Popular Science Monthly/Volume 28/December 1885/Neuter Insects
By CHARLES MORRIS.
IN the later editions of Darwin's "Origin of Species" he has answered with remarkable ability nearly all of the several weighty arguments brought against his theory. Some seemingly insuperable objections have been met with an array of facts before which they quite break down. Thus, several instances of extraordinary organs in certain species or types of animals, which it was claimed could not possibly have originated through natural selection, he has shown to be connected by intermediate variations with ordinary organs, which variations are useful at every point of their development, so that the strange appendages might easily have arisen through minute gradations of change.
There is one objection, however, which he can scarcely be said to have answered so happily. This is that in reference to neuter insects—the specially developed working-ants, for instance. As he himself acknowledges, the phenomenon of neuter insects appeared to him at first insuperable, and actually fatal to the whole theory, since these neuters often differ widely in instinct and structure from the males and females, yet, being sterile, they are incapable of hereditarily reproducing their characteristics. In working-ants the difference from the sexual forms is often very great, as in the shape of the thorax, the lack of wings and sometimes of eyes, and in instinct. The difference in instinct is still greater in the hive-bee. Nor is this the whole of the difficulty. In some species of ants there are two and even three distinct castes, well defined, and each with specialities of structure.
Yet, as it is quite impossible that these sterile females could transmit their peculiarities to descendants, and as no such peculiarities exist in the structure of the males and developed females, hereditary influence would seem to vigorously oppose their reproduction, and it seems quite extraordinary that the sexual forms should produce offspring so markedly unlike them. The case is as remarkable as if the offspring of a lion and lioness should be a cat or a leopard, or if a sheep should produce an antelope.
Darwin seeks to explain this difficulty by considering that selection may apply to the family as well as to the individual, and that chance peculiarities of structure, which proved useful to the community, may have been preserved by selection, the tribes in which such useful aberrant forms appeared surviving, while tribes more normal in reproductive power perished. Illustrative facts tending in the same direction are given, and there is certainly a degree of force in this argument, though it can scarcely be accepted as wholly satisfactory.
It is probable that Darwin did not give to this question as full a consideration as to many others, or his remarkable power of analysis could not have failed to perceive other important bearings of the subject. A full review of the phenomena of larval development certainly seems to remove the mystery of the neuter ant and bee from the position of an anomaly to that of an ordinary method of structural unfoldment. If the appearance of sexual organs and powers is the final step to maturity, then all neuters are larval forms, although in every other respect their development may be complete; and they are subject to the same modifying influences as are all larvæ. It is one of the most common conditions of invertebrate life-development for the unfolding offspring to stop at certain stages of growth, and devote itself for a while to nutrition, ere resuming its course of structural development. Such "resting-stages" are those in which there exist specially favorable conditions of nutrition, or of adaptation of the larval form to the conditions of the food-supply. The most notable instances are those seen in the extraordinary larval forms of some of the Echinodermata, and the little less remarkable larval structure of some of the insects and crustaceans. In certain cases several successive larval forms, each deviating considerably from the normal type of the animal, appear.
Yet these peculiarities of structure have never yet been advanced as stumbling-blocks in the way of natural selection. The caterpillar, for instance, while resembling the moth or butterfly in its more deep lying peculiarities, displays remarkable external deviations, and assumes organs and instincts still more anomalous than those shown by neuter ants. The larval star-fish presents an instance of still stranger anomaly. Only the stomachal region and its immediate surroundings pertain to the type, and all the rest of the structure is accessory. When the development of the star-fish is resumed the new form grows out of this internal region of the body of the bipinnaria, or larval form, whose external parts are discarded as useless, or absorbed as food by the new creature. This is the most aberrant instance of such temporary development known. No trace of the star-fish type can be perceived in its larva. It doubtless exists, but is quite masked by secondary formations. Or it may be that this larva represents an ancestral form of the star-fish, as divergent in character as is the crustacean larva of the barnacle from the mature form.
Yet this explanation of atavism, or temporary check to development at an ancestral form stage, only partly meets the difficulties of the case. There is an unquestionable new adaptation to new circumstances to be explained. Natural selection acts upon all forms which give it sufficient opportunity, without regard to whether they are larval or mature. Let us take for an instance the case of the butterfly. Here the development does not proceed continuously, from the germ to the mature form, as in some insects, but is checked for a considerable period at the caterpillar stage. The active nutrition at this stage seems to act as a check upon development, so that the caterpillar is a form upon which natural selection has full opportunity to produce its effects. Originally it may have simply rested for a time in the direct line of development, on account of finding abundant food. But, as food conditions changed, new enemies attacked it, or old foes adopted new modes of assault, one of two things was necessary for its survival. It must either lose this resting-stage and develop continuously, or it must become adapted to the new conditions. This rendered necessary changes in instinct and in structure. Where the resting-stage, as in the caterpillar, occupied a very large percentage of the total life-duration, and where the process of adaptation had millions of years for its completion, it is not surprising that structural features often very divergent from the typical form were assumed.
There is little or no reason to doubt that all the peculiarities of larval form are due to the two causes here specified: 1. A temporary check to development at some ancestral stage of the animal's unfoldment. 2. An adaptive modification of structure and habit to meet varying conditions in the environment of this stage of development.
Yet in every such case we meet with a difficulty of the same character as that existing in the case of neuter ants. These secondary adaptations are out of the direct line of the animal's development, and it is a question how they can be hereditarily transmitted. The law of phylogenetic development enables us to understand the appearance of certain embryonal peculiarities of structure which do not exist in the mature form. If development is forced to follow its original line, such ancestral features must necessarily appear, though if the development is very rapid only hints of them are perceptible; or they may become utterly obliterated, so far as our powers of observation can decide. Yet such a principle can not apply to secondary structural features, produced in larval adaptation. The latter are in no sense in the direct ancestral line of development, and it is somewhat remarkable that they are so faithfully reproduced, only to be thrown aside again as the animal resumes its temporarily checked development.
It is very evident, from the facts here cited, that the phylogenetic line is subject to disturbing influences. There is no special reason, in the nature of things, why a developing animal should repeat every stage of its ancestral growth. If never disturbed in its development it would naturally do so, since its original evolution from primeval matter lay in that line, and there has been no force since brought to bear upon it to make it deviate. But where any subsequent force causes deviation, that deviation must become persistent. There can be no possible return to the exact ancestral course.!Many such deviations have occurred. Some of them are only apparently such, arising from rapidity of development, and the slurring over of intermediate steps in the line of growth. But many of them are results of subsequent adaptation. Such is the case with many of the peculiarities seen in the unfoldment of the mammalian embryo. It has deviated from the ancestral line to meet certain special requirements of the situation. It can never return to that line. The mere fact that an ancestor once existed, with certain characteristics, has in itself no controlling force upon the development of the embryo.
The secondary adaptations of larval forms have the same bearing upon development as have peculiar ancestral conditions. They become characteristic steps in the line of development to maturity. The sexually mature animal has passed through them all in its growth from the germ, and conditions of the same character are implanted in its own germs, and must unfold in their development. There is no longer an exact phylogenetic line. Many of the ancestral stages have become greatly modified. To the new developing animals those modified stages of growth are ancestral stages so far as it individually is concerned. Development follows this new line, although it may have become a strangely warped and irregular one, and though at certain stages of growth it may yield peculiar organs or tissues which are discarded as useless, or consumed as nutriment, at later stages. The true line of growth in such cases is restricted to the more deep-lying and important parts of the organism, and though, at certain stages of growth, forces appear which produce a special growth of secondary tissue, this is reabsorbed or discarded when the development is resumed. Marked instances of such discarded tissue are seen in the pupal development of certain insects, and in the case of the star-fish development above referred to.
We have paid some little attention to the characteristics of larval growth for two reasons. Their true bearing on the mystery of evolution has been little attended to, and the above-given hypothesis of explanation has not heretofore been offered, so far as the writer is aware. The second reason is that they bear a much closer relation to the phenomenon of neuter insects than might at first sight appear. The neuter insect has not as yet been looked upon as a resting-stage in the line of full development, and as analogous to the lower stages of larval growth. It has, indeed, a peculiarity of its own, that it fails to attain full development. And as its secondary characteristics are not participated in by the sexually mature form, but have arisen by adaptation which is still operative, the fact of their transmission becomes difficult to understand. Yet we think it may be shown to be but an extension of the principle above considered.
It is a significant fact that a neuter worker class is found only in those animal tribes in which the social principle has reached its highest development, such as the bees, ants, and termites among insects, and the hydroid polyps in the other sub-kingdoms of life. In each of these communal types of life there has been a division of duties, the work of reproduction being confined to one or a few members of the community, at least so far as maternity is concerned, while the other members have gained special adaptations to other duties. In bee communities only one queen is permitted to develop, while the remaining females continue sterile, and become adapted to-working duties. Among the ants numerous queens develop, but each surviving queen usually becomes the mother of a separate community, in which the sterile females are adapted to two or more distinct duties. The problem of the males is a singular one. Among bees and ants they arc never checked at the worker stage, but develop to become a possible burden on the community. Here among the bees a second remarkable instance of intelligent selection is displayed. The males are suffered to live as long as food is abundant, but are mercilessly stung to death as soon as there is danger of lack of food. In ant communities natural selection disposes of the surplus males. Their life-power is reduced to that required for the nuptial flight, and they die as soon as their one necessary duty is performed.
Among the termites, or white ants, we find an interesting extension of this principle. Here restriction applies to both sexes, the workers and soldiers being immature males and females. Some writers, indeed, hold that they are of no sex, but have been checked in development at the larval stage, before sexual differentiation began. And a male as well as a female survives to start the new community, each nest having its so-called king and queen. In polyp colonies we find the same thing in a less developed stage. Each sexual individual is hermaphrodite, and the king and queen powers exist in a single form. In the Siphonophora, or floating hydrozoan colonies, the partly developed forms are adapted to four distinct duties. Some of them become contracting bells, and serve for locomotion; others become stomachal tubes, and digest the food of the colony; others are tentacles, or food-catchers; and others are simply covering or protective pieces; yet in all of them the Medusa type can occasionally be recognized.
It may be well to point out here that a similar division of duties exists in all the higher members of the vegetable kingdom. Each tree is a colony, the product of buds arising in a common stem, and is thus closely analogous to a polyp colony. The analogy goes further—there is a division of duties among the members of the tree colony. Some of these members attain full development and become hermaphrodite sexual individuals. The others are restricted in development, and become adapted to several distinct duties. Thus, two distinct nutritive forms appear, the leaf-bearing individual and the root individual. But greatly restricted protective forms occasionally appear, such as the thorn, whose development is on a level with that of the covering piece in a polyp colony. Other illustrations of this principle of restriction of development and division of duties might be given, but we must go on to consider its significance.
If we consider any of the lower animal forms, it will quickly appear that structural development is checked more or less completely during active life and abundant nutrition. Insect larvæ, for instance, simply grow during their active feeding-stage. New development only begins during the inactive pupal stage, in which the tissue formed during the larval stage is modified and transformed. After the insect becomes again active, as the imago, no further development of special importance takes place; and it would appear that, if the larval stage is not allowed its full period or its complete course of nutrition, the pupal development is checked at an imperfect stage, and the imago remains immature.
Such is evidently the case in bee communities. The division of the community into males, queen, and workers seems less an operation of natural selection than of intelligent selection. It is a matter of choice among the workers whether any female larva shall develop into a worker or a queen. By giving more room for growth, and more and better food, they can produce a queen from any female larva chosen at will. By contracting the growth-space and diminishing the food, the power of development is checked, and the insect, in its pupal stage, becomes incapable of developing sexual organs and powers.
Thus in every female larva it seems evident that innate powers to become either queen or worker exist. The queen is the higher phase of development, but in attaining this stage the worker stage must be passed through. Why does it not become apparent? This is not difficult to understand, since a similar phenomenon is of very common occurrence. It is simply slurred over in a rapid course of development. The sexual organs begin to unfold, and in so doing exhaust the nutriment and the life-energy which would be needed for the full unfoldment of the worker organs. Thus the superior force checks the inferior, and the innate tendency to develop into a worker is overcome by the activity of a more energetic innate tendency. Where the latter remains aborted the worker tissues fully develop, and with them the worker instincts, since every stage of structural development seems accompanied by its peculiar instincts, as if tissue dominated instinct.
In the case of the ant we have closely similar phenomena. Here there is no satisfactory evidence of intelligent selection, though many observers believe that it exists. So far as we know, however, chance decides whether the larva shall have food enough to carry it to one or other of the worker stages, or to the queen stage. Thus numerous individuals of each stage appear. But the two or more worker castes are not completely separated, since intermediate forms exist, sufficient to make a line of insensible gradation from one form to the other or others. Here, then, we have a complete line of development, reaching from the germ to the queen, but checked at various stages, in which nutrition becomes active and secondary adaptations appear. These secondary adaptive features have undoubtedly become part of the direct line of structural unfoldment. But, as soon as a higher phase of structure begins to unfold, these lower conditions of tissue are broken down or reabsorbed, just as in the human embryo the gillarches disappear, or are transformed into organs required in the next higher stage.
If, in the insect pupal stage, the development of the higher structural stage begins simultaneously with, or immediately after, that of a lower stage, the latter is interfered with by a superior energy. It can not obtain full unfoldment, and may gain but a rudimentary formation, which may be immediately reabsorbed, to answer the demands of the superior modifying energy. Two unlike energies seem fighting for the nutriment, and the stronger wins. Instances of this principle of development are common in embryo growth, and act to check or to completely abolish the unfoldment of ancestral features. In the case of the ant we may look upon it as the cause of the lack of appearance of the worker characteristics in the development of the queen, and of their full development where the conditions are such as to prevent the innate powers of sexual unfoldment from coming into play, and to restrict development at a lower level. It need scarcely be added that in the case of these insects the check to development is final. On leaving the pupa-case, they enter upon a life of active nutrition, in which the powers of development already in operation may produce their full results, but in which the latent higher powers are definitely restrained. In all cases of insect development, and doubtless to a certain extent in all animals, a state of passivity is requisite to active transformation of tissue, while simple growth is the prevailing tendency in states of activity and abundant nutrition. In these latter states organic development may proceed, but it is simply the completion of lines of development which began in the passive state. New lines of development do not begin during nutritive activity. Of this principle many illustrations might be given, had we the space here to adduce them.
In the case already cited, of the hydroid polyps, this principle of development yields some remarkable results. In many instances the sexual individual; unfold into the full medusoid type, and leave the colony to enjoy a free life. In others they remain attached to the colony, and are more or less checked in their full development. This check to development is so great, in certain instances, that a mere bud appears, to bear the generative products. Thus the sexual, instead of being the typical form, remains as a formless protrusion of the polyp stem, or the germs may originate in this stem with no form development.
Yet this seeming anomaly is not without its explanation under the above principle. Where free Medusæ are produced, the sexual organs and products do not appear until after full development and freedom from the colony are attained. In the other cases mentioned the generative products appear earlier, and it is probably their appearance that checks further form development. The innate tendency to develop the medusa form is hindered by the early unfoldment of the superior tendency to sexual development, which exhausts the vital energies and absorbs or prevents the formation of other tissue adapted to the lower life-purposes. The needs of this highest life-power tyrannize over all lower powers, and as soon as it appears all other development ceases. In most animals it is the final step, after all lower stages are completed. Here it is occasionally the initial step, and exhausts the developmental powers before any of the lower stages have appeared.
In plants the same principle holds good. Active nutrition checks development, and unfoldment ceases at the leaf or the root stage. For full development, nutrition must be checked; when a partial resting-stage succeeds, higher transformation sets in, and the sexual bud or the flower individual appears. In many cases hints of the leaf stage of development are displayed. In others this stage is completely aborted. Thus the leaf-bearing individual, in its lack of power to reproduce itself, and in its structural and functional differences from the flower individual, is closely analogous to the case of neuter insects as compared with the sexual forms. In plants, also, we have instances of the aborted development of the sexual forms, closely analogous to those seen in the Hydrozoa. Thus, in these remarkable phenomena of development there seems to be a close relation between the tenants of the two great kingdoms of life.