Samuel Wendell Williston.

Manual of North American Diptera online

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during flight; an orienting or balancing function has
been ascribed to them. The halteres are very character-
istic of the order, always present in the winged forms,
and usually present even in those in which the true wings
are aborted or functionless, though rarely they are en-
tirel}^ absent. Not all flies are winged, as has been al-
ready intimated; rarely, among both the more general-
ized and the more specialized groups they are imperfectly
developed or wholl}^ wanting. Their absence is not of
very great classificatory importance, never more than
generic, and sometimes doubtfully that. Sometimes the
male has wings and the female is wingless ; but the num-
ber of wingless forms of any kind is very small.

In size as well as in shape, flies vary not a little. As
compared with the lepidoptera, orthoptera, neuroptera,
and even the hemiptera, flies are relatively small insects.
The largest specimen of a fly of which I have knowledge
is that figured herewith natural size, pertaining to an
indeterminable species of Mydas from South America.
The length of this specimen from the tip of the antennae

Fig. I. My das, species indet. lyife size. (Brazil.

Fig. 2. Acanthomera, species indet. L/ife size. (Venezuela.)


to the extremity of the abdomen, is sixty-seven millime-
ters, or, omitting the antennae, fifty-two millimeters; the
expanse of wings one hundred and seventeen millimeters,
or a little more than four and one-half inches. The
smallest dipteron that I have ever observed in the
examination of many thousand specimens and five
or six thousand species, is that of a cecidomyid measur-
ing a trifle less than one half millimeter, also omitting
the antennae. In other w^ords, the Mydas is more than
one million times the size of the cecidomyid. Possibly
there are still greater discrepancies between the largest
and smallest specimens of the order, but in all prob-
ability not much. The largest insect known is one
allied to the dragon flies, an extinct Devonian species
which measured about fifteen inches in length. As-
suming that the bodily proportions of the largest and
smallest hexapods are not unlike, the extremes of size or
weight are more than four hundred millions apart. In
no single family of diptera are the differences in size
anywhere nearly so great as those between the niydaid
and cecidomyid. Seldom do the differences in linear meas-
urements in any one family exceed ten fold. Among
other families of diptera the tipulids, asilids, and especial-
ly the pantophthalmids, often furnish examples of large
size, while the tabanids, syrphids and cyrtids have not a
few forms of considerable size. The largest of all the
Cyclorrhapha will be found among the Calypterae, while
the Acalypterse are rarely much above the average in
size, and many are small, or very small.

Giantism in any group of animal life is a specialization,
and is, in general, an indication of approaching decadence;
enduringly small races are never the descendants of
giants, for decrease in size means lessened vitality and
incipient extinction. No strong or dominant group of
flies, like the Tachinidse, Dolichopodidae, Syrphidse, or



Bombyliidse, has ever had in the past a larger average
bodily size than is found among their living representa-
tives. On the other hand, those families composed to-
day chiefly of large forms are ones already past their
prime. These conclusions seem established for the larger
forms of life, and I believe that they are in the main also
applicable to insects.

Many curious resemblances of external form are ob-
servable among diptera belonging to widely different fami-
lies and of remote relationships. Some of these, perhaps
many, have a protective value, in flies of like habits; or
they may have been the results of like environmental

Ceria. Ceriomydas. Conops.

Fig. 3. Examples of mimetic resemblances in flies. Ceria (Syrphidse);
Ceriomydas (Mydaidse); Conops (Conopidse) . Slightly reduced.

In figure 3 are given photographs of three wasp-like
flies of very different families and of considerable size,
all of them associated in the same fauna, and curious^'-


resembling each other; several others equally striking
might have been associated in the same group.

The habits of flies are very diverse. Many are prop-
erly spoken of as flower flies — that is, insects whose sub-
sistence, for the most part or entirely, is obtained from
the honey or pollen of flowers, and the sweetened sap
of plants. Among these are many which are swift in
flight, spending most of their time during sunny hours
upon the wing or resting lightly upon leaves and flowers ;
they all love the warmest sunshine. Some mature flies
feed upon ordure or decaying material of whatever nature
it may be. Many others, including whole families like
the Asilidse, Dolichopodidse and Empididse, find their
subsistence in the juices of other insects, and are often
predaceous in the highest degree. Others, like the female
mosquitoes, blackflies and horseflies, are bloodsucking
in habit, though not exclusively so, and are often very
annoying to man and other warm-blooded animals. The
whole group of Pupipara, with the exception of the Brau-
lidse, are of this kind, living parasitically upon and suck-
ing the blood from mammals and birds. In the previous
edition of this work I expressed the opinion that, upon
the whole, the order of diptera is beneficial to man's econ-
omy, since so many of its members, whether in the larval
or adult conditions, are either useful scavengers, destroy-
ing that which otherwise might cause distress, or prey
upon other and injurious insects. Since the publication
of that edition, however, the many marvelous and import-
ant discoveries of the parasitic habits of certain ver}^
small protozoans which find their intermediary hosts in
certain flies, by whose instrumentality they are trans-
mitted to man and some of his most valued domestic ani-
mals, will require the complete reversal of that opinion.
Yellow fever, malaria, and filariasis, transmitted in the
saliva of mosquitoes, and the fatal 'sleeping sickness'


(trypanosomiasis), likewise resulting from the predatory
bites of the tsetze ^y, are among the most virulent or
widespread diseases of mankind. Perhaps we may justly
say that the order, so far as man is concerned, is the
most pestilential of all animal life.

In the larval condition the habits of flies are even more
diverse than are those of the adult insects. Brief refer-
ences to the larval habits will be found in the discussion
of the families. Suffice it here to say that the larvae or
'maggots' of diptera are, for the most part, vegetable
feeders, but not a few feed upon living or decaying ani-
mal matter; and many are parasitic within the bodies of
other insects, whether larvae or adults, snails, reptiles,
birds and mammals, and possibly also amphibians and


In the following pages I endeavor to give such defini-
tions and descriptions of the mature insect as will enable
the student to understand and appreciate, not only the
present work, but all other systematic works upon dip-
tera. I have not thought it desirable to consider at length
many interesting subjects connected with them, such as
their internal anatomy, embryology, larval habits, etc.,
as being rather apart from the chief object of the work —
an introduction or aid to the study of S3^stematic dipter-



The head in diptera is very variable in shape, reaching
its most reniarkable development in the Diopsidse and
Nycteribiidse. It is frequently more or less spherical,
but usually the posterior surface or occiput is flattened
or concave, giving a more hemispherical appearance.
The face is rarel}^ produced into an elongated rostrum
or snout, and the front part ma}^ be produced into a con-
ical prominence. . Its relative size is also variable, some-

HEAD. 2 1

times distinctly wider than the thorax, at other times
small. In the Nycteribiidae it may be folded back into a
groove on the dorsum of the thorax, but with these ex-
ceptions, it is always attached to the thorax by a freely
movable neck. The taxonomic characters furnished by
the head are second only in importance to those of the

Eyes. The large compound eyes a're present in all dip-
tera, save some Pupipara. In the majority of males,
especially of the Orthorrhapha, they are contiguous on
the upper side of the head, between the insertion of the
antennae and the hind margin, for a longer or shorter
distance; insects having such contiguous eyes are called
holoptic (Osten Sacken). In many males, however (all
the Acalypterse and several families of the Orthorrhapha,
as well as numerous genera of other families), and in
all females, with but few exceptions (certain Cyrtidae,
Orphephilidse, Blepharoceridse, Bombylidse, Platype-
zidse, etc.), the eyes are separated more or less broadly
by the front: such insects are called dichoptic ( Williston ) .
Rarely the eyes may be contiguous below the antennae,
or both above and below, as in certain cyrtids, etc. In
not a few flies, especially those of the aerial, bristleless
kinds, the upper facets of the e^^es are larger and more
conspicuous than the lower ones, separated by
a distinct line, or even entirely divided. This peculiar-
ity is rarely seen in the female or even in the dichoptic
male, though the dichoptic Asilidae may have the anterior
facets somewhat enlarged in both sexes. Those flies hav-
ing such enlarged facets usually have the eyes in life
brilliantly and beautifully colored with green and purple
markings — markings often characteristic of the various
species, and the general pattern even of the genera.
Unfortunately such markings are obliterated by dessica-


tion, though they may be somewhat revived temporaril}^
by the aid of moisture.

Most flies have the eyes bare, or pubescent only when
seen under high magnification. Not rarely, however,
the whole or part of the eyes is covered with erect short
pile, a character which usually, perhaps always, finds its
greatest development in the male sex. The pubescence
or pilosity may be sparse or dense, short or long, and is
usually, though not always, of generic importance.

Ocelli. On the upper part of the front in the middle,
between or a little back of the compound eyes, there are
three simple, small eye lenses, present in most diptera,
and called ocelli. They are by no means constant among
all the genera of some families, or even among all the
species of some genera. They are usually situated in
the form of a triangle with the apex in front ; sometimes
they are located in a nearly straight line transversely;
or, the middle one may be rarely absent, and the other
two situated, one on each side, near the compound eyes.

Front. The space between the eyes in all dichoptic flies,
limited by the upper margin of the head and the line
drawn through the root of the antennse, is called the front.
It may be wide or narrow, excavated or convex, etc.

Vertex. The uppermost part of the front, near the mar-
gin of the occiput, which is here called the vertical margin.

Vertical triangle. The triangle at the upper part of the
head, between the eyes in holoptic flies. It bears the
ocelli, which may be situated on a triangle indicated by
grooves or depressions or colorations, called the ocellar

Frontal triangle . In holoptic flies, the triangle between
the eyes and the root of the antennse, the apex of which
is above. Sometimes the term is applied to a triangle
indicated by color or depression in the dichoptic front.

Ptilinum. In the Cyclorrhapha an inflatable organ ca-



pable of being thrust out through the frontal suture just
above the root of the antennae, and which is used by the
imago in springing off the cap to the puparium when
about to extricate itself.

Frontal lunule. An oval or crescentic space just above
the root of the antennae in cyclorrhaphous flies, bounded
by the frontal suture.

Epistoma or Peristoma. The oral margin and an indefi-
nite space immediately contiguous thereto; not often
now used.

Antennal fovea or groove. A groove or grooves in the
middle of the face, as though for the lodgment of the an-
tennae, bounded on the sides by ihe facial iddges.

Cheeks or ^jowls'. The space back of the face and below
the eyes.

Orbits. The space immediately contiguous to the eyes,
sometimes indicated by structural characters, at other
times indefinite. It is called facial, frontal, etc., from
the position.

Clypeus. A part of the mouth structure, often visible
below the margin of the mouth in front as a more or less
viscr-shaped piece.

Fig. 4. Mouthparts of female 7rt;(^rt;;2//j. After Washburn . ?;//>, maxil-
lary palpus; ni, mandible; w.r, maxilla; lb, labium.



The mouth-parts of diptera are wholly suctorial. The}^
differ not a little in different flies, as might be supposed
from their diverse habits. In some they are adapted for
piercing animal or vegetable substances, and are, in con-
sequence, firmer and more slender; in others, and by far
the greater number, they are short and soft, with a thick-
ened extremity used for the attrition of small particles of
solid substances. Grains of pollen have been observed in
the digestive organs of the Syrphid^ and other flower
flies, but, as a rule, fluids only serve as food. Many have
the proboscis wholly retractile into the oral cavity, and
furnished with one or even two hinges, by which when at
rest it may be folded up. In others the proboscis is not
retractile, and projects either in front, or downward or
backward, beneath the body. While it is usually short,
it may be much longer than the body. Finally, a few
species have the mouth-parts vestigial and take no nour-
ishment in the adult state.

The more commonly accepted homologies of the mouth-
parts are as follows: labium, maxillae, maxillary palpi,
mandibles, hypopharynx, and labrum or labrum-epiphar-
ynx. The labial palpi are thought to be wholly wanting,*
or represented by the labella. The labium is always
present, more or less fleshy and provided with muscles.
It is grooved or channeled on the upper side to receive
the other parts, with the exception of the maxillary palpi,
which are free. This sheath is often nearly complete,
the thin margins touching each other above. At its tip
are the pair of joints of variable size called the 'lips'
or labella. The maxilla and mandibles are sometimes

*This has very recently been contested by Wesche, who asserts that
either the labial or maxillary palpi may be present and functional; but
no cases are known of both pairs being functionally present. I am
inclined to be skeptical.



absent, the mandibles most frequently; when present
they are always slender and firm. The hypopharynx is
unpaired and slender, grooved on the upper side and
sometimes converted into a nearly complete tube. The
labrum, also unpaired, is usually elongate and grooved
on the under side, forming by apposition with the hypo-
pharynx a complete tube. The mandibles are frequently
absent ; in fact I do not know of their occurrence in any
flies with a simple third antennal joint, and they maybe
absent in the male when present in the female, as in the
Tabanidse. They are always piercing organs, thin, firm,
chitinous and usually slender. The two maxillae, like-
wise piercing organs, find their highest development in
such predaceous flies as the Asilidse. Like the mandi-
bles they are chitinous and slender. In some they are
more or less flattened, and may have curiously shaped
projections at the tip; usually they are bristle-like.
They lie with the maxillae within the sheath of the labi-
um, at either side of the labrum and hypopharynx. In
some cases the labrum is short, and serves only as a cov-
er for the proximal part of the hypopharynx, but usually
it is as long as or longer than the hypopharynx and has
a simple groove on the under side. The hypopharynx
is always present in flies in which the mouth-parts are
functional. It is, more often, a slender, firm organ,
grooved upon the upper side, which by apposition with
the labrum forms a distinct tube. In some, however,
it may form an almost complete tube in itself.

Leaving out of account the degraded, but highly spe-
cialized Pupipara, the labium is always a sheath for all
the other organs except the palpi, but is separable at the
will of the insect. It is not used in piercing ; it is either
bent backward in the middle, as in the mosquito, or the
piercing parts are thrust out at the extremity as in most
of the predaceous flies. To facilitate this protrusion of


the piercing parts, the proximal portion is more or less
membranous and retractile ; or, the inner organs may be
capable of elongation, being coiled up in some cases, as
in Pangoyiia, within the pharyngeal cavity. The pair of
organs at the extremity, the lips or labella, are very va-
riable in shape, position and function. In the mosquito,
for instance, they serve merely as a pair of fingers to
guide the piercing parts. In many of the flower-flies with
long proboscis, they are small, oval, divaricable organs,
that seem to be chiefly sense organs, as they are usually
provided with hairs inserted in small, semi-translucent
spots on the outer sides and margins. In the greater
number of flies, however, the labella are of considerable
size, and are provided with radiating ridges on the inner,
opposable sides. These pseudotracheae, as they are call-
ed, serve as means of attrition, by which the insects rub
off particles of food from firm substances. Sometimes
the labella are long and slender and folded back under
the labium when at rest. In the Asilidse and some others,
they are rigid and horny.

Perhaps the most important of all the mouth-parts,
from the systematic stand point, are the maxillary palpi.
They are inserted at the inferior basal part of the pro-
boscis, on a thin plate which bears the maxillae, and
are always extricated. Their study has been much neg-
lected, and doubtless thorough comparative researches
will reveal not a few characters of value in classification.
They are variously described as being composed of from
one to five joints.* There are never more than four articu-
lated joints, the basal joint being merely a process of the
plate bearing the maxillae. The tendency in diptera is
toward their entire loss, and in the more highly special-
ized families there is never more than one joint. They

^Theobald says some Culicidae have six joints, but his statement
needs confirmation. .


may be reduced to the merest vestiges even in flies which
are mere or less predaceous in habit and which have the
mouth-parts, with the exception of the mandibles, other-
wise well developed ; they may indeed be absent or pres-
ent in otherwise closely related genera of flies. They are
seldom much elongated, save among some of the Nemo-


Fig. 5. Antennae, i, Tipnlidae {Polyinera, female); 2, Tipiilidse
{R/npidia, male); 3, Culicidse {Aedes, male); 4, Tabanidae {Stibaso-
iua)\ 5, Empididse [Drapetis); 6, Syrphidae {Volucella)\ 7, Tachinidae

No other organs furnish so many or so important char-
acters in the classification of Diptera as do the antennae,
or feelers as they are sometimes called. The number,
shape and arrangement of the joints or segments offer
not only the best of specific characters in nearly all cases,
but also not rarely generic, family and even subordinal
characters. Only in very exceptional instances is the
number less than three, and it is probable that, even


in those, there is only a partial atrophy of the basal joint.
Some Cecidomyidse and Leptidse ( Rhachicerus ) have as
many as twenty-eight distinct joints, and it has been said
there are as many as thirty-nine in some Cecidomyidae
(Cerodoziaf), but I can find no certain evidence support-
ing such a statement. Through all the Cyclorrhapha
the number three, or very rarely less, is constant. In
the nematocerous Orthorrhapha the number is usually
from eight to sixteen, though in rare cases there m.ay be
as few as six. In such antennae the first two joints are
called the scape, and they are always more or less differ-
entiated from the remaining segments which compose the
flagellum. When the antennae are long, or more or less
filiform, the joints of the flagellum often bear whorls or
verticils of hairs, especially in the males, and these hairs
are sometimes of very peculiar structure, sometimes loop-
ed; in such antennae, also, the joints may have a long
and abundant plumosity.

The scape in all flies usually bears bristles or bristle-
like hairs; the joints of the flagellum seldom. Upon the
structure of the antennae alone, however, it is difficult, if
not impossible, to distinguish any of the larger groups of
diptera, since those with long and multiarticulate antennae
merge very gradually into forms with shorter antennae
and fewer joints. Or, the separate joints of the flagel-
lum may be fused so closely as to be distinguishable
with difficulty, so that the third joint, or the basal part
of the flagellum, may appear to be of a simple, undiffer-
entiated structure, though upon closer examination found
to be composed of distinct segments or annuli; in such
cases the term complex is applied to the apparent joint.
This peculiar structure will be readily understood by the
examination of the antennae of a common horsefly (fig.
5, 4), where the enlarged third joint is observed to be
composed of a large basal piece and a terminal portion


of four closely united segments. By the comparison of a
horsefly's antennae with that of a Bibio and that of a mos-
quito it will be easily understood that the complex third
joint is merely the closer fusion of the real joints of the
flagellum, though I know of no instance of a complex
joint having more than eight segments. Nor is the horse-
fly's so-called third antennal joint homologous with that
of the housefly, but rather with the joint and the 'arista'
combined. Either the complex or the simple third joint
may terminate in a more or less slender, and more or less
freely differentiated portion called the style, or in a bris-
tle-like, elongated portion, called an arista. It is ver}^
evident, however, that the st5de or arista represents
merely attenuated distal joints of the flagellum, since a
close examination of them will invariably, or almost in-
variably, disclose from two to five segments (fig. 5, 7),
though some may be very small or almost completely
atrophied. The arista therefore as might be supposed,
is not sharply distinguishable from a slender style; in-
deed the arista always, or almost always, has its basal
portion thickened more or less, in some cases so much so
as to form a real though short style provided with a long
bristly extremity; and the style is often provided with a
short bristly end. An examination of the antennae of the
Leptidae and Stratiomyidse, figured further on, will make
these statements clear. The arista or style is of course
not at all apparent in the Nemocera, since the antennal
structure is here generalized, and it maybe entirely want-
ing among the Brachycera, either because the distal
flagellar joints have not become at all differentiated, as
may be observed in Xylophagus of the Leptidae, or because
there has been an actual atrophy of the distal part of the
antennae ; but the cases are rare where some of the flag-
ellar joints beyond the basal one are not observable in
flies, albeit very rudimentary. Whenever the style or



arista is composed of numerous segments, the basal piece
of the complex third joint has necessarily fewer segments,
since both together never have more than eight. The
style, as the term is usually applied, is always terminal;
while the arista may be either terminal or dorsal, usually
the latter, and it may even be inserted close to the base
of the third joint. It is also apparent here, and this is
the rule among the Cyclorrhapha, that the basal position
is in reality due to the greater development of the under
side of the joint, b}- which the width has greatly exceeded
the length. The st3de is only rarely present in the Cy-
clorrhapha ( Ceria, Conops) , and in but few known instances

Online LibrarySamuel Wendell WillistonManual of North American Diptera → online text (page 2 of 25)