( Originally Published 1917 )
The butterflies furnish the best known examples of insect transformations. The change from the egg to the caterpillar or larva, from the caterpillar to the pupa or chrysalis, and from the chrysalis to the butterfly or imago is doubtless the most generally known fact concerning the life histories of insects. It is a typical example of what are called complete transformations as distinguished from the manner of growth of grasshoppers, crickets, and many other insects in which the young that hatches from the egg bears egg a general resemblance to the adult and in which there is no quiet chrysalis stage when the little creature is unable to eat or to move about.
The Growth of the Caterpillars
Caterpillars are like snakes in at least one respect: in order to provide for their increase in size they shed their skins. When a caterpillar hatches from the egg it is a tiny creature with a soft skin over most of its body but with rather a firm covering for its head. While we might fancy that there could be a considerable increase in size provided for by the stretching of the soft skin it is easy to see that the hard covering of the head will not admit of this. So the story of the growth of a caterpillar may be told in this way :
A butterfly lays an egg upon a Ieaf. Some days later the egg hatches into a larva, which is the technical name for the second stage of an insect's life. In the case of the butterfly we call this larva a caterpillar. The little caterpillar is likely to take its first meal by eating the empty egg shell. This is a curious habit, and a really satisfactory explanation of it seems not to have been made. Its next meal is likely to be taken from the green tissues of the leaf, commonly the green outer surface only being eaten at this time. The future meals are also taken from the leaf, more and more being eaten as the larva gets older.
After a few days of this feeding upon the leaf tissues the little caterpillar becomes so crowded within the skin with which it was born that it is necessary to have a larger one. So a new skin begins to form beneath the first one. Con sequently the latter splits open in a straight line part way down the middle of the back just behind the head. Then the new head covering is withdrawn from the old one and the caterpillar wriggles its way out of the split skin and finds itself clothed in a new one. At first all of the tissues of the new skin are soft and pliable and they easily take on a larger size as the body of the caterpillar expands. A little later these tissues become hardened and no further expansion is possible.
This process of skin shedding is called moulting. The cast skin is often called the exuviae. The period of the caterpillar's life between the hatching from the egg and this moult is often called a stage or instar that is, the caterpillar up to the time of this moult is living in the first caterpillar stage or instar.
During the actual moulting the caterpillar is quite active in freeing itself from the exuviae. But as soon as it is free it is likely to rest quietly for some hours while the tissues of the new skin are hardening. Then it begins feeding upon the leaf again and continues taking its meals at more or less regular intervals for several days. By that time it will again have reached its limit of growth within this second skin and the process of moulting must be repeated. It takes place in the same way as before and the caterpillar enters upon the third instar of its larval life.
This process of feeding and moulting is continued for several weeks, the number of moults being usually four. During the later stages the increase in size is more marked each time the skin is shed, until the caterpillar finally reaches its full growth as a larva and is ready for the wonderful change to the quiet chrysalis in which all its caterpillar organs are to be transformed into the very different organs of the butterfly.
In the case of butterfly larvae one of the most interesting features of the growth of the caterpillar is that of the remarkable changes in colors and patterns of marking which the caterpillar undergoes. One who had not followed these changes would often be at a loss to recognize cater-pillars of slightly differing sizes as belonging to the same species. These changes commonly show a remarkable adaptation to the conditions of life, and generally tend to the concealment of the caterpillar upon its food plant. The stages of growth of the green clouded swallowtail caterpillar are illustrated on plate opposite page 80.
Before each moult the caterpillar is likely to spin a silken web upon the leaf surface. It then entangles its claws in the web to hold itself in place while the skin is cast.
A week or ten days after the last moult of its caterpillar growth the larva commonly becomes full fed and ready to change to the chrysalis state. The details of the way in which this is accomplished vary greatly with different butterflies, as will be noted in the stories of many species later in this book. In general, however, the caterpillar provides a web of silk which it spins against some surface where the chrysalis will be secure and in this web it entangles its hind legs.
Sometimes there is the additional protection of a loop of silk over the front end of the body. After the legs have become entangled the the caterpillar hangs downward until the skin splits open along the median line of the back and gradually shrinks upward until it is almost free, showing as it comes off a curious creature which has some of the characteristic features of a chrysalis. It is seldom at this stage of the same shape as the chrysalis. When the caterpillar's skin is nearly off this chrysalis-like object usually wriggles its body quickly in a manner to entangle a curious set of hooks attached to the upper end in the web of silken thread. This hook like projection is called the cremaster, and it serves a very important purpose in holding the chrysalis in position.
As soon as the cremaster is entangled in the web the cast skin usually falls off and for a very short period the creature hanging seems to be neither caterpillar nor chrysalis. It is in fact in a transition stage between the two, and it very soon shortens up and takes on the definite form of the chrysalis, the outer tissues hardening into the characteristic chrysalis skin.
From the fact that this chrysalis skin shows many of the characteristic features of the future butterfly it is evident that the change from the caterpillar to the butterfly really began during the life of the larva. The nature of the process by which this change takes place has Iong been a puzzle to scientists. For the making of a butterfly is one of the most wonderful phenomena in the outer world, and it has challenged the attention of many acute observers. Some two centuries ago the great Dutch naturalist, Swammerdam, studied very carefully the development of many insects, especially the butterfly. He found that i he placed in boiling water a caterpillar that was ready to pupate or become a chrysalis, the outer skin could easily be removed, revealing beneath the immature butterfly with well-developed legs and antennae. From these observations he was led to believe that the process of growth was simply a process of unfolding; that is, as Professor Packard has expressed it, "That the form of the larva, pupa, and imago preexisted in the egg and even in the ovary; and that the insects in these stages were distinct animals, contained one inside the other, like a nest of boxes or a series of envelopes one within the other." This was called the incasement theory and it was held to be correct by naturalists for nearly a century. It was discredited, however, about a hundred years ago, but not until another fifty years had passed was it definitely replaced by another and much more convincing theory propounded by Weismann.
According to Weismann's theory, which is now well established, the process of development internally is a much more continuous one than the external changes would indicate. So far as the latter are concerned we simply say that a caterpillar changes to a chrysalis and a chrysalis to a butterfly, the transition in each case requiring but a very short time. Internally, however, it has been going on almost continuously from the early life of the caterpillar. The various organs of the butterfly arise from certain germinal disks or "imaginal" buds, the word "imaginal" in this case being an adjective form of imago, so that the imaginal buds are really simply buds for the starting of growth of the various organs of the imago or adult. As the caterpillars approach the chrysalis period these imaginal buds rapidly develop into the various organs of the butterfly. This process is helped along by the breaking down of many of the tissues of the larva, this broken-down tissue being then utilized for the production of the new organs. About the time the chrysalis is formed this breaking-down process becomes very general, so that the newly formed chrysalis seems largely a mass of creamy material which is soon used to build up the various parts of the butterfly through the growth of the imaginal buds.
The Change to the Butterfly
There is probably no phenomenon in the world of living creatures which has attracted more attention than the change of the chrysalis into the butterfly. It is not strange that this is so. We see upon a tree or shrub or wall an inert, apparently lifeless object, having no definite form with which we can compare it with other things, having neither eyes nor ears nor wings nor legs an object apparently of as little interest as a lifeless piece of rock. A few minutes later we behold it again and note with astonishment that this apparently inanimate being has been suddenly transformed into the most ethereal of the creatures of earth, with an exquisite beauty that cannot fail to attract admiration, with wings of most delicate structure for flying through the air, with eyes of a thousand facets, with organs of smell that baffle the ingenuity of man to explain, with vibrant antennae, and a slender tongue adapted to feeding upon the nectar of flowers the most ambrosial of natural food. So it is not strange that this emergence of a butterfly has long been the theme both of poets and theologians and that it attracts the admiring attention of childhood, youth, and age.
Fortunately, this change from chrysalis to butterfly may readily be observed by any one who will take a little trouble to rear the caterpillars or to watch chrysalids found outdoors. The precise method of eclosion, as we call this new kind of "hatching," varies somewhat with different species but in general the process is similar in all.
Those chrysalids which have a light colored outer skin are especially desirable if we would watch this process. One can see through the semi-transparent membrane the developing butterfly within, until finally, just before it is ready to break out, the markings of the wings and body show distinctly. If at this time the chrysalis is placed in the sunshine it is likely to come out at once, so that you can observe it readily. It usually breaks apart over the head and the newly released legs quickly grasp hold of the empty skin as well as of the support to which it is attached. It then hangs downward with a very large abdomen and with the wings more or less crumpled up, but decidedly Iarger than when they were confined within the chrysalis. The wings, however, soon begin to lengthen as they are stretched out, probably through the filling of the space by the body juices. Commonly, the hind pair of wings be-come full size before the front ones. In a short time the wings attain their full size, the abdomen becomes smaller, through the discharge of a liquid called meconmm, and the butterfly is likely to walk a few steps to a better position where it will rest quietly for an hour or two while body and wing tissues harden. After this it is likely to fly away to lead the free life of a butterfly.
These changes from larva to chrysalis and from chrysalis to adult in the case of the Monarch Butterfly are illustraed on the plates opposite pages 32-33. A little study of these photographs from life will help greatly to an understanding of the process.
Some very interesting observations have been made by Mr. J. Alston Moffat upon the method of the expansion the wings. In summarizing his investigations he writes:
"When a wing is fully expanded, and for an hour or two after, the membranes can be easily separated. Entrance for a pin-point between them is to be found at the base of the wing where the subcostal and median nervures come close together. The membranes are united at the costal and inner edges, which have to be cut to get them apart; but they are free at the outer angle. At that time the nervures are in two parts, half in one membrane and half in the other, and open in the centre. The fluid which has been stored up in the pupa enters the winglet at the opening referred to, expanding the membranes as it passes along between them, and the nervures at the same time, and when it has extended to every portion of the wing, then it is fully expanded. The expanding fluid is of a gummy consistency, and as it dries, cements the membranes together, also the edges of the half nervures, and produces the hollow tubes with which we are so familiar."