The following paper by Dr. Fritz Müller[222] forms the third of
a series of communications on Brazilian butterflies published in
“Kosmos,” and as it bears upon the investigations made known in the
third essay of the present work, I will here give a translation, by
permission of the publisher, Herr Karl Alberts.
“ACRÆA AND THE MARACUJÁ BUTTERFLIES AS LARVÆ, PUPÆ, AND IMAGINES.
“In a thoughtful essay on ‘Phyletic Parallelism in Metamorphic
Species,’ Weismann has shown that in the case of Lepidoptera the
developmental stages of larva, pupa, and imago vary independently,
and that a change occurring in one stage is without influence upon
the preceding and succeeding stages, so that the course which has
been followed by the individual stages in their developmental history
has not been in all cases identical. This want of agreement may
manifest itself both by unequal divergence of form-relationship, and
by unequal group formation. With respect to unequal form-divergence
the caterpillars are sometimes more closely related in form than their
imagines, and at other times the reverse is the case. With respect
to unequal group formation again, two cases are possible; the larvæ
and imagines may form groups of unequal value, the one stage forming
higher or lower groups than the other, or they may form groups of
unequal size, _i.e._, groups which do not coincide but which overlap.
Form-relationship and blood-relationship do not therefore always agree;
the resemblances among the caterpillars would lead to a quite different
arrangement to that resulting from the resemblances among the imagines,
and it is probable that neither of these arrangements would correspond
with the actual relationships.
“Starting from this fact, which he establishes by numerous examples,
Weismann proceeds to show most convincingly that an innate power of
development or of transformation, such as has been assumed under
various names by many adherents of the development theory, has no
existence, but that every modification and advancement in species has
been called forth by external influences.
“A most beautiful illustration of the want of ‘phyletic parallelism,’
as Weismann designates the different form-relationships of the
larvæ, pupæ, and imagines, is furnished by the five genera _Acræa_,
_Heliconius_, _Eueides_, _Colænis_, and _Dione_ (= _Agraulis_). This
instance seems to me to be of especial value, because it offers the
rare case of pupæ showing greater differences than the larvæ and
imagines.
“The species of which I observed the larvæ and pupæ are _Acræa Thalia_
and _Alalia_, _Heliconius Eucrate_, _Eueides Isabella_, _Colænis Dido_
and _Julia_, _Dione Vanillæ_ and _Juno_; besides these I noticed the
pupa of _Eueides Aliphera_.
“The following remarks apply only to these species, although we may
suppose with great probability that the whole of the congeneric
forms--excepting perhaps the widely ranging species of _Acræa_--would
display similar characters to their Brazilian representatives.
“The imagines of the five genera mentioned form two sharply defined
families, the _Acræidæ_ and the butterflies of the Maracujá group.[223]
The latter comprises the three genera _Heliconius_, _Eueides_, and
_Colænis_, which differ only in very unimportant characters; _Eueides_
is distinguished from _Heliconius_ by its shorter antennæ, and
_Colænis_ differs from _Eueides_ in having the discoidal cell of the
hind-wings open. The genus _Dione_ is further removed by the different
structure of the legs, and the silvery spots on the underside of
the wings. Certain species resemble those of other genera in a most
striking manner, and much more closely both in colour and marking, and
even in the form of their wings, than they do their own congeners. This
is the case with _Acræa Thalia_ and _Eueides Pavana_, with _Heliconius
Eucrate_ and _Eueides Isabella_, and with _Eueides Aliphera_ and
_Colænis Julia_, which are deceptively alike, and the last two are
connected with _Dione Juno_, at least by the upper side of the wings.
The difficulty of judging of the relationships of the single species is
thus much aggravated; it cannot be said how much of this resemblance
is to be attributed to blood-relationship, and how much to deceptive
imitation.
“As larvæ all the Brazilian species must be placed _in one genus_, as
they agree exactly in the number and arrangement of their spines (4
spines, not in a transverse row, on segments 2 and 3; 6 spines, in a
transverse row, on segments 4-11; 4 spines, not in a transverse row,
on the last (12th) segment). They differ from one another much less
in this respect than do the German species of _Vanessa_, such, for
instance, as _V. Io_ or _Antiopa_ from _V. Polychloros_, _Urticæ_,
and _Atalanta_.[224] The larvæ of _Acræa Thalia_ are certainly
without the two spines on the head which the others possess, and, on
the other hand, they have a well-developed pair of spines on the
first segment, which, in most of the other species, are completely
absent; but this does not justify their separation, since the head
spines of _Heliconius_, _Eueides_, and _Colænis Dido_, which are of
a considerable length, are shorter than those of the next segment in
_Colænis Julia_, and _Dione Vanillæ_, and in _Dione Juno_ they dwindle
down to two minute points, this last species also bearing a short pair
on the first segment. The larva of _Dione Juno_ is thus as closely
related to that of _Acræa Thalia_ as it is to that of its congener
_Dione Vanillæ_.
“If it were desired to form two distinct larval groups this could not
be effected on the basis of their differences in form, but could only
be based on their food-plants. The larvæ of _Heliconius_, _Eueides_,
_Colænis_, and _Dione_ live on species of Maracujá (Passiflora);
those of _Acræa Thalia_ and _Alalia_ on Compositæ (_Mikania_ and
_Veronia_). These larval groups would agree with those founded on the
form-relationships of the imagines, but unlike the imaginal groups,
which can be formed into families, they would scarcely possess a
generic value.
“If we arrange the single species of caterpillars according to their
resemblances, this arrangement does not agree with that based on the
resemblances of the imagines, even if we disregard the different values
of the groups. The result is somewhat as follows:--
[Illustration:
IMAGINES.
(Nymphalideous butterflies with tufts on wing-veins.)
/---------------------------------------------------\
(Families.) MARACUJÁ-GROUP. ACRÆIDÆ.
/---------------------------\ |
/-------------------\ /-------------------\ |
(Genera.) Heliconius. Eueides. Colænis. Dione. Acræa.
| /-------\ /-------\ |
(Species.) Eucrate. Isabella. Dido. Julia. Vanillæ. Juno. Thalia.
| | | | | | |
\----------+---------/ \--------/ \------/
\--------------------------------------/
LARVÆ.”
]
* * * * *
[Here follow the remarks on the habits of the larvæ in connection with
their colours, &c., which have already been quoted in illustration of
the use of the spiny protection (note 133, p. 293). From these facts
the author draws the conclusion that the form-relationships of the
caterpillars depend rather upon their mode of life than upon their
blood-relationships, assuming the latter to be correctly expressed by
the arrangement of the imagines at present adopted.]
[Illustration: Figs. 1-4. Pupæ of _Acræa Thalia_; _Heliconius Eucrate_;
_Eueides Isabella_, and _Colænis Dido_; life size.]
“A glance at the above figures of the pupæ of _Heliconius Eucrate_
(Fig. 2), _Eueides Isabella_ (Fig. 3), and _Colænis Dido_ (Fig. 4),
will show how great are the differences between these pupæ as compared
with the close form-relationship of all the Maracujá butterflies, and
with the no less close resemblance of their larvæ. A family which
comprised three such dissimilar pupæ would also be capable of including
that of _Acræa Thalia_ (Fig. 1).
“The pupa of this last species has nothing peculiar in its general
appearance, but possesses the ordinary pupal form; it is tolerably
rounded, without any great elevations or depressions; a minute pointed
projection is situated on the head over each eye-cover, and a similar
process projects from the roots of the wings. Its distinguishing
characters are five pairs of spines on the back of the abdominal
segments. These spines are found also in _Acræa Alalia_, but appear
to be absent in other species, _e.g._ in the Indian _A. Violæ_.
Last summer, among some batches of _Thalia_ larvæ--each batch being
the progeny from one lot of eggs--I found certain individuals which
differed from the others in having much shorter spines, and these
changed into pupæ in which the five pairs of spines were proportionally
shorter than usual, thus being an exception to the rule that changes in
one stage of development are without influence on the other stages. I
may remark, by the way, that this law, enunciated by Weismann, can only
be applied to imagines and pupæ with certain restrictions. The skin of
the pupa forms a sheath or cover for the eyes, antennæ, trunk, legs,
and wings of the imago, and if these parts undergo any considerable
modification in the latter, corresponding changes must appear in the
pupa. This is shown, for instance, by many ‘Skippers’ (_Hesperidæ_),
in which the extraordinarily long trunk necessitates a sheath of a
corresponding length. The colour of the pupa of _Acræa Thalia_ is
whitish, the wing-veins with some other markings and the spines are
black; metallic spots are absent.
“In the pupa of _Heliconius Eucrate_ the laterally compressed region of
the wings is raised into a large projection, the antennal sheaths lying
on the edges of the wings are serrated and beset with short pointed
spines; instead of the minute projections of _Acræa Thalia_, the head
bears two large humped processes; the body is raised on each side into
a foliaceous border carrying five spines of different lengths, the
foremost pair, directed towards the head, being the longest. The pupa
is brown, and ornamented with four pairs of brilliant metallic spots,
one pair close behind the antennæ, and three pairs, almost coalescent,
on the back before the longest pair of spines. A short spine projects
from the middle of each of the latter somewhat arched metallic patches.
“In the pupa of _Colænis Dido_ (which resembles that of _Colænis
Julia_, and to which may be added those of _Dione Vanillæ_ and _Juno_)
the spines are absent, the wing region is but moderately arched, and
the antennæ marked only by small elevations; instead of the leaf-like
border, there are on each side of the back five knotty or humped
processes. The metallic spots are similar in number and position to
those of _Heliconius Eucrate_; those on the back have a wart-like
process in the middle, instead of a spine.
“The pupæ of _Heliconius_ and _Colænis_ when moving their posterior
segments rapidly, as they do whenever they are disturbed, produce a
very perceptible hissing noise by the friction of these segments,
this sound, which is especially noticeable in the case of _Heliconius
Eucrate_, perhaps serving to terrify small foes. (So loud is the sound
produced in this manner by the pupæ of _Epicalia Numilia_, that my
children have named them ‘_Schreipuppen_.’)
“The pupæ of _Heliconius_ and _Colænis_ thus differ to a much greater
extent than the imagines or larvæ, and the same holds good for
_Eueides_ in a much higher degree as compared with its above-mentioned
allies. The larvæ of _Eueides_ have no distinctive characters, and
even the generic rank of the imagines is doubtful; as pupæ, on the
other hand, they are far removed (even by their mode of suspension)
not only from the remainder of the Maracujá group and from the whole
of the great Nymphalideous group (_Danainæ_, _Satyrinæ_, _Elymniinæ_,
_Brassolinæ_, _Morphinæ_, _Acræinæ_ and _Nymphalinæ_), but from almost
all other butterflies. The larva pupates on the underside of a leaf;
the pupa is fastened by the tail, but does not hang down like the pupæ
of the other _Nymphalidæ_,--its last segments are so curved that the
breast of the chrysalis is in contact with the underside of the leaf.
I am not acquainted with any other pupa among those not suspended by
a girdle which assumes such a position. Something similar occurs,
however, in the pupa of _Stalachtis_, which is without a girdle, and
according to Bates, is ‘kept in an inclined position by the fastening
of the tail.’ By this peculiarity Bates distinguishes the _Stalachtinæ_
from the _Libytheæ_ with pupæ ‘freely suspended by the tail.’
“Besides through this peculiar position of the body, the pupa of
_Eueides Isabella_ is distinguished by short hooked and long narrow
sabre-like pairs of processes on the back and head. Its colour is
whitish, yellowish, or sordid yellowish-grey; in the last variety both
the four long dorsal processes and the surrounding portions, as well as
the points of the other processes, remain white or yellowish. The pupa
_Eueides Aliphera_ is very similar, only all the processes are somewhat
shorter, the four longest (dorsal) and some other markings being black.
“Now if, as Weismann has attempted to show for larvæ and imagines,
the form-divergence always ‘corresponds exactly with the divergence
in the mode of life,’ the question arises as to what difference
in the conditions of life has brought about such a considerable
form-divergence between the pupæ of such closely allied species as the
Maracujá butterflies. In pupæ which do not eat or drink, and which
have neither to seek in courtship nor to care for progeny, it is only
protection from foes that can concern us. But in the pupæ of nearly
allied species of which the larvæ feed on kindred plants in the same
districts at the same periods of the year, can the enemies be so
different as to produce such a considerable divergence in form? One
might answer this question in the negative with some confidence, and
affirm that in this case the difference in the pupæ does not result
from the ‘divergence in the mode of life,’ or from the difference
in the external conditions, but is accidental, _i.e._ a consequence
of some fortunate variation induced by some external cause, which
variation afforded protection against common foes--to one species in
one way, and to the other species in some other way; this course,
once entered upon, having been urged on by natural selection, until
at length the wide divergence now shown is attained. How in the case
of any of the species the peculiarity in colour or form can actually
serve as a protection, I must confess myself at fault in answering.
Only in the case of the pupa of _Eueides Isabella_ will I venture to
offer a supposition. That it is not green like other pupæ which suspend
themselves among foliage (_Siderone_, _Epicalia,_ _Callidryas_, &c.),
but contrasts more or less brightly with the dark green of the leaves,
precludes the idea of concealment; on the other hand its colour is
too dull to serve as a conspicuous sign of distastefulness. In either
case the meaning of the wonderful processes of the pupa would remain
unexplained.
“We are thus compelled to seek another possibility in mimicry, by which
foes would be deceived by deceptive resemblance. But what is the object
imitated? Dead insects overgrown by fungi are often found on leaves,
the whitish or yellowish fungi growing from their bodies in various
fantastic forms. Such insects of course no longer serve as tempting
morsels. The processes of the pupa of _Eueides_ suggest such fungoid
growths, although I certainly cannot assert that to our eyes in broad
daylight the resemblance is very striking. But the pupæ hang among the
shadows of the leaves, and a less perfect imitation may deceive foes
that are not so sharp-sighted; protective resemblance must commence
moreover with an imperfect degree of imitation.”