx as
the part of the proboscis through which the food passes, and he therefore termed
it the sucker {jmgoir). He called the labellae lips {lhvres\ and writes of them
that they are traversed by parallel channels, or flutes {cannelures), which extend
toward the middle of the labellae ; but, further on, he incorrectly assumes that
this channeling is due to a large number of parallel vessels, which distend with
liquid, when one presses the head of a fly, or when a fly wishes to use its
labellae. Altho Reaumur does not directly say, in any place, that the proboscis
of the fly is extended by means of injecting blood or air into it, yet the idea
that the proboscis was extended by inflation was evidently often in his mind,

• Tome 4, part 1, p. 256-297, pi. 16-18. "Des trompes a levres grosses et chamues
des mouches k deux ailes."



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while writing about the proboscis, and he often uses the term inflate (gonjlery in
regard to the proboscis or its parts. He writes (p. 259), '^ The fly can increase
the volume of its trunk and can diminish it to a certain extent." * He further
writes (p. 260), " One easily compels a fly to show its entire trunk, finely extended
and well inflated; one has only to press between the two fingers the thorax,
either laterally or from above and below; it seems as if one obliged the fly,
immediately, to put out the tongue." ** E^aumur regarded the proboscis of
Musca to be composed of, or folded into, two joints.

Gleichen,'* in 1764 (p. 19-21), describes the proboscis of Musca dojnestiea.
He regards the proboscis as made up of three parts, longitudinally, namely a
basal sac, a middle tube, and the proboscis proper, the latter being only the
labellae. He writes (p. 19) that " The extension of the proboscis probably arises
from the air, which is driven by the fly into the sac; and from this into the
[middle] tube, finally into the lips. The elevation and depression of the proboscis
is, however, brought about by the process with which the [middle] tube is sup-
ported." *** Gleichen writes further (p. 21), " The fly can indeed, as I have
already mentioned, drive air between the membranes of the proboscis into the
lips, but cannot take in air with them." f

Gleichen did not succeed in finding the part now known as hypophaiynx in
Mmca domestica. He regarded the basal sac, or basal part of the proboscis, as
a pumping organ.

Gebstfeldt's statements* (p. 24-26) regarding the mouth-parts of Musca may
be noticed briefly as follows : the under lip, according to him, was what will be
later described as labrum-epipharynx ; his hypopharynx, as mentioned in the
historical notes (p. 7) of this paper, was really the epipharynx, separated
by the action of caustic potash, from the labrum ; the real hypopharynx was not
discovered by Gerstfeldt; the rudiments of the mandibles appear, according to
Gerstfeldt, as the dorsal comer, on each side, of the fulcrum; the maxillae form

* "La Mouche peat augmenter le volume de sa trompe & le diminuer jusqu'a un
certain point."

•* " On force ais^ment une Mouche i montrer sa trompe toute entiere, bien etendue &
bien gonflee; on n*a qu'4 presser entre deux doigrts, soit les deux cotes, soit le dessus & le
dessous du corcelet; il semble qu^on oblige sur le champ la Houche k tirer la langue.''

**♦ "Das Ausstrecken des Riissels riihret vermuhtlich von der Luft her, die von der
Fliege in das Sackchen, und von diesem in die Rohre, biss in die Le£ze, getrieben wird.
Das aufrichten und senken des Riissels aber wir^ mit dem Hornbeinchen, womii die Robre
gefasset ist, bewirket"

f " Die Fliege kan zwar, wie ich bereits erwahnet, die Luft zwischen den Hauten
des Riissels in die Lefzen treiben, aber nicht damit Luft schopfen.*'



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the lateral portions of the ftdcram and support above the maxillary palpi; the
labium, as submentum, mentum, etc., extends anteriorly from the base of the
fulcrum and terminates in the labellae. Gerstfeldt's yi^ws of the homologies of
the different parts of the fulcrum seem utterly untenable, as will appear later.

LowKE,** in 1870 (p. 41-51, pi. 2-3), describes and figures the proboscis and
its parts in Mmca vomitoria. He describes fiilly, and in general accurately, the
fulcrum, which he regards to be homologous with the ftdcrum of bees. The
labrum-epipharynx, Lowne thinks, is composed of the labrum united with the term-
inal lobes of the maxillae, and he terms the organ as a whole the operculum,
because it shuts like a lid over the other mouth-parts. Lowne correctly describes
the hypopharynx under the name of tongue. The labium, which he calls the
canula, is considered to be the united labium and mentum. I do not here go
into further details in regard to Lowne's work, because I shall have occasion to
notice it often in the following pages.

Macloskib, in 1880, describes the proboscis of the house-fly, Mv^ca domestica,
using for the most part Lowne's terminology, and discusses briefly several points
about its homologies and functions. Among other things he again afSrms the
statement that inflation plays a prominent part in the extension of the proboscis;
a statement criticized by Suffolk,'^ in 1869. Macloskie writes (p. 157-158),
** It is easy to dispose of Mr. Suffolk's hasty criticism. Immerse the head of the
fly in caustic potash, which destroys the muscles, the chitine of the membranous
sheath and the tracheal tubes remaining intact, and you can still protrude the
organ by slight pressure. Further, when the proboscis is pressed out and all its
parts distended, pierce with a needle the swollen air sacs under the tip, and at
once the tip collapses upon the mentum. If you tear the membrane about the
base of the proboscis that part collapses. If you press the head over nrach, the
membrane-sheath sends out bulging processes which soon burst, sending bubbles
of air through the water in which you are examining it." I have verified the
experiments mentioned by Macloskie above, besides trying others, and I think that
an important factor in the cause of the extension of the proboscis of Musca is
the injection of air into it. Macloskie also homologizes the fulcrum of the diptera
with the endocranium of the cockroach (Blatta), a view that I am not prepared
to affirm or to deny; but his conclusion that the so-called middle section is the
true base of the proboscis, the so-called basal portion being a part of the head,
and its organs internal, rather than true mouth-parts, this latter conclusion I
hope to be able to prove in the concluding part of this paper.

Menzbieb (p. 62-66) describes the organs which constitute the proboscis of
Mmca, and which are, according to him, the following: a labrum united to an



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epipharyni, a bypopharynx, a labium, and two maxillary palpi; mandibles and
maxillae are entirely absent. Menzbier says that the fulcrmn is of ^'chitinized
processes of the wall of the pharynx." * Menzbier criticizes severely (p. 26-27)
several absurd statements made by Lowne, in regard to the proboscis of Muscch


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Online LibraryGeorge DimmockThe anatomy of the mouth-parts and of the sucking apparatus of some Diptera → online text (page 4 of 7)