THYSANOPTERA

NO. 9 INSECT METAMORPHOSIS — SNODGRASS 35 At an early stage of the labial transformation in Anax Junius the

VIII. THYSANOPTERA

Malpighian tubules increase in size.

These

changes are mostly retro- gressive

from

the larval condition.

On

^{the other hand, the reproduc-}

tive organs develop gradually to the definitive functional state,

and

there isa thoroughgoing reconstructionof thelarval musculature into that of theadult.

In heraccountof themuscle transformationinthe

male

of

Pseudo-

coccus

Makel

distinguishes five different groups of muscles, as fol- lows^: (i) Larval muscles that

go

over with little or

no

change into the

imago;

(2) larval muscles that

undergo

such changes as splitting, uniting, or a change of position; (3) ^larval muscles destroyed

by

histolysis

and

not regenerated; ₍₄₎ transformation muscles

formed by

addition of imaginal elements to larval muscles; ₍₅₎ muscles of the

imago

thatariseas

new

musclesinthepropupa.

To

thislast

group

belong four muscles of the thorax,

and

seven oblique intersegmental muscles of the

abdomen,

together with

two

muscles connected with the external genitalorgans.

The metamorphosis

of the muscular sys-

tem

as given by

Makel

is based

on

a detailed comparative study of the musculature in the larva, pupa,

and

adult.

It is clearthatthe transformationof the

male

coccid is a true holo- metabolous metamorphosis,

and

that the larva isa specializedjuvenile stage. It

may

be questioned, however, that the coccid

pupa

is

com-

parable to the

pupa

of the higher holometabolous insects.

The

^pres- ence of

two

pupal stages having ageneral resemblance to the

winged nymphal

stages of other

Hemiptera

suggests that the so-called pupal instars of the

male

coccid pertain to the juvenile period of the life history

and

not to that of the imago.

The work

of

Wigglesworth

(1948, 1951a)

on

the

hormonal

control of transformation in the reduviid

Rhodnius shows

that the juvenile

hormone

controls the

nymphal

status

up

totheimago,

and

ifthis is true in other

Hemiptera

the coccid

pupa

is not a part of the imaginal stage.

Holometabolism

can bedefinedonly asatype of

metamorphosis

; thefact thatitoccurs

among

the

Hemiptera

in the

male

coccid,

and

also in the

Thysanop-

tera does not taxonomically relate these insects to each other or to such holometabolous insects as Coleoptera, Lepidoptera, Diptera,

and

Hymenoptera.

no.

9 INSECT METAMORPHOSIS — SNODGRASS

₄₅

it

would seem

that in like

manner

it could

grow

into an adult thrips without

any

radical process of transformation.

However,

after

two

active, feedingnymplike stages (fig. 7

A,B)

the

young

thrips

becomes

Fig. 7.

—

Life-history stages of athysanopteron,Scirtothripscitri Moulton (out- linesfrom Horton, 1918).

A, first instar. B, secondinstar. C,propupa. D, pupa. E, adult female.

inactive, ceases to feed, moults,

and

enters a quiescent stage

known

as a

propupa

(C).

The propupa

ⁱⁿ turn is followed by a second rest- ing stage

termed

the

pupa (D), from which

after a final moult the adult

emerges

(E). In the Terebrantia the

wings

appear in the propupal stage as straplike outgrowths,

which become

still

more

ex- tended in the pupa. In the Tubulifera the

propupa

differs little in

external appearance

from

the second

nymph,

since in this suborder the

wings

do notappear until the pupal stage. In

most

of the

Tubu-

lifera,however, thereisa second pupal stageseparated

from

the first

by

a moult,

making

five

immature

instars in all, but according to Priesner (1926) a propupal stage is absent in

some

species

and

in others thereis only one pupal stage.

The few

external changes other than the

growth

of the

wings

that take place during the postembryonic development of the

Thysanop-

tera are of littleconsequence. In

some forms

theantennaeare reduced in the

propupa and

their segmentation

becomes

indistinct. In the pupal stagetheantennaeelongate, theirsegmentation

becomes

distinct, the

form

of the

head

approaches that of the imago, the

compound

eyes increase in size, the ocelli appear,

and

the sexes are

now

distin- guishable.

Most

^{of these} changesare merely those that

any

ametabo- lous

nymph might go

through in its development to maturity.

The

resting stages in the life history of a thrips, however, suggest that internal changes are going on, and, in fact, a reconstruction of

some

of the internal organs takes place during the propupal

and

pupal stages that is entirely comparable to the transformation processes of holometabolous insects.

These

changes inthe thrips affect thealimen- tary canal, the salivary glands, the fat tissue, the muscular system,

and

ina lesserdegreethe nervous system.

The

alimentarycanal of Liothripsoleae, accordingto Melis (1935), does notdiffer essentially in external

form

during preimaginal stages

from

that of the adult, but the cellular structure of the mesenteron

becomes

highly unstable

and

is in a continuous state of reorganiza- tion.

On

^{the other hand,} in Parthenothrips dracaenae, as described

by

Miiller (1927), ^the alimentary canal undergoes changes ⁱⁿ shape

and

size as well as cellular reconstruction during the propupal

and

pupal stages. Inthe

two nymphal

instarsthe longtubular ventriculus is looped

forward upon

itself

and

then turns back to join the intes- tine; in the

propupa

the

whole

canal

becomes

^a simple straight tube with

no

ventricular loop; in the

pupa

the ventricular loop reappears but only as a short lateral fold

from

the middle of the tube; in the

imago

the ventriculus isagain bent

forward on

itselfas in the

nymph, and

thereis a secondary ^{small loop} in thedescending arm. Since the alimentary canal of the adult

becomes

practically the

same

as that of the

young

thrips the intervening changes

might seem

useless, except that,as theinsecttakes

no

food duringthepropupal

and

pupalinstars, theventricularchanges

may

be simply

economy

adaptationstoa lack of need for a digestive organ. In Parthenopthrips, Miiller says, there is

one renewalof the

midgut

epithelium.

At

thebeginningof

metamor-

NO.

9 INSECT METAMORPHOSIS — SNODGRASS

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