32 DIPTERA OF NORTH AMERICA. [PART IV

cross-vein, then alongthe discal cell,

between

the

two

intercalary veins, to the posterior

margin

(see 1. c. fig. 3, the

wing

of

Empis, which

in this respect resembles that of the Tipulidse,

and com-

pare itto the

wing

of Ortalis

and

to the

diagram which

I give below). Thus, if

we

force

upon

the Tix>ulidse the terminology introduced originally for the families of Diptera

with

a less de- veloped venation,

we meet with

inextricable difficulties.

But

there is

no more

reason for doing this than for following the opposite course,

adopting

a terminology for the Tipulidae first

and

forcing ^it afterwards

upon

the Muscidse. It is perfectly arbitraiy at

which end

of the

system

of Diptera

we

begin to trace out the

homologies

of the venation. This study of the

homologies

has

two

distinct aims*in

view

: the scientific

aim

of

showing

that the ground-plan of the venation isthe

same

inall

the families of the order;

and

the practical

aim

of

adopting

a terminology for descriptive purposes.

We

cannot carry out a terminology

on

solely theoretical

grounds

;

we

will

have

to vary the detailsof it according tothe peculiarities of structure occur- ringindifferent families,the

main

plan

remaining

thesame. This

is

done

in all the

departments

of zoology,

and

I

do

not see

why

the venation oftheDiptera should be treateddifferently.

In accordance

with

these views, I call fifth longitudinal vein the

whole

vein immediately following the second basal cell

and

thelast of the posterior cells; I call great cross-vein (in contra- distinction from the posterior cross-vein of the Muscidse) the cross-vein connecting the fifth vein with the vein preceding it.

T\\Q

fourth

vein, I look

upon

as including the discalcell

between

its

two main

branches.*

The

posterior of thesebranches is almost

always

forked (the posterior

branch

of this fork corresponds to

Mr. Loew's

2^osteriorintercalaryvein, v,in the

wing

of

Empis,

Ilonogr. I, p. xxiv, fig. 3)^;

and

the cross-vein, connecting this fork withthe anteriorbranch, closes the discal cell; hence,

when

the discal cell is open,

through

the disappearanceof thiscross- veinitcoalesceswiththesecondposteriorcell(asinTab.I, fig. 1), orwith the third,

when

there are five posteriorcells (as in Tab.

II, fig. IT).

Such

is the case with the majority of the genera

which have

the discal cell open, as

Orimarga, EmjJeda,

Crxjpto- labis, Ej'ioptera (subgenera: EriojJtera

and

Molopjhilus), Plec-

' IntinsIfollow Dr. Suliiuer's views.

INTRODUCTION. 33

troinyia, Dicranota,

and

Rliaphidolahis. In those

genera where

the discal cellis

open

in

some

species only, or in

some

specimens of certain species, the

same

rule prevails; it coalesces with the

second

posterior cell,

when

there are foursuch cells,

and

withthe third,

when

there are five

(compare

the

genus Dicranomyia).

Cases,

where

the anteriorbranch of the fourth veinis forked

and

the posteriornot;in otherwords, where, withfourposteriorcells,

the discalcell coalesces

with

the thii'd posterior cell (as in Tab.

I, fig. 15)^; such cases are rare,

and

occur

more commonly

only in the section Erioptei'ina

(compare

the general

remarks on

this section); outside of it, the

genera Thaumadoj^tera and

Elliptera (Tab. I, fig. 10) only possess t-his character. In Dic-

ranomyia pubipennis

0. S., also,

when

the discal cellis open, it

coalesces with the third posterior cell;

a

singular exception from

among

all the Dicranomyise, Outside of the Tipulida;

brevipalpi, this structure

may

be observed in

Pfychoplera

(Tab.

II, fig. 19).

The

occurrence of five posterior cells, without

any

fork

on

the posterior

branch

of the fourth vein, can take place only

when

the anterior

branch

of this vein has a double fork.

This isthe case

with Dolichopeza

; but I

have

not

met

with

any

instance of this kind

among

the Tipulidse brevipalpi, except in the

Limnophilina.

It is

worthy

of notice, that in this section

where

the discal cell is, as a rule,

always

closed,

whenever

an

abnormal specimen

is

met

with,

where

this cell is open, the branching of the fourth vein is very apt to

appear

like that of Dolichopeza.

The

fork ofthe anterior

branch

ofthe fourth vein is

formed by

the insertion of the vein

which Mr. Loew

calls theanterior inter- calary vein (uin

Monogr.

I,p. xxiv, fig. 3). Itis the addition of this vein

which

raises the

number

of posteriorcellsto five.

The

small cross-vein usually forms the inner

end

of the first posterior cell. In

some

rare cases the inner end of the sub-

marginal

cell is in

immediate

contact with thediscal cell (as in the

wing

of

Triogma,

Tab. I, fig. 7),

and

in such cases there is,

of course,

no

smallcross-vein. This structure characterizes the genera

Triogma and Paratropeza

Schiner; italsooccurs in

most specimens

of the

North American Cylindrotoma

nodicornis

and

adventitiouslyinthe

genus Rhamphidia.

I call prsefurca (aterm which has

been

used

by Mr.

Ilaliday in

Walker's

Ins.Brit. Dipt. Ill,p. 304) the portionof the second

3 June, 1868.

34

DIPTERA

OF

NORTH AMERICA. [part

IV, vein

between

itsorigin

and

the emissionofthe thira longitudinal vein.

The

petiole of thefirst submarginal cell isthe portion of the secondlongitudinalvein

between

thetip ofthe prsefurca

and

the inner

end

of that cell.

In

order to describe the relative position of the tips of the veins

and

of cross-veins, I

have used

the terra opposite;

two

points are opposite each other

when,

pro- jected

on

the longitudinal axis of the wing,they

appear

^equi- distant

from

its basis.

The

following

diagram

explains the other terms,

which have

been

used by me

^:

—

e

f

Diagramofawingwith two submarginalandJive posteriorcells

(Cladura iiuUvisa),

1.Costal.

2. Subcostal.

3. Marginal.

3*. luner marginal.

4. First subniari'inal.'

Cells.

5. Secondsubmarginal.

6-10. Firstto fifthposterior.

11. Discal.

12 Firstbasal.

13. Secondbasal.^*

14.Anal.

15. Axillary.

16. Spurious.

Veins.

bI. Auxiliary.

cm. Firstlongitudinal.

hno. Second longitudinal.

hi. Prtefurca.

kn. Anteriorbranchofthe second

1.vein.

k0. Posteriorbranchofthesecond

1.vein.

ik.^{Petiole of}thefirstsubmarginal

cell.

ip. Thirdlongitudinal.

dqrst.Fourthlongitudinal.

qr Forkofitsaiiteriorbranch:the posteriorbranchof this fork, endinginr, isMr.Loew'san- tenor intercalaryvein.

St. Forkoftheposteriorbranchof thefourth vein; thebrancli ofthis fork, endingin(, is Mr. Loew'sposteriorinter- en laryvein.

fu.Fifthlongitudinal.

fv. Sixth longitudinal.

gw. Seventh longitudinal.

' In

my

paper: Description of some

new

Genera and Species ofNorth American Lininobina, Proc. Phil. Entom. Soc. 1865,p. 225, I have called this cellthe secondmarginal;theproperterm,however,inaccordancewith theterminologyoriginallyadoptedbyMacquart,isJirstsubmarginal.

INTRODUCTION.

Cross-veins.

35

a;. Humeral.

sex. Sabcostal.

icxx.Marginal,

X*.Small, or anteriorcross-vein.

X**.Greatcross-vein.

Other termswhich have beenused.

When

theveins

between

the

end

of the proefurca (i)

and

the greatcross-vein are

more

or less in a line, I designate

them by

the collective terra central croi<s-veins.

Yeins

orcross-veins not

found

in the ordinary venation

and

therefore not separately

named, have been

called

supernumerary, when

they are of constant occurrence

and

distinguisha

genus

or a species; adventitious,

when

their occurrence is accidental in

abnormal

specimens only.

Y.

Comparison of

^the

^North American and of

^the

European TiPULiD^ of

^theeight sections described inthis volume.

The knowledge

of both faunas is far

from

perfect,

and

in this country, as well as in

Europe, almost

everyyear brings with it the discovery of

some

of the

more

rare

and more

interesting forms.

Only

the general features of thesefaunas can therefore be

compared

with a certain degree of confidence,

and

our state- ments, with regard to the details, the numerical proportions of the species,

and

the

comparison

of the smaller

genera

must, in a certain measure, be considered as onlyprovisional.

What

^strikes usmost,

when we compare

the

number

of

Euro- pean and North American

species in the eight sections of the Tipnlidae described in the present volume, is the remarkable agreement, in this respect,

between

the

two

faunas.

The com-

parisonofthe

number

^ofspeciesoccurringin

Germany

(according to Dr. Schiner's enumeration),

with

those of the Atlantic slope of this continent (as far as represented in

my

collection) stands thus^:

—

LargeGroups.