Popular Science Monthly/Volume 15/August 1879/Neuter Insects
By PHILIP WOOLF, M. D.
HOW the workers of many insects became sterile is an interesting question, though one difficult to answer. Those who believe in special creation solve this problem, as they solve so many other difficulties, by stating that insects are sterile because they were created sterile. The majority of educated persons, however, require to be convinced by some more tangible argument; the conclusion that things are because they are, having lost the only merit it ever possessed, the annihilation of thought.
To state the problem that all may understand it: there are many insects, as the bees, ants, wasps, and termites, that are divided into three castes, males, females, and workers; the latter are sterile, and it is asked how, according to modern theory, these workers can have arisen? It is said that, since they do not propagate their kind, no spontaneous variations can be produced for natural selection to work on, and hence that there is no other explanation of their existence than that they are specially created—that is, created in opposition to law.
To simplify the initial inquiry, we will suppose that workers differ from females only in that they are not fertile, the secondary differences being reserved for discussion toward the end of this paper.
That a creature may develop, it is essential that it be supplied with a sufficiency of food. Even with human beings food is all-important; if with these a sufficiency of mineral ingredients be not assimilated, we have the disease called rickets; insufficiency of vegetable food will cause scurvy; insufficient nutriment dwarfs the body and mind generally. The growing body must have enough material out of which to elaborate tissue, or the tissue, muscle, nerve, bone, can not develop. Now let it be supposed there are two larvæ or grubs of the bee, which under similar conditions will reach similar degrees of development; further suppose that one larva, which may be called A, receives an over-abundance of food, while the other larva, which may be called B, receives a quantity of food just sufficient for its more important wants, what will happen? The larva A will reach its full development—it will be a queen-bee; the larva B, on the other hand, not having food enough for all its wants, and furthermore having within a given time to change its conditions, it must use the nutriment for the building up of its more important organs, must work it up into tissues and structures that are essential to all bees. Its head, body, legs, and wings must be perfected before its reproductive system; the individual must profit before the race; and, if the food is only sufficient for individual functions, the race functions must suffer, the reproductive system must remain in an incipient state.
This conclusion is no mere theory, for among the higher animals, and even with man himself, insufficient nourishment first produces its effects on the reproductive system. Loss of blood induces abortion; badly prepared or insufficient food decreases or entirely checks the production of offspring. The distinction first clearly formulated by Dr. Carpenter is now a commonplace of science: "There is a certain degree of antagonism between the nutritive and reproductive functions, the one being executed at the expense of the other. The reproductive apparatus derives the materials of its operations through the nutritive system, and is entirely dependent on it for the continuance of its functions. It may be universally observed that, when the nutritive functions are particularly active in supporting the individual, the reproductive system is in a corresponding degree undeveloped, and vice versa."
With bees the effects produced by food are clearly shown: a larva which otherwise would turn into a neuter is supplied with a different kind of food and it is converted into a queen. Huber obtained queen bees by placing some of the "royal food" in cells inhabited by the larva? of workers. Kleine performed the same experiment, placed a portion of the royal food on the inner margin of the worker cell and produced queens. With bees, then, we see that the neuter is only a female with the reproductive organs partially developed, a conclusion further enforced by the fact that the neuter sometimes even lays eggs which develop into the drone or male bee. In this case then, at least, it may be said that the fecundity or sterility of bees depends almost entirely on the nature of the food given to the larvæ. With the true ants also, the so-called neuter is only a partially developed female, although there are no recorded observations to show that the difference between the fertile and the infertile insect is due to difference in food.
To advance now a step further: insects, originally, were either male or female; there were no neuters. For reasons presently to be given, we conclude that the males of even social insects, in addition to the duty of fecundating the female, also undertook the duty of defending the nest against warlike intruders. It may be also safely concluded that originally the females of even social insects, in addition to the duties of maternity, also undertook the labors of building the nest and taking care of the young—customs that still prevail more or less with many insects. Thus, the females of the solitary bees, after impregnation,during the winter. With the warm days of spring they awaken, build their nests, and die. Among wasps a similar state of things obtains. When the winter approaches, the entire colony, with the exception of a few pregnant females, dies. In the spring these females begin building a new nest, laying eggs, and so producing an efficient corps of assistants to aid them in their future labors.
Supposing, then, that originally the females undertook most of the work now done by the neuters; supposing, also, that at the beginning of spring there are only a few females, or even one female, to commence the work of building the nest and feeding the future progeny. With many duties to perform, it is not unwarranted to conclude that some duties may not be completely performed. The immediate success of the colony depends, not so much on the number of males or of females, as on a body of efficient assistants. Now, when one or a few insects have to feed many, some of the larvæ receive an abundance, some barely a sufficiency of food; and, on the theory previously advanced, we may see how neuter insects arose. The fact that quality as well as quantity of food is essential to future fertility does not add to the difficulty, since, as it takes more time to produce the highly nutritious than the somewhat less nutritious food, the conditions are still the same. If efficient assistants could be produced at a less expenditure of labor and in a shorter time than females, a supposition countenanced by facts, natural selection begins to work. In a given neighborhood those insects which produce a corps of assistants soonest, and with least expenditure of labor, will stand a better chance of obtaining food—of surviving—than those insects which give the same amount of attention to each larva, rearing them all into perfect insects—a benefit also accruing to the earlier hatched larvæ themselves, as they sooner reach maturity, and can thus obtain an abundance of food without rivalry. These earlier produced neuters have their reproductive systems only slightly less developed than egg-bearing insects; but natural selection, acting on the nurse, will cause them to grow less and less. Reproductive organs that do not produce are of no use to their possessors; and, as this slightly less developed but useless reproductive system would require more attention and more food from the fertile nurse than would those larvæ in which it was a mere rudiment, natural selection, by working on the instinct of the nurses, would modify, alter, or even suppress what was of no use. Or we may suppose that in the distribution of food to the larvæ the action is direct, some receiving such a minimum quantity of food that the reproductive organs remain rudimentary from want of material to build them up.
The necessity of producing offspring quickly in early spring would give rise to the instinct to feed the first produced brood of larvas on food insufficient in quality, or in quantity, or in both. The eggs, then, first laid by the female would develop into neuters. Neuters being sterile females, they would inherit the instincts of true females when they in turn took charge of the young. Whatever may be the worth of this theory, it throws some light on the curious fact that with some insects, as the bees and ants, the sexes are produced at different times. With bees the queen first lays eggs which produce neuters; then, at a later period, eggs producing males. According to Gould, the female of the Formica sanguinea—the red ant—lays eggs which will produce females, males, and workers at three different periods. That the habit is so is well known; but on the theory here supported can not we see how the habit arose, and the reason why such a habit exists?
Thus far food alone has been supposed to affect the development of insects, but there are several secondary factors; size of cell being one. Thus with the bee, the cell in which the queen is hatched is larger, differently formed, and in weight said to be equivalent to one hundred ordinary cells; the cells from which emerge the males are also larger than the cells of neuters. Now, the extra labor necessary to produce these cells in founding a new colony, the extra labor in attending to the inmates, and the non-necessity of having males or females at this early stage of colonial existence, are other reasons why the first-laid eggs produce only neuters. At present and in this place these points can be only touched on; in the future they will receive more elaboration.
If neuters have arisen in the manner suggested, how has their inflexibility of character been maintained? By natural selection modifying the instincts of the nurse. What at first arose through the incapability of one or several insects taking care of many insects, became through the action of natural selection, by the survival of those insects which produced a given result with least labor to themselves, an established instinct. Further questions may be now asked: Why is it that any other insects besides neuters have been produced? As they can not propagate their kind, how have they become gifted with the instincts to take care of and to feed the young, provide food for the colony, build nests, etc.?
To the first question a sufficient reply is, that, if nothing but neuters were produced, there would be no insects. The other questions may be answered by an amplification of a statement already made. Neuters, as a rule, are sterile females; the exceptions to this will be considered further on; they inherit the instinct of females and perform their duties with the one exception of ovulation. To give a few details: Originally there were no neuters; and as females were numerous, each female would lay comparatively few eggs. How the neuters arose we have already seen; at first they were few, gradually increasing until they equaled and finally outnumbered the females. At the same time the labors of the female became more restricted; as they decreased in number they must, in order to keep up the colony, lay more eggs: as a result, the extra time devoted to ovulation was so much time taken from cell-building, nursing, etc. Applying this theory to facts, we can see why with the increase of neuters the duties of the queen-bee have grown less and less until they consist of nothing but ovulation. The queen-bee is a queen only in name, receiving just such extra care that her time may be entirely devoted to propagating the species. At such time when each hive only contained one reigning queen, this female had to assume the reproductive functions of her twenty thousand sterile sisters, and it is not strange that she has no time to build or to feed the young. Under no other conditions could she lay her two thousand to three thousand eggs a day. The case of the termites or so-called white ants is more striking, the female laying eighty thousand eggs in the course of a day, or very nearly one egg a second continuously. What other duties can this huge animated egg-sac perform?
With existing social insects, as a rule, the male does little or no work but that of fertilizing the females, but before the appearance of neuters we may suppose he had other duties. With many beetles the male performs a large share of the labor; the male of the burying beetle, for example, excavating the grave in which its prey is buried and in which the female deposits her eggs. With many insects the male defends the nest or burrow from the attacks of invaders, of which take an example quoted by Mr. Darwin in his "Descent of Man": "The two sexes of Lethrus cephalotes (one of the Lamellicorns) inhabit the same burrow. If during the breeding season a strange male attempts to enter the burrow, he is attacked; the female does not remain passive, but closes the mouth of the burrow and encourages her mate by continually pushing him from behind." That males are not always passive, the work just quoted abundantly proves. Now when neuters are produced from males their duties differ from the neuters produced from females, and, as we have supposed that the duties now performed by sterile females were once performed by fertile females, so we may now conclude that the duties performed by sterile males were once performed by the perfect males. The worth of this conclusion will be presently seen.
The remaining difficulty is to account for the fact that with many insects the neuters differ considerably from the fertile insects. Thus, for example, with the termites or white ants; these are the perfect males and females, the soldiers, which are aborted males, and the workers, which are aborted females. The males and females have wings, the neuters are wingless; the workers undertake architectural duties, act the part of nurses, etc., while the soldiers defend the nest from attacks. Both workers and soldiers are blind, but whereas the workers have a somewhat circular head and small jaws, the soldiers have a comparatively enormous head, and strong resisting mandibles. In what manner then, or through what cause, could the head of this soldier termite differ so greatly from either that of the perfect male or perfect female? Or, since it is impossible in any given case to explain all the details satisfactorily, let it be asked how it is that so many neuter insects differ from their parents.
Neuters are either sterile males or sterile females, and in many cases do not differ greatly from their fertile progenitors; the social bees and wasps are examples of this. On the theory advanced, it has been supposed that originally the neuter differed only from the perfect insect in that it had a rudimentary reproductive system. Now, suppose, to take an imaginary example, that in a colony of ants there are only males and females; that the duty of the male is the defense of the nest against encroaching enemies, and that the duties of the females are to build the nest, lay eggs, and take charge of the young. Of course the males and females having different duties to perform will have their structures differently modified; say, in our case, the male has a largely developed head like the soldier-termite, the female a head like the worker-termite. Going a step further, neuters begin to appear, the aborted male still performing soldier duty, the aborted female still attending to its domestic duties. The neuters continuing to increase as we know they have increased, and the true males and females decreasing in number as we know they have decreased, a state of affairs is reached in which it is essential to the welfare of the colony that the male should confine himself to fertilizing the female, the female principally confine herself to laying eggs.
Thus far, the fertile and infertile males, the fertile and infertile females, have resembled each other; but disuse of parts induces retrograde metamorphosis, or modification or suppression of useless parts. If the males no longer use their heads and jaws to protect the community, these parts would decrease in size; if the females no longer assist in building, if their entire duty is to lay eggs, their wings, legs, jaws, etc., will decrease; the surplus of force thus entailed being added to the reproductive system. Thus, then, there would have been produced the four castes found among the termites; the soldier representing the typical male of the species, the workers the typical females minus the perfected reproductive organs. But—and here is the great difficulty—how can the changed male have given his lost organs to his sex, and the female have transmitted her original but now modified peculiarities to the workers; especially as neuters do not propagate, and hence can not transmit their characters to progeny? Another law solves a portion of the difficulty: peculiarities acquired at any period of life are apt to appear in the same sex at the same time of life. Disuse having wrought its changes on our fertile insects after they had reached their perfect form, we can not expect them to appear in offspring which never reach this form. The modifications of structure were produced that the reproductive system might be benefited; why, then, should they take place in those insects which have a rudimentary and hence useless reproductive system?
We have now to ask whether those larvæ which are to produce fertile insects resemble, in any stage of their existence, the larvæ which are to produce neuters; whether, for example, the fertile male termite resembles, at any time, the infertile soldier. The reply to this is partly positive, partly negative; the larva of the female termite resembles very nearly the larva of the worker; but there is no great resemblance between the male and the soldier larvæ; there is a greater resemblance between the pupæ.
The fact alone that the female of social bees, and the male and female of white ants, should be presented under two forms is no novelty in insect history. In the aphides or plant-lice a similar state of affairs obtains: there is the perfect and imperfect female. Even as high in the scale as butterflies dimorphism is not uncommon. Mr. Wallace has discovered two forms of the female of Papilio Memnon, an inhabitant of the East Indies, one of which has tailed wings, the other of which is tailless. Several butterflies have three kinds of females, or are trimorphic. Hence the mere differences between neuter and perfect insects are nothing unusual, considered as differences. Were the workers fertile and thus able to propagate their peculiarities, the difficulties would vanish; but the problem why a fertile female should give birth to two or three distinct forms is still shrouded by mysteries, accept what explanation we may.
Our alternative that the workers are the type from which the males and the females have diverged is only an hypothesis, but in view of the facts it is the only alternative left us; since the neuters themselves can not have diverged from any type on account of their sterility.
The conclusions, then, reached in this paper are, that in many cases the differences between fertile and infertile insects are due to the quality or quantity, or both, of the food given to the larvæ. This conclusion is of worth, since it is supported by Huber, Smith, Woodbury, and others; though these naturalists only apply it to the social bees. The suggestion of Professor Wyman, that the difference in development is due to the difference in the time the eggs are laid after fertilization, seems to be opposed to facts; especially to the experiment of Kleine, who reared worker-larvæ into queens by feeding them on royal food. The other conclusion is, that the neuters represent the type from which the true males and females have diverged; that in those cases where food is powerless, the neuter retains its immutability for the reason that its development is arrested at a certain stage; that is, it does not go beyond the state reached by the typical progenitor, while the perfect males and females go beyond this stage, and that the differences between them and the neuters were inaugurated at this time; that changed conditions have been potent in producing such differences; that the differences are only inherited at that advanced period of progression in which they were initiated. In other words, to render this conclusion plain to the general reader, we believe that if the neuter-worker of the white ant, for example, were to progress in development, it would turn into the fertile female; if the neuter soldier of the white ant were to continue on the line of development, it would become a fertile male. This does not give support to the theory that the worker and soldier are immature male and female; that they are the perpetual babies, while the perfect insects are adults—since we believe that in their way the neuters are as adult as their parents. This proposition may be rendered clearer by a symbol, which may be represented by the letter Y. The stem of this letter will stand for the typical insect represented at the present day by the neuter, and the two arms, respectively, will represent the male and the female, which, after the typical insect reached a stable form, diverged into new routes of progression.
At some future time we hope to work out this subject more elaborately, and, from the observations and facts already collected, it is believed that the theory can be defended if not vindicated. Our present purpose, however, has been accomplished—to introduce to the general reader a subject which has perplexed, and still perplexes, our greatest naturalists.