Samuel Wendell Williston.

Manual of the families and genera of North American Diptera online

. (page 2 of 18)
Online LibrarySamuel Wendell WillistonManual of the families and genera of North American Diptera → online text (page 2 of 18)
Font size
QR-code for this ebook

plete tube. The labrum, also unpaired, is usually elongate
and grooved on the under side, forming by apposition with
the 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 may be 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, likewise piercing organs,
find their highest development in such predaceous flies. as the
Asilidse. Like the mandibles they are chitinous and slender.
In some they are more or less flattened, and may have curi-
ously shaped projections at the tip; usually they are bristle-
like. They lie with the maxillae within the sheath of the
labium, at either side of the labrum and hypopharynx. In some
cases the labrum is short, and serves only as a cover 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, Arm organ, grooved upon the upper side, which by
apposition with the labrum forms a distinct tube. In some,
however, it may form almost a complete tube in itself.

Leaving out of account the degraded, but highly specialized
Pupipara, the labium is always a sheath for all the other or-
gans 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
j)ortion is more or less membranous and retractile; or, the
inner organs may be capable of elongation, being coiled
up in some cases, as in Pangonia, within the pharyngeal


cavity. The pair of organs at the extremity, the lips or la-
bella. are very variable 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 Avith hairs inserted into small, semi-translu-
cent 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 called, 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 standpoint, are the maxillary palpi. They are
always inserted at the inferior basal part of the proboscis, on
a thin plate which bears the maxillae, and are always extri-
cated. Their study has been much neglected, and doubtless
thorough comparative researches will reveal not a few charac-
ters of value in classification. They are variously described as
being composed of from one to five joints. Probably there
is never more than four articulated 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
most highly specialized families there is never more than one
articulated joint. They may be reduced to the merest rudi-
ments, even in flies which are more or less predaceous in hab-
its and which have the mouth-parts with the exception of the
mandibles, otherwise well-developed. They are never greatly
elongated, save among some of the Nematocera.

Without going into Prof. Smith's arguments, he shows with
what seems to be much force, that the real homologies of the
dipterous mouth-parts are as follows :



Labrum. .




Maxillary palpi.






Maxillary palpi.



The tliorax is composed, as in other insects, of three parts,
the ^ro-thorax, the meso-thorax and the 7neta-thovax, but the
first and the last are so aborted as to present but few ana-
tomical characters. The prothorax is perhaps most readily
distinguished in the Nematocerous flies, forming a rounded
collar back of the neck. The metathorax is not seen at all
from above ; the scutelluin, cut off by an impressed line, usu-
ally a semi-oval body, really belongs to the mesothorax, the
dorsum of which is often called the mesonotum.

Transvei'se suture, an impressed line usually running
straight across the mesonotum and terminating a little in front
of the root of the wings. It is more or less incomplete in
the middle.

Humerus or humeral callus, the anterior superior angles of
the mesothorax, usually a more or less rounded callus.

Post-alar callus, a more or less distinct rounded swelling,
situated between the root of the wing and the scutellum.

Pre-alar callus, a similar, but usually less prominent, pro-
jection situated before the root of the wings on the sides of the
mesonotum, just back of the outer ends of the transverse suture.

Scutellar bridge, a small ridge on either side of the scutel-
lum, connecting it with the mesonotum.

Presutural depression, a depression, usually triangular in
shape, at the outer ends of the transverse suture, near the
dorsopleural suture.

Supra-alar groove, a groove on the mesothorax immediately
above the root of the wings, along the inner margin of which
there are, usually, characteristic bristles.


Dorsopleural suture, the suture running from tlie humerus to
the root of the wings, separating the mesouotum from the
pleura. Mik proposes for it the rather more appropriate term
of notopleural suture.

Sternopleural suture, the suture below the dorsopleural
suture, nearly parallel with it and separating the mesopleura
from the sternopleura.

Mesoyleural suture, the suture running from the root of the
wings downward and separating the mesopleura from the

Mesopleura, the space situated in front of the root of the
wings, between the dorsopleural and sternopleural sutures.

PteropAeura, situated below the root of the wings, back of
the mesopleural suture.

Sternopleura, the lower part of the pleura, below the sterno-
pleural suture and above the front coxae.

Hypopleura, the space over the middle and hind coxae, be-
low the metapleura and pteropleura.

Metapleura, the " sides of the metanotum ", a more or less
swollen space at the outside of the metanotum and between it
and the pteropleura and hypopleura.

Metanotumfh, the oval, arched portion behind, beneath the
scutellum. It is frequently the best developed in the flies
with long, slender abdomen, as the Tipulidse.

Halter es, balancers or poisers, rudimentary posterior wings,
a slender organ with a dilated head, situated below each

Tegulce or alula", a pair of membranous scales situated
above the halteres and back of the root of the wings, one
above the other. The lower one or both may be rudimentary
or absent; the upper one moves with the wings and is called
the antitegula by Osten Sacken. Comstock, however, objects
to this use of tegula, saying that the term was first used
for the cup-like scale above the root of the wing in certain
hymenoptera, and should be reserved for that organ.



The three pairs of legs are attached to the prothorax, meso-
thorax and metathorax, and are called, respectively the front,
middle and hind pairs. When the front and middle pairs are
spoken of together they should be called, for the sake of ex-
actness, the anterior legs ; when the middle and hind pairs
are collectively meant, the posterior legs. The legs are com-
posed of five parts :

Coxa, the part attaching the legs proper to the thorax.

Trochanter, the short, small, ring-like portion between the
femur and coxa.

Femur, almost invariably the longest and stoutest portion
of the legs, often provided with tubercles, spines or projec-
tions or sometimes greatly thickened ; usually slender.

Tibia, the next part succeeding the femur, and like it often
with various ornamentations or projections. When it ter-
minates in one or more distinct, short bristly spines, it is
said to be spurred.

Tarsus, the distal division of the legs, composed (except in
some abnormal forms) of five joints, of which the first, that
next to the tibia, is called the metatarsus. On the terminal,
or fifth joint, are the

Ungues or claws, usually two, curved movable booklets on
the under side of the last tarsal joint, at the base of which
below, are a pair of

Pulvilli, two pad-like, fleshy cushions attached to the last
joint of the tarsus below the claws, usually present, but often
absent among the Orthorrhapha and often much larger and
better developed in the male than the female. Between them,
among a number of the families of the Cyclorrhapha, is the

Empodium, a median appendage on the under side of and
between the claws, either in the form of a pad, like the pul-
villi, when it is called pulvilliform, or like a bristle or spine ;
rarely it is alone present and the pulvilli wanting.



The abdomen is composed of a variable number of segments,
more or less closely fused together. The normal number for
insects, nine, are rarely all visible (the Tipulidse are exam-
ples). They are counted from the base on the upper side. In
not a few cases the lirst two are so closely fused together, and
the first one abbreviated, that the nomenclature leaves some
doubt in the mind of the student. The upper part of the ab-
domen may be especially indicated by the word dorsum, but
in general, the venter or under part is alone thus contradistin-
guished. The male genitalia, which in many cases are of
complicated structure, and of much value in classification, are
known collectively as the hopopygiurn, though Bergroth has
proposed the name propygiitm for them. The ovipjositor of the
female abdomen very frequently projects from the abdomen,
and is sometimes extremely long ; its structure is often char-
acteristic of genera or families. The more detailed descrip-
tion of the parts of both these organs it is unnecessary to go
into here ; they may be studied in the different families, where
they often find useful application in the se^jaration of species.


Until within recent years but little attention had been paid
by writers on systematic dipterology to the number and ar-
rangement of the bristles among flies. Osten Sacken, who
proposed the term chwtotaxy, to designate the science of their
arrangement, published an epochmaking paper on the subject
in 1881. Since that time their use has steadily increased
with the most happy results. That they will become more
and more useful as their study in different families progresses,
there can seem to be no doubt. For that reason, it behooves
the student to become familiar with the nomenclature already
in use. The following are the most important terms :

Cephalic Bristles.

Vertical, two pairs, inner and outer, inserted more or less


behind the upper and inner corner of the eye, erect, or the
inner pair convergent, the outer pair divergent.

Postvertical, in the middle of the upper part of the occiput,
generally small or absent.

Ocellar, situated close to the ocelli, usually directed for-
ward ; often absent.

Fronto-orbital bristles, placed on each side of the front, near
the orbit, immediately below the vertical bristles. There
may be one, two or none on each side.

Lower fro7ito-orhital, situated on the lower part of the front,
above the antennae, along the orbit, and not quite in line with
the fronto-orbital bristles. They are not of frequent occur-

Vibrissa, a stout bristle situated near the oral margin on
each side. Of important use in the classification of the

Facial, a series of bristles on either side of the middle por-
tion of the face, above the vibrissse, especially conspicuous
among many genera of the Tachinidse, but usually absent in
the Acalyptratse.

Thoracic Dorsal Bristles.

Humeral, one or more bristles inserted on the humeral

Post-humeral, usually two, inserted immediately above the
dorso-pleural suture, between the humeral callus and the root
of the wing, on the bottom of the pre-sutural depression.

Pre-sutural, one or more bristles situated immediately in
front of the transverse suture, above the pre-sutural depression.

Supra-alar, usually three bristles, one on the post-alar cal-
lus, one on the alar frenum and the third in front of the second
on the edge of the supra-alar depression.

Intra-alar, a row of two or three bristles between the supra-
alar group and the dorso-central bristles.

Dor so-central, two or four longitudinal rows on the inner
part of the dorsum, sometimes represent by the


Prescutellar, a transverse row of bristles in front of the

Thoracic Pleural Bristles.

Propleural, bristles inserted immediately above the front

Mesopleural, bristles inserted on the mesopleurse, in the
angle formed by the dorso-plenral and mesopleural sutures.

Sternopleural, one or several bristles situated on the sterno-
pleura, below the sternopleural suture.

Pteropleural, bristles inserted on the pteropleurse, rarely
present and difficult to distinguish.

Metapleural, bristles inserted on the metapleurse, especially
conspicuous in the Asilidae-, and named by Lynch the

Trichostical bristles, a fan-like row on the metapleurse, con-
spicuous in some families.

Hypopleural bristles, a row or tuft of bristles on the hypo-
pleura, occurring in the Calyptratse.

Abdominal Bristles.

Marginal bristles, bristles inserted on the posterior margin
of the segments, especially conspicuous in many Tachinidse.

D'lscal bristles, bristles, usually one or more pairs, inserted
on the middle of the segments before the hind margin.

Lateral bristles, one or more bristles situated on or near
the lateral margins of the segments.

In addition, a number of terms are used to designate the
inclination of the bristles, often important in describing the
cephalic bristles. The more important of these are erect, when
standing vertically, or nearly so; proclinate, when directed
forward ; reclinate, when directed backward ; divaricate or
divergent when directed outward from the middle line ; con-
vergent when directed inwardly ; decussate or cruciate when
crossing each other.



To understand the neuration or venatio7i of the wings the
student may select a common large horse-fly (Tabanidse).
Observe near the middle of the wing directed transversely, a
large, oblong, five or six sided cell, surrounded on all sides by
other cells. This is the discal cell and is present in nearly
all flies. Somewhere on the vein (fourth longitudinal), that
bounds this cell in front, will be seen a short connecting vein,
directed anteriorly, the anterior or small cross-vein, which
affords, in most cases, a key to the neuration, no matter how
intricate. It always connects the fourth longitudinal vein
behind with the third longitudinal vein in front (in a few rare
cases the second longitudinal vein); the cell behind it is the
discal, in front, between the second and third longitudinal
veins, the submarginal ; on the outer side the first posterior ;
on the inner side the first basal. Just back of the first basal
cell and separated from it by the fourth longitudinal vein, is
the second basal cell ; back of the second basal and separated
by the fifth longitudinal vein, is the third basal or anal cell.
Back of the anal cell and including the free posterior proximal
portion of the wing is the axillary cell. In the horse-fly the
anal cell is seen to run back obliquely to near the posterior
margin of the wing, where it terminates acutely, that is, the
anal cell is closed near the border of the wing; should the
two veins that close it run separately into the margin of the
wing, then the cell is said to be open. Counting from the
third longitudinal vein (posterior branch) backward along the
posterior border of the wing, to the vein that closes the anal
cell outwardly, the student will count five different cells, the
first of which, as already said, borders on the first basal cell,
the second and third on the discal cell, the fourth on the
discal and second basal cell, and the fifth on the second basal
and anal cells ; these cells are called the 2^osterior cells, and
are numbered from before back; the first is bounded by the
third and fourth longitudinal and the anterior cross-vein ; the


second by the fourth longitudinal in front, the anterior inter-
calary vein behind, and the posterior cross-vein at the outer
side of the discal cell ; the third is bounded by the anterior*"
intercalary, and the fifth longitudinal vein and the posterior
cross-vein; the fourth is bounded by the fifth longitudinal
vein in front and the posterior intercalary vein behind ; the
fifth by the posterior intercalary; the fifth longitudinal at the
outer end of the second basal cell, and the posterior basal cross-
vein at the outer end of the anal cell. The short vein which
separates the second basal cell from the discal cell is known
as the anterior basal cross-vein. Now, following the third
longitudinal vein outwardly it will be found to give off an an-
terior branch which runs forward to terminate in the anterior
border of the wing, or costa ; the cell included between this
fork and the vein itself behind is the second submarginal cell.
The second longitudinal vein borders the first submarginal
cell in front and terminates in the costa ; between it and the
first longitudinal vein which has a similar course and termi-
nation is the marginal cell ; between the first longitudinal
vein and the costa there is another parallel vein with nar-
row cells on each side of it, the vein is the auxiliary, and the
cell before it the costal, behind it the subcostal. Finally, near
the root of the wing there is a short cross-vein connecting the
auxiliary vein with the costa; it is known as the humeral
cross-vein. .

The student is urged to procure a copy of Comstock's Man-
ual of Insects for use in connection with this work. The
figures there given of the various parts of the external anato-
my of diptera and especially of the neuration, are the truest
to nature that I have ever seen. The nomenclature of the
wing-neuration there adopted, or some modification of it, is
destined to supplant the Meigenian or Schinerian terminolo-
gies now almost exclusively used. I have not adopted it in
this work for two reasons : first, that it has not yet been fully



crystallized into a permanent shape ; second, because nearly
all the existing literature has the nomenclature here employed
and to use a new one would largely defeat the object of the
work in the hands of the beginner, I give below the ter-
minology used by Comstock, together with the equivalent
terms here used and the equivalent ones in the terminology
of Schiner, for the horse-fly.

Auxiliary vein . . .



First longitudinal .



Second longitudinal

1112,3 .


Third longitudinal .

III5 .


Fourth longitudinal



Fifth longitudinal .



Sixth longitudinal .



Anterior branch of third vein


Anterior intercalary


Posterior intercalary . . .


Costal cell ....



Subcostal cell ....



Marginal cell ....



First submarginal cell



Second submarginal cell


First basal cell


Second basal cell


Anal cell ....


Axillary cell ....


First posterior cell . . .


Second posterior cell


Third posterior cell


Fourth posterior cell .


Fifth posterior cell .


Discal cell ....


Flies differ very much in the nature of their covering.
Many are nearly or quite bare; others have a thick, woolly
covering of closely set, long fine hair ; while others still have
an abundant covering of long stout, heavy bristles' or^macro-
chcetce. Undoubtedly the kind of covering has much relation


with the habits of the mature insects, yet just what the rela-
tions are is not yet well understood. Osten Sacken has ob-
served that the eremochsetous flies (i. e. diptera in which
there is a general absence of bristles, as for example the
Stratiomyidse, Leptidae, and Tabanidiae) are for the most part
holoptic in the male sex and at the same time are principally
aerial flies, flying swiftly and with the habit of hovering,
using their legs only for alighting. On the contrary the
chsetophorous flies (as the Muscidse, sens, lat., Phoridae, Doli-
chopodidge, Asilidse, etc.) use their legs as much as, sometimes
more, than the wings for locomotion, and rarely have the eyes
contiguous in the male sex. Probably the development of the
macrochsetae reaches its highest extent among the Tachinidse,
as for instance in Dejeania, Saundersia, etc., and the Dexiidae
{^Hystrisiphona, etc.), where the abdomen may be almost
wholly covered with long and erect, very rigid spines.

As concerns other forms of covering, the usage of writers
is not very exact ; the terms hair, pile, pubescence and tonieii-
tum are used with a wide degree of latitude. In general, how-
ever, pile should be restricted to indicate close, thickly set,
fine hair, as in the pile of velvet, while hair may mean longer,
and less abundant. Pubescence should mean very short, tine
hairs, while tomentum can only be correctly applied to recum-
bent, flattened scale-like or stubble-like hairs, which gradually
merge into dust or pollen, which is so generally present in
flies, and upon which the determination of many species must
largely depend.


For the following brief account of the internal anatomy of
Diptera I am indebted to Prof. V. L. Kellogg.

The special features of the internal structure of the Diptera
are the high degree of concentration of the nervous system
attained in some of the members of the order, the expansion
of the two main tracheal trunks in the base of the abdomen


to form air sacs, the presence of a sucking stomach as in the
Lepidoptera, the constant number (four, rarely five) of the
Malphigian tubes, and the absence of a bursa copulatrix in the

The alimentary canal presents behind the oesophagus an
expansion which is a crop or sucking stomach. The ventri-
culus, or true stomach, lying behind it, has usually two coeca ;
and the long, slender, Malpighian vessels are, in almost all
species, four in number, a surprising constancy compared with
the condition in other groups of specialized insects. The ves-
sels open singly into the alimentary canal in some flies and in
others they unite in pairs before reaching the canal and open
into it by two ducts.

The heart is of the usual type, with two chambers in the
more specialized families, owing to the concentration of the
body. In the larva of Corethra the heart is a simple, elongate
tube without chambers.

The two main tracheal trunks expand at the base of the
abdomen into conspicuous air sacs similar to those found
among Hymenoptera, Lepidoptera, the lamellicorn beetles and
some other insects. The two pairs of spiracles of the thorax
are provided with " vocal cords ", and a considerable part of
the humming sound is produced by these structures. The
abdominal spiracles of some flies are as primitive as are to be
found among insects, being simply unlipped openings.

The condition of the nervous system varies greatly within
the order. In the elongate, more generalized Nematocerous
forms there are five or six abdominal ganglia, and three dis-
tinct thoracic ganglia. From this condition to that shown by
the Muscidae, where all the thoracic and abdominal ganglia
are united into one large ganglion in the thorax, a most in-
structive series of gradatory forms is presented. In the
Empididse, which stand intermediately as regards the concen-
tration of the ventral cord, the two anterior thoracic ganglia

2 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Online LibrarySamuel Wendell WillistonManual of the families and genera of North American Diptera → online text (page 2 of 18)