EXP. 1.--In July, 1875, eggs of _P. Tharos_ were obtained on _Aster
Nova-Angliæ_ in the Catskill Mountains, and the young larvæ, when
hatched, taken to Coalburgh, West Virginia. On the journey the larvæ
were fed on various species of _Aster_, which they ate readily. By the
4th of September they had ceased feeding (after having twice moulted),
and slept. Two weeks later part of them were again active, and fed for
a day or two, when they gathered in clusters and moulted for the third
time, then becoming lethargic, each one where it moulted with the cast
skin by its side. The larvæ were then placed in a cellar, where they
remained till February 7th, when those that were alive were transferred
to the leaves of an _Aster_ which had been forced in a greenhouse, and
some commenced to feed the same day. In due time they moulted twice
more, making, in some cases, a total of five moults. On May 5th the
first larva pupated, and its butterfly emerged after thirteen days.
Another emerged on the 30th, after eight days pupal period, this stage
being shortened as the weather became warmer. There emerged altogether
8 butterflies, 5 males and 3 females, all of the form _Marcia_, and
all of the variety designated C, except 1 female, which was var. B.[60]
EXP. 2.--On May 18th the first specimens (3 male _Marcia_) were seen on
the wing at Coalburgh; 1 female was taken on the 19th, 2 on the 23rd,
and 2 on the 24th, these being all that were seen up to that date, but
shortly after both sexes became common. On the 26th, 7 females were
captured and tied up in separate bags on branches of _Aster_. The next
day 6 out of the 7 had laid eggs in clusters containing from 50 to
225 eggs in each. Hundreds of caterpillars were obtained, each brood
being kept separate, and the butterflies began to emerge on June 29th,
the several stages being:--egg six days, larva twenty-two, chrysalis
five. Some of the butterflies did not emerge till the 15th of July.
Just after this date one brood was taken to the Catskills, where they
pupated, and in this state were sent back to Coalburgh. There was no
difference in the length of the different stages of this brood and the
others which had been left at Coalburgh, and none of either lot became
lethargic. The butterflies from these eggs of May were all _Tharos_,
with the exception of 1 female _Marcia_, var. C. Thus the first
generation of _Marcia_ from the hibernating larvæ furnishes a second
generation of _Tharos_.
EXP. 3.--On July 16th, at Coalburgh, eggs were obtained from several
females, all _Tharos_, as no other form was flying. In four days the
eggs hatched; the larval stage was twenty-two, and the pupal stage
seven days; but, as before, many larvæ lingered. The first butterfly
emerged on August 18th. All were _Tharos_, and none of the larvæ had
been lethargic. This was the third generation from the second laying of
eggs.
EXP. 4.--On August 15th, at Coalburgh, eggs were obtained from a female
_Tharos_, and then taken directly to the Catskill Mountains, where they
hatched on the 20th. This was the fourth generation from the third
laying of eggs. In Virginia, and during the journey, the weather had
been exceedingly warm, but on reaching the mountains it was cool, and
at night decidedly cold. September was wet and cold, and the larvæ
were protected in a warm room at night and much of the time by day, as
they will not feed when the temperature is less than about 8° R. The
first pupa was formed September 15th, twenty-six days from the hatching
of the larvæ, and others at different dates up to September 26th, or
thirty-seven days from the egg. Fifty-two larvæ out of 127 became
lethargic after the second moult on September 16th, and on September
26th fully one half of these lethargic larvæ commenced to feed again,
and moulted for the third time, after which they became again lethargic
and remained in this state. The pupæ from this batch were divided into
three portions:--
A. This lot was brought back to Coalburgh on October 15th, the weather
during the journey having been cold with several frosty nights, so
that for a period of thirty days the pupæ had at no time been exposed
to warmth. The butterflies began to emerge on the day of arrival, and
before the end of a week all that were living had come forth, viz., 9
males and 10 females. “Of these 9 males 4 were changed to _Marcia_ var.
C, 3 were var. D, and 2 were not changed at all. Of the 10 females 8
were changed, 5 of them to var. B, 3 to var. C. The other 2 females
were not different from many _Tharos_ of the summer brood, having large
discal patches on under side of hind wing, besides the markings common
to the summer brood.”
B. This lot, consisting of 10 pupæ, was sent from the Catskills to
Albany, New York, where they were kept in a cool place. Between October
21st and Nov. 2nd, 6 butterflies emerged, all females, and all of the
var. B. Of the remaining pupæ 1 died, and 3 were alive on December
27th. According to Mr. Edwards this species never hibernates in the
pupal state in nature. The butterflies of this lot were more completely
changed than were those from the pupæ of lot A.
C. On September 20th 18 of the pupæ were placed in a tin box directly
on the surface of the ice, the temperature of the house being 3°-4° R.
Some were placed in the box within three hours after transformation and
before they had hardened; others within six hours, and others within
nine hours. They were all allowed to remain on the ice for seven days,
that being the longest summer period of the chrysalis. On being removed
they all appeared dead, being still soft, and when they had become
hard they had a shrivelled surface. On being brought to Coalburgh they
showed no signs of life till October 21st, when the weather became
hot (24°-25° R.), and in two days 15 butterflies emerged, “every one
_Marcia_, not a doubtful form among them in either sex.” Of these
butterflies 10 were males and 5 females; of the former 5 were var. C, 4
var. D, and 1 var. B, and of the latter 1 was var. C, and 4 var. D. The
other 3 pupæ died. All the butterflies of this brood were diminutive,
starved by the cold, but those from the ice were sensibly smaller
than the others. All the examples of var. B were more intense in the
colouring of the under surface than any ever seen by Mr. Edwards in
nature, and the single male was as deeply coloured as the females, this
also never occurring in nature.
Mr. Edwards next proceeds to compare the behaviour of the Coalburgh
broods with those of the same species in the Catskills:--
EXP. 5.--On arriving at the Catskills, on June 18th, a few male
_Marcia_, var. D, were seen flying, but no females. This was exactly
one month later than the first males had been seen at Coalburgh. The
first female was taken on June 26th, another on June 27th, and a
third on the 28th, all _Marcia_, var. C. Thus the first female was
thirty-eight days later than the first at Coalburgh. No more females
were seen, and no _Tharos_. The three specimens captured were placed
on _Aster_, where two immediately deposited eggs[61] which were
forwarded to Coalburgh, where they hatched on July 3rd. The first
chrysalis was formed on the 20th, its butterfly emerging on the 29th,
so that the periods were: egg six, larva seventeen, pupa nine days.
Five per cent. of the larvæ became lethargic after the second moult.
This was, therefore, the second generation of the butterfly from the
first laying of eggs. All the butterflies which emerged were _Tharos_,
the dark portions of the wings being intensely black as compared with
the Coalburgh examples, and other differences of colour existed, but
the general peculiarities of the _Tharos_ form were retained. This
second generation was just one month behind the second at Coalburgh,
and since, in 1875, eggs were obtained by Mr. Mead on July 27th and
following days, the larvæ from which all hibernated, this would be
the second laying of eggs, and the resulting butterflies the first
generation of the following season.
Thus in the Catskills the species is digoneutic, the first generation
being _Marcia_ (the winter form), and the second the summer form. A
certain proportion of the larvæ from the first generation hibernate,
and apparently all those from the second.
_Discussion of Results._--There are four generations of this butterfly
at Coalburgh, the first being _Marcia_ and the second and third
_Tharos_. None of the larvæ from these were found to hibernate. The
fourth generation under the exceptional conditions above recorded (Exp.
4) produced some _Tharos_ and more _Marcia_ the same season, a large
proportion of the larvæ also hibernating. Had the larvæ of this brood
been kept at Coalburgh, where the temperature remained high for several
weeks after they had left the egg, the resulting butterflies would have
been all _Tharos_, and the larvæ from their eggs would have hibernated.
The altitude of the Catskills, where Mr. Edwards was, is from 1650
to 2000 feet above high water, and the altitude of Coalburgh is 600
feet. The transference of the larvæ from the Catskills to Virginia
(about 48° lat.) and _vice-versa_, besides the difference of altitude,
had no perceptible influence on the butterflies of the several broods
except the last one, in which the climatic change exerted a direct
influence on part of them both as to form and size. The stages of
the June Catskill brood may have been accelerated to a small extent
by transference to Virginia, but the butterflies reserved their
peculiarities of colour. (See Exp. 5.) So also was the habit of
lethargy retained.[62] The May brood, on the other hand, taken from
Virginia to the Catskills, suffered no retardation of development.
(See Exp. 2.) It might have been expected that all the larvæ of this
last brood taken to the mountains would have become lethargic, but
the majority resisted this change of habit. From all these facts it
may be concluded “that it takes time to naturalize a stranger, and
that habits and tendencies, even in a butterfly, are not to be changed
suddenly.”[63]
It has been shown that _Tharos_ is digoneutic in the Catskills and
polygoneutic in West Virginia, so that at a great altitude, or in a
high latitude, we might expect to find the species monogoneutic and
probably restricted to the winter form _Marcia_. These conditions are
fulfilled in the Island of Anticosti, and on the opposite coast of
Labrador (about lat. 50°), the summer temperature of these districts
being about the same. Mr. Edwards states, on the authority of Mr.
Cooper, who collected in the Island, that _Tharos_ is a rare species
there, but has a wide distribution. No specimens were seen later
than July 29, after which date the weather became cold, and very few
butterflies of any sort were to be seen. It seems probable that none of
the butterflies of Anticosti or Labrador produce a second brood. All
the specimens examined from these districts were of the winter form.
In explanation of the present case Dr. Weismann wrote to Mr.
Edwards:--“_Marcia_ is the old primary form of the species, in the
glacial period the only one. _Tharos_ is the secondary form, having
arisen in the course of many generations through the gradually working
influence of summer heat. In your experiments cold has caused the
summer generation to revert to the primary form. The reversion which
occurred was complete in the females, but not in all the males. This
proves, as it appears to me, that the males are changed or affected
more strongly by the heat of summer than the females. The secondary
form has a stronger constitution in the males than in the females. As
I read your letter, it at once occurred to me whether in the spring
there would not appear some males which were not pure _Marcia_, but
were of the summer form, or nearly resembling it; and when I reached
the conclusion of the letter I found that you especially mentioned that
this was so! And I was reminded that the same thing is observable in
_A. Levana_, though in a less striking degree. If we treated the summer
brood of _Levana_ with ice, many more females than males would revert
to the winter form. This sex is more conservative than the male--slower
to change.”
The extreme variability of _P. Tharos_ was alluded to more than a
century ago by Drury, who stated:--“In short, Nature forms such a
variety of this species that it is difficult to set bounds, or to know
all that belongs to it.” Of the different named varieties, according
to Mr. Edwards, “A appears to be an offset of B, in the direction
most remote from the summer form, just as in _Papilio Ajax_, the var.
_Walshii_ is on the further side of _Telamonides_, remote from the
summer form _Marcellus_.” Var. C leads from B through D directly to the
summer form, whilst A is more unlike this last variety than are several
distinct species of the genus, and would not be suspected to possess
any close relationship were it not for the intermediate forms. The
var. B is regarded as nearest to the primitive type for the following
reasons:--In the first place it is the commonest form, predominating
over all the other varieties in W. Virginia, and moreover appears
constantly in the butterflies from pupæ submitted to refrigeration.
Its distinctive peculiarity of colour occurs in the allied species
_P. Phaon_ (Gulf States) and _P. Vesta_ (Texas), both of which are
seasonally dimorphic, and both apparently restricted in their winter
broods to the form corresponding to B of _Tharos_. In their summer
generation both these species closely resemble the summer form of
_Tharos_, and it is remarkable that these two species, which are the
only ones (with the exception of the doubtful _Batesii_) closely allied
to _Tharos_, should alone be seasonally dimorphic out of the large
number of species in the genus.
Mr. Edwards thus explains the case under consideration:--“When _Phaon_,
_Vesta_, and _Tharos_ were as yet only varieties of one species, the
sole coloration was that now common to the three. As they gradually
became permanent, or in other words, as these varieties became species,
_Tharos_ was giving rise to several sub-varieties, some of them in time
to become distinct and well marked, while the other two, _Phaon_ and
_Vesta_, remained constant. As the climate moderated and the summer
became longer, each species came to have a summer generation; and in
these the resemblance of blood-relationship is still manifest. As the
winter generations of each species had been much alike, so the summer
generations which sprung from them were much alike. And if we consider
the metropolis of the species _Tharos_, or perhaps of its parent
species, at the time when it had but one annual generation, to have
been somewhere between 37° and 40° on the Atlantic slope, and within
which limits all the varieties and sub-varieties of both winter and
summer forms of _Tharos_ are now found in amazing luxuriance, we can
see how it is possible, as the glacial cold receded, that only part of
the varieties of the winter form might spread to the northward, and but
one of them at last reach the sub-boreal regions and hold possession to
this day as the sole representative of the species. And at a very early
period the primary form, together with _Phaon_ and _Vesta_, had made
its way southward, where all three are found now.”