THE CASE OF THE RED SILVERWING
Having dealt briefly with the transformations of butterflies in
general, it may be well to examine more closely into those of a
single representative species. For this purpose I have chosen _Dione
vanillae_, known as the Red Silverwing, and have described each stage
and transition in considerable detail. The following paragraphs are
extracts from the daily records of a study I made in southern Kansas,
the northern limit of _vanillae’s_ usual range.
OVIPOSITION
“One of the females, alighting upon the upper side of a leaf within
ten inches of my face, suddenly elevated the forward part of the body,
brought the wings together vertically, curved the abdomen slightly
forward, and drew its tip slowly across the surface of the leaf for a
distance of approximately 12 mm. Remaining quiet for an instant, it
lifted the abdomen, showing an egg firmly attached to the leaf. The
eggs are laid singly, usually in the upper middle of the leaf. I have
never seen more than one upon the same leaf.”
THE EGG
“The egg measures about 1.12 mm. in height and .7 mm. in diameter
at the widest part, barrel-shaped, with fourteen vertical ribs. A
brilliant yellow at first, it assumes after about thirty-six hours a
reddish brown color. A few hours later an irregular, whitish broken
ring, not quite circling the egg, appears about one-third of the
distance from the top. When within an hour or so of hatching the shell
becomes very thin and transparent and reflects the light with a sort of
frosted-glass effect. The large black head of the larva inside gives
the upper one-third of the egg a black, metallic appearance, while the
yellowish body may be seen curled up in the lower part. The incubation
period seems to vary greatly with the temperature, from forty-seven
hours to seven days.”
THE EMERGENCE OF THE CATERPILLAR
“At 2:15 p. m. the larva was clearly visible through the shell. Slight
bodily movements were noted, then the very large, shining black head
was thrust out at a point on one side of the egg, just below the top.
The entire structure rocked and swayed slowly back and forth, inclining
toward the side from which the head projected.
“At 2:19 the larva emerged very slowly, head first. The head and the
final segment appeared very large. The entire length was about 1.5
mm. The body yellowish red; prolegs and caudal segment lemon yellow;
the head, legs and spines black. The latter bear no visible branches,
and appear as stiff, black hairs, each growing out of a slight black
protuberance.
“The beautiful egg-shell remains erect, the form being unchanged.
There is a ragged hole in it, but it is scarcely noticeable, and
appears hardly large enough for the egress of the caterpillar.”
THE CATERPILLAR
“The newly emerged larva does not venture far from the egg-shell and
does not move about much for the first five or six hours. It sometimes
devours the shell, but this is not usually the case. At the age of six
hours it appears darker, and the black spots from which the hairlike
spines protrude have become more conspicuous. Has not eaten any of the
leaf, and has increased in size very little if at all.”
“At the age of twenty-seven hours the larva attains a length of 2.3
mm. and has eaten several small holes in the center of a large leaf,
gnawing through the leaf to the transparent epidermis on the opposite
side, which is left intact. Usually works from the upper side, but by
no means invariably.”
“These smaller larvae seem to be always attached to the leaf with silk,
although the threads are very few and fine. More mature larvae feed
differently; they grip the petiole with the prolegs, and eat from the
edge inward and forward, swinging the head toward the midrib, with
practically no lateral motion.”
“The caterpillar is cylindrical and bears six rows of black, branching
spines, twelve in each row. As regards color, there are besides the
reddish, newly hatched creature described above, two well-defined
types: the orange-drab and the drab-orange. In the first the body
appears orange with three narrow drab stripes, and a very narrow
lateral line just above the prolegs. In the second the drab markings
become very much more prominent, so that the body now appears drab with
four narrow orange stripes. The narrow drab dorsal line of the younger
caterpillar becomes very conspicuous in the mature larva, separating
the dorsal surface into two distinct orange areas. As the orange-drab
type seems to embrace all of the smaller larvae, and as all those about
to pupate belong to the drab-orange type, I have assumed that the color
change is a matter of maturity.”
MOULTING
“1:30 a. m.--Larva about 21 mm., inactive and very dark, extended
on lower side of leaf, front of body high, head bent, holding with
third, fourth and anal prolegs. A few silk threads lie flat on leaf
near caudal end of body. After some little rippling of muscle (no
violent motion) the skin of the body separates from that of the head.
The former is very tight; slowly, segment by segment, it is skinned
backward until finally it is left, a crumpled, prickly black ball
about 4 mm. in diameter, fastened to the leaf by the aforementioned
silk. Meanwhile the head covering, which is all of a piece and bears
two large spines, slips forward until it appears to be held in the
mandibles as a hat might be held between the teeth. After a moment
it falls to the ground. The caterpillar now appears somewhat short
and a trifle thickened. The head, legs and prolegs are yellow,
the body orange. The spines are only half size, blunt, yellow and
semi-transparent. The branches appear as black hairs lying flat along
the sides of the spines.”
“11:33 a. m.--The spines have now attained practically their normal
size, springing out with astonishing rapidity. They are still blunt,
yellow and semi-transparent, but the black branches have reached normal
size and assumed their proper position. The two head spines, which were
curled backward, have taken on the usual angle. The head, body and
prolegs now appear orange rather than yellow, the head being lightest.
The drab dorsal stripe has appeared, but is very narrow. The row of
depressions, one in the dorsal center of each segment, is much more
conspicuous than usual.”
“11:50 a. m.--Appears quite normal again, except that the spines are
not quite as black, showing grayish toward the tips. The black frontal
markings appear. Caterpillar rests motionless.”
“12:10 p. m.--Turns about and begins to devour the cast-off skin. With
the lens I watched it begin at the tip of one of the big spines and
consume it clear to the root, apparently in about three mouthfuls. The
head covering is never eaten.”
“12:30 p. m.--Skin practically consumed. Larva rests motionless, a few
of the surplus spines resting beneath the head and thorax.”
PUPATION
“10:00 a. m.--Caterpillar, 37 mm. long, leaves food plant and begins
to wander restlessly about.
“1:00 p. m.--Extended motionless, clinging to under side of window
frame.
“2:30 p. m.--Same position.
“3:30 p. m.--Begins to lay a sparse, ragged network of very fine
threads flat to the surface, covering a space of perhaps an inch
square. These threads become thinner toward the periphery of the net,
where they are hardly visible without a lens.
“4:00 p. m.--Spins a little white silk button in the center of the
network. The head moves slowly out horizontally in all directions from
the button about one-half inch, swinging always back to the center. The
body is fully extended, the abdomen being away from the button.
“4:30 p. m.--Interrupted the button-making to raise high the caudal
third of the body, spreading the anal appendages apart several times.
When a pellet of excrement appeared, the caterpillar turned about and
pulled it out of the anal opening with the mandibles, holding it thus a
moment before allowing it to fall to the floor.
“4:45 p. m.--Resting, body extended, clasping the button with the third
prolegs.
“6:00 p. m.--Fastened to the button. The body is extended horizontally,
clinging to the under surface of the window frame with the prolegs;
head drawn back, legs not touching the surface.
“8:00 p. m.--Hanging from the button. The body is thickened a trifle
and shortened to 31 mm. Color much lighter; hangs motionless except
for very slight movements of the legs and prolegs.”
“9:00 a. m.--No change.
“10:00 a. m.--The brilliant colors have practically disappeared,
leaving the body a pearl-gray color, against which the black spines
show up with startling distinctness. Movements slight and infrequent.
The dorsal thorax is now the lightest part of the body.
“11:00 a. m.--Begins to wriggle a little.
“11:10 a. m.--I note that the spines appear closer together at the
caudal end of the body; then see that the skin has split down the back
and that the light-colored head of the chrysalis is protruding. By a
series of vigorous wrigglings the skin is slowly forced up to where the
tip of the abdomen is attached to the button. Then comes the violent
struggle which finally loosens the skin, which hangs for a moment
against the ventral abdomen, then falls to the floor. The entire moult
is complete in less than five minutes.”
THE CHRYSALIS
“When the chrysalis first appears it is nearly cylindrical; no dorsal
depression, no ventral bulge. The head is bent forward, and the whole
thing has a compact, slug-like appearance. An hour or so later the
head is no longer bent, and bears two double projections, set wide
apart; the wing cases are bulging and prominent; on the dorsal side,
opposite the middle of the wing covers, is a U-shaped depression; there
are projections on the third, fifth, sixth, and seventh abdominal
segments, those on the third being the largest. At first the head,
thorax and wing covers are translucent and nearly white; the abdomen
is a little darker and bears a ventral bluish stripe on the fourth,
fifth and sixth segments. The four pairs of dorsal protuberances are
amber-colored.
“An hour later the head and thorax darken somewhat, and opaque whitish
veins appear in the wing covers. Distinct black lines mark the outer
borders of the fore wings, and a black V-shaped mark shows the position
of the larger of the three discal spots.
“Another hour and the translucent appearance is gone, the abdomen and
the dorsal thorax become much darker, the wing covers grayish tan,
while the bluish ventral stripe turns gray or white.
“From this time forward the chrysalis does not change appreciably
in form or color (although the latter varies greatly) until about
twenty-four hours before the emergence of the butterfly, when the
entire body becomes very dark.
“The average length of the chrysalis is 28 mm., and the white ventral
stripe is usually the most conspicuous marking. After the butterfly
has emerged the color variations of the chrysalis still persist in the
empty shells, some being much darker than others.
“Not infrequently the cast skin of the larva is found attached to the
anal portion of the chrysalis. This skin is black excepting the head,
which is gray, and the black branching spines loom large because of
the contraction of the empty skin.
“Sometimes the chrysalids turn bright yellow; these are usually
infested by parasites. I have opened many of these pupae, but have
never found the parasitic larva itself; the pupae and adults I have
often taken, the latter being a small (2.3 mm.), green hymenopterous
insect which I have not as yet had the opportunity to classify. Some
ten or a dozen of these insects emerge from a small round hole, usually
in the wing cover, in early August.”
PUPAL MOVEMENTS
“The chrysalis seems unable to bend the body toward the back or toward
the wing covers; sidewise, however, it can turn until the body is
extended nearly parallel to the horizontal surface from which it is
suspended. Six chrysalids which hung in a north window in September,
1919, and which I observed for fourteen days, were noted to be
invariably and unanimously pointed inward (toward the warm room, that
is) every morning. During the warmer part of the day there was no
uniformity in position. Six specimens--fourteen days; it appears to be
more, perhaps, than mere coincidence. The pupal sensibility to light
and temperature stimulii should be worth investigation.”
THE APPEARANCE OF THE BUTTERFLY
“Two or three weeks after pupation (the period varies with the
temperature, from eight days in August to twenty-seven in November)
the chrysalis turns nearly black, and a diagonal fissure appears on
either side, extending from the back of the head down along the antenna
cases, nearly to the middle ventrum.
“About twelve hours later, after some little wriggling, the ventral
triangle formed by the covering of the head, antennae and mouth parts
falls open trap-door fashion, the antennae covers serving as hinges and
the flexing point being about one-third of the distance up the wing
covers from the abdomen. There is also a dorsal cleavage following
the medial dorsum to the first abdominal segment, then the outline of
the wing covers to a point half way to the end of the antenna covers.
The crumpled-winged imago wriggles out and mounts the empty shell, to
which it clings by the four hinder limbs, turning the entire body back
and forth as if mounted on a pivot. In each of these turns the body
describes an arc of nearly 90 degrees, the body being held stiff. The
angle of the body is about 45 degrees from the vertical, the head being
uppermost. The proboscis is usually partially unrolled. In five or six
hours the soft, wrinkled wings spread and harden, and the insect is
able to fly. A thin, transparent liquid, and sometimes a thick, reddish
substance, are voided by the newly emerged butterfly.”