EPHEMEROPTERA

NO. 9 INSECT METAMORPHOSIS — SNODGRASS 21

V. EPHEMEROPTERA

The young

mayfly (fig. 3

A)

is distinctly

more

specialized in its adaptation to life in the water than is the

young

stonefly. Still, the

young

mayfly is simply a juvenile insect of generalized structure; it

has

compound

eyes

and

frontal ocelli, well-developed legs,

mouth

parts of the biting type,

and

duringits

growth

it develops

wing

pads that increase in size

up

to the last moult. In these characters the

young

mayfly hasthe developmentalstatus of

an

orthopteroid

nymph, and

that it

was

primarily a land-inhabiting

nymph may

be

deduced from

the presence of

an

elaborate tracheal system in both the adult

and

the larva. Since the

young

of the earliest

known

fossil mayflies,

found

in the Permian, already

had

gills, the mayfly larva has

come down

tous with surprisingly

few

changes.

The

larval gills of the mayflies are organsof particularinterest be- cause of their apparent likeness to the abdominal styli of Thysanura.

In

modern

species the gills are present

on

the sides of, at most, the firstseven segments of the

abdomen

;larvae

from

the lower Permian, however,

had

nine pairs of gills,

and some

Jurassic species

had

eight.

The

gillsare highly variable in

form

in different species, but they are borne singly

on

lateral lobes of the abdominal segments (fig. 3

B,C)

interpolated

between

the tergal

and

sternal regions.

The

gill-bearing lobes fall directly in line with the bases of the thoracic legs

(Cx), and

thus

may

be likened to the stylus-bearing plates of Machilis.

Moreover

each gill is

movable by

muscles arising in the supporting lobe (C,D).

The movements

of the mayfly gills has been

made

the

no. 9

INSECT METAMORPHOSIS — ^SNODGRASS

₂₉

Fig. 3.

—

^{Characters of larvae of} Ephemeroptera.

A, Ephemerella sp. B, Ephoron sp., part of thorax and abdomen, showing gill-bearing lobes in linewith coxae of legs. C, diagrammatic cross section of abdomen. D, a singlegill, showingmuscles arising insupportingbody lobe. E, base ofgill, with tracheal trunk and muscles. F, Ephemerella sp., showing ad- hesive disc on venter of abdomen. G, Prosopistoma foliaceus Fourcroy, dorsal

(from Vayssiere, 1890). H, same, ventral (from Vayssiere, 1890).

ib-4b, bmcls, branchial muscles; Brn, branchia, gill; Cx, coxa of leg; Cx?, gill-bearing lobe of abdomen; en, respiratory entrance; ex, respiratory exit; /,

lateral body muscle; S,sternum^; T,tergum; Tra,trachea.

subject of

a

special study

by Eastham

_(1938, 1939). ^{Just as the} thysanuran styli

do

not appear until after the first moult, so the gills of the mayfly larva are absent in the first instar. It is said

by

Ide (1935) ^{that all the gills} appear with the first moult in

some

species, but that in other species

most

of

them may

be delayed until several moultslater.

Gills of the simplest

form

are slender processes penetrated by tracheae,othersare fringedwith longfilaments,

some

are lamelliform,

and most

of

them

are branched.

According

to Ide (1935) ^{all the}

gills are at first uniramous,

and some

that eventually

become

lamel- liform

grow

out first inthe

form

of filaments. It

would

appearto be true, therefore, as Spieth (1933) ^says, that the primitive gills of the ancestral mayflies

were

simple slender tubular structures, into

which

the tracheae enter,

and

that the

compound

gills of the present-day

forms

havearisen as modifications of the primitivetype. If the

mod-

erngills do represent styli,

we may

suppose that the

young

mayfly in its

primary

terrestrial life

may have had

abdominal styli similar to those of the

Thysanura and

Diplura,which,

when

ittooktothewater,

were

readily converted intogills.

That

themayfly gillshave been de- rived

from

styli, however, is merely a theoretical concept, but con- sidering that the

Ephemeroptera

are relatively primitive insects the conceptissufficiently reasonabletobe acceptedas not too improbable.

Unlike styli, however, the mayfly gills are discarded at the moult to the subimago.

Some

remarkable larval modifications occur in connection with the gills. In the

genus

Baetisca, described

by

Vayssiere (1934), ^the

mesonotum

is extended posteriorlyto themiddle of the sixth

abdomi-

nal

segment

to

form

a carapace covering the gills

and

the meta- thoracic

wing

pads, the pads of the first

wings

being fused with its

under

surface. In

Prosopistoma

(Vayssiere, 1882, 1890) a carapace

is even

more

extensively developed (fig. 3

G) and

covers a respira- tory

chamber

enclosing the gills,

which

is shut in ventrally

(H) by

the pleural regions of the thorax

and

lateral extensions of the first fiveabdominal sterna.

The Prosopistoma

larvathus resembles a small crustaceaninappearance.

Water

has entrancetothe respiratory

cham-

ber

by way

of lateral openings

(H,en) between

the carapace

and

the sternum,

and

is dischargedthrough a

median

dorsal aperture (G,ex) inthenotchof the posterior

end

of the carapace.

A

preliminary stage inthedevelopmentof acarapaceis suggestedin the larva of

Epheme-

rella

(A)

in

which

the

mesonotum

including the fore

wing

pads is

extended posteriorly over the base of the

abdomen and

completely covers the hind

wing

pads.

NO. 9 INSECT

METAMORPHOSIS — ^SNODGRASS

31