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Psyche 8:15-29, 1897.

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YSYGHTE.
IMAGINAL DISCS IN INSECTS.
BY HENRY 5. PRATT, PIT. D,, HAVERFORD COLT.EGB, PA. [Annual address of the retiring president of the Cambridge Entomolo~cal Club, 8 January, iSwl. Among insects which have a complete
metamorphosis the organization of the
larva is in general similar to that of the adult or imago, but the larva
always
lacks certain organs which the imago
possesses. The larval beetle or butter-
fly lacks wings, the larval bee has
neither wings nor legs which is also true of the larval gnat, while the larval fly has neither legs, wings nor head; and
in each case the imago possesses these
organs. As we ascend the scale of
development from the less highly to the
more highly specialized insects having
complete metamorphosis, we find a
constantly increasing sum of differences between the larval and the iinaginal
forms, iind a correspondingly increas-
ing number of organs which are pos-
sessed by the imago and not by the
larva. This drawing apart is due, on
the one hand, to the higher specialization of the imago and its consequent further
departure from the ancestral stage of its ontogeny represented or suggested by
the larva, but also largely to the retro- gressive development of the larva itself. In the highest insects, where the imago
is a highly specialized animitl capable
of living only in a certain restricted
environment, the larva is perhaps as
highly specialized as is the imago :
its environment is as sharply restricted and its structure departs as far from
the phyletic type or stage it represents as is the case with the imago.
In the coleoptera, to
consider first
one of the less highly specialized groups of holometabolic insects, the environ-
ments of the larva and of the imago are
usually quite similar, or perhaps they
are exactly the same ; the organs of the two forms are correspondingly similar,
and the transformations which must be
accomplished on the body of the larva
to produce the imago are but slight.
The imago differs from the larva prin-
cipally in that it has acquired wings,
elytra, compound eyes. and external re-
productive organs, but all the larval
organs with the exception of the mid-
gut become imaginal ones without great
change. The midgut in all holomettibo-
lie insects undergoes a complete trans-
formation during the metamorphosis,*
In the lepidoptera, to come to a
somewhat more highly specialized
group, the larval and imaginal eirviron- ments are apparently widely different
* Kowalevsky, A.
Beitrage zur Kenutnis der nacliein-
bryonalen Entwicklung der Miisciden.
Zeit. f. wiss. 2001.
45. 3. ~887. P. 56s.
Paths 8 01530 (prc.1903). hfp //psyche aitclub orgWS-0315 html



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from each other.
But, after all, both
larva and imago live on vegetable food,
and more than this, in the higher mem-
bers ofthe group at least, on very nearly the same kind of vegetable food, the
leaves and flowers of phanerogamous
plants. The larva eats and lives among
the grccn leaves, while the imago finds
its nourishment in the flowers which are modified leaves. So that we shall not
be surprised if we find that the transfor- nations which result in the production
of the butterfly from the caterpillar are not as great as the apparently great dif- ference between the two forms might
suggest. As is the case with the
coleoptera, the imago acquires wings,
compound eyes, and external reproditc-
tive organs, and all the larval organs
with the exception of the miclg~~t pass
directly into the imaginal organs, al-
though some of them fire highly modi-
fied in the process ; but there is no
complete making over, no general his-
tolysis.
In the hymenoptera the larva and
imago live in general on the same kind
of food. But the conditions of colon-
ial and family life which prevail among
the higher members of the order have
resulted in a marked retrogressive de-
velopment on the part of the larva, so
that it is very different structurally, from the imago. Not having to find its own
food, to protect itself, or to escape from enemies, it has lost its extremities. In the nematoceran diptera other conditions have produced similar results, and we
also find apoclous larvae. The transfor- m~itious, now, which the pupae of these
insects must undergo to become imagin-
es are much greater than is the case in
the coleoptera or lepidoptera. Not only
must wings be acquired during the
metamorphosis but legs as well, and the
larval organs require a much greater
modification before they can serve as
imaginal organs. But yet no complete
making over, no general histolysis *
takes place.
When we come to the brachycerai~
diptera, the most highly specialized in- sects in my opinion, we find the great-
est structural differences between the
larva and the imago to be met among
insects. The environment of the larv;~
is as a rule totally different from that of the imago, and the larval structure
correspondingly different from the imag- inal. Thelarva, too, has undergone an
extensive retrogressive development.
In the case of the nematocera, as I have just said, the larva is without legs, but iu the brachyccra the retrogression has
gone much further and the larva has
neither legs nor head. There are also
great internal differences. In the meta- morphosis, consequently, a very differ-
ent animal must come out of the pupa-
rium than went into it. The imago
must acquire not only external repro-
ductive organs, compound eyes, wings,
legs, and a head, none of which the
larva possessed, but also internal organs very different from those of the larva.
A complete making over accomp:inied
by general histolysis is the result.
* Canin, M. The post embryonic development of insects. (Ruasnn) Warsaw 1876, Reviewed in Zeit. f. Wiss. Zool. ABd, 1877; p. 3%.




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A dissection of an old larva or a young
pupa of either of the insect orders
above n~entioned wonlrl show that thcse
new organs which the insect is to ac-
quire during its metamorphosis are
really already present, not as fully
formed organs, however, but in the form
of rudiments or anlagen. In the body
cavity of the caterpillar, for instance, buried beneath the dorsal meso- and
metathoracic i~~te~~iinetit are two pairs of small disc-like islands ofcells. These -emtiin i~~~functional and inactive dur- ing tlic caterpillar's lifetime although growing constantly, but dtiringits meta- ~norpliosis they develop into the two pairs of wings of the butterfly.* Similar cell- islands are present in the larval coleop- tero1i.f The larval hymenopteron also
possesses them f ; while beneath its
vcntrill tlioracic integument arc three
other pairs of cell-islands whose fate it is to furnish the imaginal legs. In the
neniatocera, also, ventral and dors;~l
pairs of cell-isliiiicls tire present in the larval. as thc observations of Weisniaiii~ on Corethra f first showed. This classic investigation demonstrated the presence
of three dorsal pairs of cell-islands as well as three ventral pairs. They tire sit- uated in the body cavity of the larva just beneath the integument, a dorsal and a
ventnil pair in the prothorax, destined
to form the pupal spiracles and the
4: Weismanu. Die Metamorphose von Corethfil pluniicor- is.
Zcit. f. wiss. Zool. 16 Bd 1866. y. 45,
imaginal prothoracic legs, respectively ; a dorsal and a ventral pair in the meso- thorax, destined to form the wings and
the mesothoracic legs; and a dorsal and
a ventral pair in the metatliorax, des-
tined to form the balancers and the
~nctaLlioracic Icgs.
Ill tlic brachyccra Weisinann 5 was
again the first to prove the existence of these cell-islands. It was in 1864 that
he published in his account of tlie post- embryonic development of the mnscicls
the first correct and extended observa-
tions on these peculiar cell-islands in any insect. He called them imag'inal discs.
He showed that in Musca six pairs are
present in tlie larval thorax, not near
the surface as in Corethra, but in the cen- ter of the larva, and that their fate is exactly the same as in Corethra. In
addition- to thcse thoracic imaginal discs lie described two large cephalic discs
situated in the forward portion of the
thorax and connected with the larval
pharynx, the fate of which is to form
the imaginal head. Weisnianii also
showed 11 that but a small portion of the larval body passes directly into the im- aginal body, ~ L I L that most of it iincler- gocs disi~~tcgraf.io~i so that tlie different tissues entirely lose their identity, after which the imaginal body is built up from the iinaginal discs. To this process, the entire significance of which was not,
however, understood until later, he gave tlie name histolysis.
- ~
$ Wekm.iin, A. Die ~adtfcuibr~onale F.nlv.iclc. d. Musei~ den imch Red). an Musca voin. u. Sarcaphoga carnaria. Zeit. f, wiss, 2001. 14 Bd. Heft 3. 1864. p. 221 I! 1. c. p. 322 and 336.




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18 PSYCHE.
[February 18,~.
These observations of Weismann published the first of his epoch-mak- opened a new field of investigation in
the development of insects.
This has
been entered by several eminent inves-
tigators ofwhom Ganin, Kiinckel d'Her-
culais, Viallanes, Kowalevsky, and Van
Rees have been the mosi. successful.
Ganin * in 1876 demonstrated the pies-
ence of many other imaginal discs than
those which Weismann described.
Weismann supposed that the hypoder-
mis of the larval abdomen went di-
rectly with some modifications to form
that of the imago. Ganin showed, now,
that in the hypodermis of each of the
eight abdominal segments are four cell-
islands, two dorsal and two ventrsil,
which resemble in every respect the
tissue of the imaginal discs ; that they are in fuct imaginal discs and are des-
tined to form the starting points for (lie growth of the imaginal abdominal hyp-
oclermis. Ganhi also discovered similar
discs in the epithelium of the midgut
whose fate it is to form, in the same
way, the imaginal inidgut, and also
the important fact that each imaginal
disc in the larva is made up of two
kinds of embryonic tissue, ecloderm
and mesoden11 or ~nesencl~ym. In
1875 Kiinckel d'Herculais f found in
the last abdominal segment two pan's
of imaginal discs OF the external geni-
tal organs. In 1883 Metsclinikoff j:
t Kilnckel d'Herculais. Rechercl~es sur 1'urganisation ct le d&elopperaeiit des Volucelies.
Paris, 1875, p. 1.19,
ing observations on tlie destruction of
tissues in certain invertebrates by leu- cocytcs or as be culled them phagocytes. lie discussed Ganin's paper and espe-
cially his statement that diiring the
histolysis of the pupal inuscid the larval organs are destroyed by amoeboid mes-
oderm cells. These cells he suggests
are none other than phagocytes. In
1884 Van Rees å and in 1885 Kowalev-
sky 11 proved the correctness ofthis posi- tion ; they showed that the process of
histolysis is the learing down and diges- tion of the function:^! larval tissues by phagocytes and the building up of hag-
inal tissues from i~naginal discs.
In 1888 Van Rees 7 published Ins ex-
tensive paper on the post-embryonic de-
velopment of muscicls, and completed
our knowleilge of tins phenomenon.
He showed that when the muscidian
larva has entered upon the p~~piil stage, histolysis is inanguralccl by the destriic- lion of the larval muscles, they becom-
ing i~tifn~~ctional directly after pupation and a natural prey to the phagocytes.
Soon the thoracic liypocler~~~is and the inner organs are attacked, and at the
same time the iinaginiil discs begin to
grow and widen out to supply the place
of the tissues which are being destroyed. The continuity of the l~ypoclermis and
~ ~
$ Van Rees. Over intra-cellulaire si-iijbvcrtceriDg en over de beteckcnisder witte bloedlichampje~. Alaaiidblad voor Saunwelenschappen. Jaarg. n, 1884.
I Kowalevsky. Beitrage z. nachembryonale Eutwickhn~g d. llnsddeii. Zool. Aiii. 8 Bd. 1885, p. 98. ^I Van Rees. Beitrage 2. Kenntniss d, iniiercn Meta- norphose v. Uusca vomhoria. Zool. Jahrb. Abth. f. Anat. I. Oiitog, 3. Bd. 1888, p. r.




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February 1897.1 F'sY.Cl?m. 19
of most of the internal organs is thus
not at any time broken. As these pro-
cesses go on, the two large head imag-
inal discs, which form two irregularly
shaped sacks extending as diverticula
from the dorsal wall of the pharynx
back to the brain, begin to pass forward dragging' the brain with them. Their
anterior ends bend and pass ventrad
enibracing the pharynx between tliesn.
At the same time their cominnnications
with the pharynx enlarge and their
luinena fuse more completely with the
pharyngial lumen. These comnim~ica-
tions now continue to enlarge : they
fuse into one single median opening
which, ever increasing in size, travels
from the anterior end of the plfarynx
posteriad, obliterating the dorsal phar- yngial wall in its course. Finally the
liimena of the discs and that of the
pharynx become completely merged
and form together a single continuous
space, and the walls of the discs and of the pharynx form a single continuous
vesicle. This is the head-vesicle of
Weismann * am1 Van Rees + which is
destined to become the i~naginal llcad.
On its anterior ventral surface epithelial thickenings appear which are destined to form the imaginal antennae and mouth
parts, while at its posterior end arc
thickenings which are to become tlie
compound eyes and which are still in
contact with the brain. The head-
vesicle remains buried within the pupal
thorax until near the end of the pupal
period when it evaginates and forms the
im:tgin;il head. This evagination has
been observed by Weismann .1 to be the
consequence of the pressure of blood
which at the right moment rushes from
the abdomen into the thorax and pushes
the head-vesicle fonvttrd.
The metamorphosis oftlie thorax goes
on simultaneously with the formation of
the head-vesicle. In proportion as the
larval hypodermis disappears under the
attacks of the phagocytes, as I have
already mentioned, the edges of the
imaginal discs grow and take its place,
forming the imaginal Iiypodermis.
As we have seen, there arc six pairs of
these discs, three dorsal and three
ventr:iI, :ind they are in the center of the larva. Each disc is, however, con-
nected with that portion of the hypo-
dermis of the segment to which it
genetically belongs and where it is des- tined to appear as an extremity, by a
very fine, hollow chord. This chord,
now, begins to shorten and its lumen to
enlarge. The disc is thus brought
nearer the surface and, as it advances, it increases in size. The lumen of the
chord then opens through the liypoder-
nits to the outside, and finally becomes so wide and the chord itself so short
that the disc is brought through the
hypodermis to the outside. The hol-
low chord has of course been obliterated by this process and the edges of the
proximal end of the disc brought into
direct connection with the liypodermis.
The disc has by this time assumed its




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20 p.7 2Th'E. [February 1897.
position as an extremity. It is an ap- active, funclional organs during the pendagc of the body wall ; it lias become irregularly cylindrical in shape and pos- sesses a number of constrictions and
folds, w11ich in the case of the ventral discs are equivalent to the joints of the future leg". The proximal cdgcs of the
discs,
those in contact with the larval
hypodern~is, grow and extend then-
selves and take the place of larval liy- podermis in proportion as tins is dc-
stroyed by pl~agocytes.
The metamorphosis of the abdomen
is retarded :ind does not begin until that of the hcad and thorax is well advanced. Then in each abdominal segment the
two ventral and four dorsal discs (Van
Rees found two additional dorsal discs
iii each segment) begin to grow and take the place of the disappearing larval liy- ioderrnis.
Kowalevsky "discovered that the discs
of the last segment do not take part in
the formation of hypodermis, but of the
enclg'ut with the rectal glands, and that they arc situated in the vicinity of the 1. LII\ . - a1 anus. The metamorphosis of the larval internal organs was correctly re- ported first by Kowalevsky f in the year preceding tlie publication of Van Recs'
paper. All of these organs are de-
stroyed by phagocytes except the
central nervous system, the heart, the
reproductive organs, and three pairs of
tlioracic muscles. These with Hie excep- tion of the reproductive oipans remain
pupal period and are not attacked by
phagocytes, but pass directly into the
imago without great change and be-
come imaginal organs. The organs
destroyed are reconstructed from imag-
inal discs in a way similar to that
ready described.
The only paper dealing with imagi-
nal discs which has appeared since
Van Rees' is one of my own published
in 18934 It contains a description of
the larva of Melophagus ovinus, a pup-
parous clipter.
The p~ipipurs are cyciorrlxiphic bra-
cl~yceni and very closely allied to the
mnscids, so that we may expect to find
the same imaginal discs in tlieir larvae as in the m~~scids. And we do, in fact,
find in general siniiliu' conditions, but there are several interesting differences. The larva is apodous and acephalous
like the muscidian, but in many respects it is muell less highly specialized;
it seems, in fact, as if it nlighl
represent the ancestral stage in dip-
teran ph~logeny at which the rnuscids
are beginning to -draw away from
their relatives, to occupy a position be- i.ween Corethra and Miisca. 111 the
position of the thoracic discs, for in-
stance, it closely resembles Corethra.
We find these discs just beneath the
integument in two very regular rows
and not in the center of the larva as in Musca. The accompanying wood cut
represents dorsal and ventral frontal
$ Pratt. Beitrage z, Kcrn-itnis d. Pupipai-en. Die Larve von I^Ieloplmgus ovinus. Arcliiv f. Naturgeschichte, Bd. 59, 1893, P. ,jl,