Animal Behaviour

I.—Definition of Instinctive Behaviour

There are probably few subjects which have afforded more material for wonder and pious admiration than the instinctive endowments of animals. “I look upon instinct,” wrote Addison in one of his graceful essays, “as upon the principle of gravitation in bodies, which is not to be explained by any known qualities inherent in the bodies themselves, nor from any laws of mechanism, but as an immediate impression from the first Mover and the Divine Energy acting in the creatures.”[21] In like manner Spence said: “We may call the instincts of animals those faculties implanted in them by the Creator, by which, independent of instruction, observation or experience, and without a knowledge of the end in view, they are all alike impelled to the performance of certain actions tending to the well-being of the individual and the preservation of the species.”[22] According to such views, instinct is an ultimate principle the natural genesis of which is beyond the pale of explanation. But similar views were, at the time these passages were written, held to apply, not only to animal behaviour, but also to animal structure. The development of the stag’s antler, or of the insect’s wing, was also regarded as “an immediate impression from the first Mover and the Divine Energy acting in the creatures.” This view, however, is, neither in the case of structure nor in the case of behaviour, that entertained by modern science. It is indeed an expression of opinion concerning the metaphysics of instinct. Leaving the question of ultimate origin precisely where it stood in the times of Addison and of Spence, modern science seeks to trace the natural antecedents of all natural phenomena, and regards structure and behaviour alike as the products of evolution, endeavouring to explain the manner of their genetic origin in terms of progressive heredity.

Omitting, therefore, all reference to problems which, however important, are beyond the limits of scientific inquiry,[23] we may take as a basis for further discussion Spence’s definition, according to which the instincts of animals are those faculties by which, independent of instruction, observation, or experience, and without a knowledge of the end in view, they are all alike impelled to the performance of certain actions tending to their own well-being and the preservation of the species.

Let us first consider the reference of instinctive actions to a faculty by which animals are said to be impelled to their performance. Paley also defined instinct as “a propensity prior to experience.” And unquestionably in the popular conception it is usual to attribute instinctive acts to some such conscious cause. But it will be more convenient, for the present, to consider instinctive behaviour from the objective point of view, as it is presented to our observation; we may then proceed to the further consideration of the conscious concomitants which may be inferred. From the objective point of view, therefore, we may agree with Professor Groos, who says[24] that “the idea of consciousness must be rigidly excluded from any definition of instinct which is to be of practical utility,” since “it is always hazardous in scientific investigation to allow an hypothesis which cannot be tested empirically.” In this we have the support of Dr. and Mrs. Peckham, whose studies of the life-histories of spiders and wasps are models of careful and patient investigation. “Under the term Instinct,” they say, “we place all complex acts which are performed previous to experience, and in a similar manner by all members of the same sex and race, leaving out as non-essential, at this time, the question of whether they are or are not accompanied by consciousness.”[25]

It may be said, however, that some reference to the conscious aspect of instinctive behaviour is implied by saying that the acts are performed without instruction or experience. But the reference at present is wholly negative. We may say, as the result of observation, that instinctive acts are performed under such circumstances as exclude the possibility of guidance in the light of individual experience, and render it in the highest degree improbable that there exists any idea of the end to be attained. But this is a very different position from that of asserting the presence of a positive faculty or propensity which impels an animal to the performance of certain actions. This it is which, from the observational point of view, is unnecessary. For the reference of a given type of observed behaviour to a “propensity” so to behave or to a “faculty” of thus behaving, is no more helpful than the reference of the development of any given type of structure to a “potentiality” so to develop. We may, therefore, without loss of precision, simplify Spence’s definition by stating that instinctive behaviour is independent of instruction and experience, and tends to the well-being of the individual and the preservation of the species.

Let us next consider the clause which affirms that instinctive behaviour is prior to experience. This is well in line with the distinction now drawn by biologists between congenital and acquired characters. It refers them to the former category, and implies that the organic mechanism by which they are rendered possible is of germinal origin. This is not, however, universally admitted. Professor Wundt, for example, approaching the subject from the point of view afforded by the study of man and the higher animals, gives to the term a wider meaning, and so defines instinct as to include acquired habits. “Movements,” he says,[26] “which originally followed upon simple or compound voluntary acts, but which have become wholly or partly mechanized in the course of individual life, or of generic evolution, we term instinctive actions.” In accordance with this definition, instincts fall into two groups. Those “which, so far as we can tell, have been developed during the life of the individual, and in the absence of definite individual influences might have remained wholly undeveloped, may be called acquired instincts.” They have become instinctive through repetition. “To be distinguished from these acquired human instincts are others which are connate.” Now, there can be no question that behaviour which has become habitual through frequent repetition is frequently, in popular speech, described as instinctive. We hear it said that the experienced cyclist guides his machine instinctively. And the word is similarly used in many like cases. But we shall find it conducive to precision and clearness of thought to emphasize the distinction between what is acquired in the course of life and what is congenital in the race. And to this end we shall regard behaviour which has “become mechanized in the course of individual life” as due to acquired habit, reserving the term instinctive for such behaviour as is independent of individual experience. We shall, in short, so far accept Spence’s definition.

In this definition, as in those of the majority of naturalists, it seems to be further implied that instinctive behaviour is of a relatively definite kind, though it is no doubt subject to such variation as is found in animal structure and organization. Mr. Rutgers Marshall, however, in a recent work,[27] protests against any such implication, and urges that “this variableness is so wide that definiteness of reaction cannot for a moment be used as a differentia in relation to instinct without narrowing our conception of the bounds of instinct in a manner to be deplored.” “The actions,” he says, “connected with the preparation for self-defence, those connected with protection of the young, with nest-building, with migration, etc., these actions are surely to be classed as instinctive; and yet they are exceedingly variable and unpredictable in detail; all that we can predict is the general trend of the varying actions which result from varying stimuli under varying conditions, and which function to some determinate biological end.”

Mr. Marshall then proceeds to argue that we are “warranted in speaking of the ethical instincts, of the patriotic instincts, of the benevolent instincts, and of the artistic instincts;” and thus leads up to the position, to be further elaborated in his work, that there exists in man a religious instinct which has fulfilled a function of biological value in the development of our race. Now, here again there is much in popular usage of the words instinct and instinctive which lends support, for what it is worth, to Mr. Marshall’s very broad conception of the range of instinct. Again and again we hear, in the pulpit and elsewhere, of the religious instinct; we hear, too, of the benevolent, patriotic, and artistic instincts, and more besides. But what we are endeavouring to define is a type of behaviour which, as such, is prior to instruction and experience. Can we affirm that patriotic and religious behaviour conforms to such a type? Is it unquestionably congenital and not acquired? If we are forced to give negative answers to these questions we must regard Mr. Marshall’s conception of instinct (one inclusive of multifarious tendencies which have a biological value) as too broad and too vague to be of any service to us at this stage of our study of animal behaviour.

What, then, shall we understand by Spence’s phrase that instinct involves the performance of “certain actions”? And how far shall we accept it? We shall take it as implying so much definiteness of behaviour as renders instinctive acts susceptible of scientific investigation, and in this sense shall accept it with some modification of phraseology. We shall freely admit, however, the existence of variations of instinctive behaviour analogous to variations in animal structure. It is the occurrence of such variations that renders the natural selection of instinctive modes of behaviour conceivable. We shall also admit some, nay much, variation in detail. Take, for example, two of the cases which Mr. Marshall cites—nest-building and migration. Both involve, not merely a simple response to a given stimulus, but a complex sequence of actions. In detail there may be much variation even among members of the same species. And yet, can it be questioned that the behaviour as a whole is in each case relatively definite? May we not even say that it is remarkably definite? May we not even go further, and assert that only on the assumption that any given instinctive act is relatively definite, can we regard it as a subject for scientific investigation, and can we hope to distinguish it from other modes of behaviour?

The next point for consideration in Spence’s definition, which we have taken as our text, is his characterization of instinctive acts as “tending to the well-being of the individual and the preservation of the species.” Here we have Mr. Marshall with us, for he too lays stress on the fact that instinctive behaviour has reference to a definite biological end. But in saying that the biological end is the objective mark of an instinct,[28] he seems to be in error. Because, in the first place, there are other “objective marks,” and because, in the second place, this objective mark is not restricted to instinctive behaviour. According to Spence, a further characteristic of instinctive acts is that they are independent of instruction or experience; and this serves to differentiate them from other modes of behaviour which are also subservient to a biological end. Intelligent behaviour, not less than that which we term instinctive, has reference to a biological end. Many intelligent acts, for example, have for their object the well-being of the individual; many subserve race preservation; these bear, every whit as much as instinctive acts, the “objective mark” which Mr. Marshall regards as characteristic of instinct. And if we turn to his subjective criterion—the absence of any conception of the biological end which the behaviour subserves—Mr. Marshall’s position is equally untenable. There are thousands of acquired modes of behaviour, dependent on instruction or experience, in which there is, on the subjective side, so far as we can judge, no conception of the biological end to be attained. What can the animal in the early stages of intelligence know of biological ends? Mr. Marshall’s subjective criterion applies just as much to a wide range of intelligent behaviour as it does to instinctive actions.

In accepting, therefore, Spence’s statement that when animals behave instinctively they perform, without a knowledge of the end in view, certain actions tending to their own well-being and the preservation of the species, we must take it in connection with the preceding limitation, remembering that they are also performed without instruction and experience.

A further point for very brief consideration is suggested by the phrase in which Spence says that animals are all alike impelled to the performance of certain actions. As it stands it is too sweeping and general. Still, we do require some explicit statement of the facts which he had in mind when he wrote the words “all alike.” And we find it with sufficient exactness in Dr. Peckham’s definition, where he comprises under the category of instinctive behaviour “all complex acts which are performed previous to experience, and in a similar manner by all members of the same sex and race.” This places congenital behaviour in line with morphological structure as a subject for comparative treatment.

One more question remains. What shall we understand by “complex acts”? In the first place, it is well to restrict the term instinctive to co-ordinated actions; and this implies the presence of nerve-centres by which the co-ordination is effected. We thus exclude the organic behaviour of plants, since there is no evidence in the vegetable kingdom of co-ordinating centres. In the second place, the co-ordination is, as we have seen, congenital, and not acquired in the course of individual experience. Young water-birds, and indeed young chicks, as soon as they are born, and have recovered from the shock of birth, can swim with definite co-ordination of leg movements. Here the definiteness is not only congenital, but connate, if we use the latter term for an instinctive activity which is performed at or very shortly after birth. On the other hand, young swallows cannot fly at birth; they are then too immature, and their wings are not sufficiently developed. But when they are some three weeks old, and the wings have attained functional size and power, little swallows can fly with considerable if not perfect skill. The co-ordination is congenital, for it is not acquired in the course of individual experience; but it is not connate, since it is not exhibited at or shortly after birth. The term deferred may be applied to such congenital activities as are thus carried out when the animal has undergone a certain amount of further development after birth.

In the third place, it is customary to distinguish between such reflex actions as have already been briefly exemplified,[29] and instinctive behaviour. It is, however, by no means easy, if indeed it be possible, to draw any sharp and decisive line of demarcation. Instinct has indeed been well described by Mr. Herbert Spencer as compound reflex action; hence the distinction between instinctive and reflex behaviour turns in large degree on their relative complexity. It would seem, however, that whereas a reflex act—such as the withdrawal of the foot of a sleeping child when the sole is tickled—is a restricted and localized response, involving a particular organ or a definite group of muscles, and is initiated by a more or less specialized external stimulus; instinctive behaviour is a response of the animal as a whole, and involves the co-operation of several organs and of many groups of muscles. Partly initiated by an external stimulus or group of stimuli, it is also, seemingly, determined in part, in a greater degree than reflex action, by internal factors which cause uneasiness or distress, more or less marked, if they do not find their normal instinctive satisfaction. This point, however, may be more profitably discussed in connection with the conscious aspect of instinct. If, then, we say that reflex acts are local responses of the congenital type due to specialized stimuli, while instinctive activities are matters of more general behaviour, usually involving a larger measure of central (as opposed to local or ganglionic) co-ordination, and due to the more widely-spread effects of stimuli in which both external and internal factors co-operate, we shall probably get as near as is possible to the distinction of which we are in search. But it must be remembered that there are cases in which the distinction can hardly be maintained.

We are now in a position to define instinctive behaviour as comprising those complex groups of co-ordinated acts which are, on their first occurrence, independent of experience; which tend to the well-being of the individual and the preservation of the race; which are due to the co-operation of external and internal stimuli; which are similarly performed by all the members of the same more or less restricted group of animals; but which are subject to variation, and to subsequent modification under the guidance of experience.

II.—Instinctive Behaviour in Insects

Since instinctive behaviour is, by definition, independent of experience, and since the animals which act instinctively are also, in many cases, able to act intelligently, it is clear that, apart from hereditary variations, we must expect to find acquired modifications of instinct. As Huber said of bees, their instinctive procedure often indicates “a little dose of judgment.” It is, indeed, exceedingly difficult, as a matter of observation, to distinguish between hereditary variation and acquired modification. For the rôle played by these two factors in any given behaviour can only be determined if the whole life-history of the individual be known, and if there be opportunities for comparing it with the complete life-histories of other members of its race. And this is seldom possible.

These considerations must be borne in mind as we proceed to a brief study of some of the instinctive modes of behaviour in insects.

Dr. and Mrs. Peckham’s investigations on the instincts and habits of the solitary wasps have been described in a volume[30] worthy to be placed by the side of Fabre’s “Souvenirs.” Their descriptions seem to glow with the warm sunshine, and are redolent of the fresh air which afforded the conditions under which the observations were conducted. We can but regret that, in extracting from their bright pages some of the salient facts, the natural delicacy and grace of their treatment must be lost. For we can only give the dry skeleton which they have clothed with the flesh of lively detail. They enumerate the following primary modes of instinctive behaviour:—

1. Stinging.

2. Taking a particular kind of food.

3. Method of attacking and capturing prey.

4. Method of carrying prey.

5. Preparing nest, and then capturing prey, or the reverse.

6. The mode of taking prey into the nest.

7. The general style and locality of the nest.

8. The spinning or not spinning of a cocoon, and its specific form when one is made.

When the young Pelopœus emerges from the pupa-case and gnaws its way out of the mud cell, with limp and flaccid wings, it responds to a touch by well-directed movements of the abdomen with thrusts of the sting, as perfect as those of the adult. There is clearly no opportunity here for either instruction or experience to afford any intelligent guidance. Stinging is an instinctive act. And it is an act of which great use is made in the capture of prey which shall serve for food to the young—it has a biological end. But the wasps of different species do not have to learn by experience what prey to attack. It is by instinct, too, that they take their proper food-supply, one caterpillars, another spiders, a third flies or beetles. So deeply seated, indeed, is the hereditary preference, that no fly-robber ever takes spiders, nor will the capturer of spiders change to caterpillars or beetles. Some keep to a few species or genera, while Philanthus punctatus preys chiefly or entirely on bees of the genus Halictus.

Romanes[31] thought that the manner of stinging and paralyzing their prey might “be justly deemed the most remarkable instinct in the world.” Spiders, insects, and caterpillars are stung, he says, “in their chief nerve-centres, in consequence of which the victims are not killed outright, but rendered motionless; they are then conveyed to a burrow and, continuing to live in their paralyzed condition for several weeks, are then available as food for the larvæ when these are hatched. Of course the extraordinary fact which stands to be explained is that of the precise anatomical, not to say also physiological knowledge which appears to be displayed by the insect in stinging only the nerve-centres of its prey.” Eimer[32] thought that it “is absolutely impossible that the animal has arrived at its habit otherwise than by reflection upon the facts of experience.” “At the beginning,” he says, “she probably killed larvæ by stinging them anywhere, and then placed them in the cell. The bad results of this showed themselves; the larvæ putrified before they could serve as food for the larval wasps. In the mean time the mother wasp discovered that those larvæ which she had stung in particular parts of the body were motionless but still alive, and then she concluded that larvæ stung in this particular way could be kept for a longer time unchanged as living motionless food.”

Now, since these wasps, when they have stored their nests and laid an egg on one of the victims, close it up once and for all, and take no further interest in it or its contents, there seems no opportunity, at any rate in the existing state of matters, for the acquisition of that experience on which Eimer relied. But both his explanation and Romanes’s difficulty are based on the following assumptions: first, that the victims are instinctively or habitually stung in the chief nerve-centres; secondly, that when thus stung they are not killed but remain paralyzed for weeks; and thirdly, that the marvellously definite and delicate instinctive behaviour is in direct relation to the uniform result of prolonged paralysis and consequent preservation of the food in the fresh state. But Dr. Peckham’s careful observations and experiments show that, with the American wasps, the victims stored in the nests are quite as often dead as alive; that those which are only paralyzed live for a varying number of days, some more, some less; that wasp larvæ thrive just as well on dead victims, sometimes dried-up, sometimes undergoing decomposition, as on living and paralyzed prey; that the nerve-centres are not stung with the supposed uniformity; and that in some cases paralysis, in others death, follows when the victims are stung in parts far removed from any nerve-centre. “We believe,” he says, “that the primary purpose of the stinging is to overcome resistance, and to prevent the escape of the victims, and that incidentally some of them are killed and others are paralyzed.”

If, therefore, as will probably be shown to be the case, these conclusions are found to be generally true for this interesting group of insects, the mystery of “the precise anatomical, not to say also physiological knowledge which appears to be displayed” by these wasps turns out to be one of our own fabrication. It melts away in the light of fuller and more searching investigation.

It must not be supposed, however, from what has been said, that the behaviour in the act of stinging is altogether indefinite. On the contrary, each species proceeds in a relatively definite manner with some variation or modification of method. Philanthus punctatus, for example, stings the bees, on which she preys, under the neck, and the thrust is at once fatal. Dr. Peckham further notes that he was only successful in getting the wasps to sting when they were hunting; those that had not yet begun to store the nests paid no attention to the bees. This is an example of that internal factor to which reference was made in the last section. Marchal observed that Cerceris ornata runs the end of her abdomen along the under surface of the thorax of the bee, and delivers her thrust at the division of the segments—that is, where the sting can enter. The action does not imply any physiological knowledge. In general she begins at the neck. Spiders are usually, but not always, stung on the ventral surface. To give but one more example, Dr. Peckham observed in three cases the procedure of Ammophila urnaria which preys on caterpillars, and often, after stinging, bites the neck in several places, this process being termed malaxation. In three observed captures, all the caterpillars being of the same species and alike in size, the thrusts were given on the ventral surface near the middle line, between the segments. In the first, seven stings were given at the extremities (there being thirteen segments), the middle segments being left untouched, and no malaxation was practised. In the second, seven stings were again given, but in the anterior and middle segments, followed by slight malaxation. In both these cases the first three thrusts were in definite order, behind the third, the second, and the first segments successively. In the case of the third caterpillar, only one thrust was given, between the third and fourth segments—that is to say, in the position of the first stab in the other cases,—and after this one thrust there was prolonged malaxation. Of fifteen stored caterpillars examined, some lived only three days, others a little longer, while a few showed signs of life at the end of a fortnight. In more than one instance the second of the two caterpillars stored in each nest died and became discoloured before the first one was entirely eaten. The larva under such circumstances ate it with good appetite, and then spun its cocoon as if nothing unpleasant had occurred.

The mode of carrying their booty is in these wasps instinctive, and relatively uniform. Ammophila urnaria grasps the caterpillar, near the anterior end, in her mandibles, and carries or drags it beneath her legs, walking forwards. It is generally but not always with the ventral surface uppermost. Pompilus takes hold of her spider anywhere, but always drags it over the ground, walking backwards. Oxybelus clasps her fly with her hind legs; Bembex with the second pair. Each works after her own fashion in a way that is relatively uniform for each species.

The general style of the nest, its mode of construction, and its method of closure, are always performed, says Dr. Peckham, by each species in a similar manner, not indeed in circumstantial detail, but quite in the same way in a broad sense. Variation or modification is always present, but the tendency to depart from a nest of a given type is not excessive. Some dig in the ground curved tunnels, with or without one or more chambers. Others bore into decaying wood; others use straws, or make tunnels in bramble stems; while the mud-daubers build cells in which to store the food and lay the egg. This is sometimes deposited on the first, sometimes on the last, sometimes on some intermediate victim, but generally in much the same place and position. Ammophila, for instance, lays it on the side of the sixth or seventh segment—that is to say, in about the mid position.

Some species first capture their prey, and then make the nest in which it is to be entombed. Others first prepare the nest, and then carry or drag their prey to it—often from considerable distances—quite irrespective of what seems to us the more appropriate method of the two under the particular circumstances of the case. And the way in which the victim is dragged into the nest is similarly a matter of inheritance. Each way is characteristic of the species concerned, and would be an important part of any definition of the animal based upon its modes of behaviour. For example, a Sphex places her grasshopper just at the entrance of the nest, which she then enters herself before dragging in her prey by the antennæ. When the wasp was in the hole, Fabre moved the victim a little way off; the wasp came out, brought the grasshopper to the entrance as before, and went in a second time. This was repeated about forty times, each time with the same result, until the patience of the naturalist was exhausted, and the persistent wasp took her booty in after her appropriate fashion. She must place the grasshopper close to the opening; she must then descend and examine the nest, and, after that, must drag it down. Nothing less than the performance of these acts in a certain order satisfies her instinctive impulse.

In a private letter, from which he kindly allows me to quote, Dr. Peckham says: “We have recently made some experiments on this wasp (Sphex ichneumonea). First we allow her to carry in her prey undisturbed, to see how far she was faithful to the traditions of her ancestors, and to observe her normal methods. On the next day, when she had placed her grasshopper just at the opening of the nest, and while she was below, we drew it back to a little distance. She came out, and we both repeated our operations four times—she running down into the nest, always after getting the grasshopper into position, and we as regularly drawing it away. The fifth time she changed her plan, seized it by the head and backed into the nest with it. The next day, at the fourth trial, she straddled it and walked head first into the nest with it; and on the fourth day, at the eighth trial, she backed in with it as on the second day.” These interesting observations show that the wasp has sufficient intelligence to modify her procedure in accordance with an unwonted situation. The “consecutive necessity,” as it has been termed, has a potent influence, but is not absolute.

Fabre notes a case of similar consecutive necessity in the case of the mason bee, Chalicodoma. If while a bee is provisioning its nest with honey and pollen the structure be destroyed, she sometimes breaks open a completed cell, and, having done so, goes on bringing more provision, though the cell already contains a sufficient store of food; and only when she has completed the superfluous storing does she deposit her egg and seal up the cell. So, too, when the cell is removed in an early stage of construction, and another completed cell already partially stored is substituted, the bee, instead of simply adopting the new cell, goes on building until the cell is as much as one-third beyond the usual height; then, and not till then, does she proceed in due course to the next stage of the instinctive procedure, the provisioning of the cell.

From our general knowledge of animal nature, we should expect to find parasitic forms ready to take advantage of the material stored by such insects as the solitary wasps and the mason bees. It is said that Chalicodoma provides nourishment to the larvæ of some sixteen unbidden guests. A parasitic bee (Stelis nasuta) breaks open a closed cell, and, after depositing its eggs, seals it up again with mortar. Since her eggs and larvæ develop more rapidly than those of the mason bee, they are first served with the store of provision, while the rightful owner is done out of its inheritance. By a curious act, of what appears to us like retributive justice, these parasitic larvæ sometimes fall a prey to another parasite, also a hymenopterous insect named Monodontomerus, the larvæ of which prey on the young of both bees. Another genus of the same family, Leucopsis (Fig. 13, F), also succeeds in piercing with its ovipositor, at a suitable spot, the walls of the Chalicodoma cell, and suspends its curious hooked egg (Fig. 13, G) on the delicate cocoon within which the chrysalis lies. Fabre found in some cases as many as five of these parasitic eggs on a single cocoon. But he never found more than one larva in any cell that he examined. The following is an epitome of his conclusions and inferences. From the parasitic egg is hatched a minute arched grub, with relatively large head and mandibles, and provided with a number of bristles, which aid it in progression (Fig. 13, H). It does not, however, at once attack the bee larva, but makes a series of excursions, the object of which is to reach and destroy any other parasitic eggs. This was not actually observed, but the eggs were found to have been destroyed, and there was seemingly no other means of destruction under the conditions maintained. The larva, this done, changes its skin and takes on a new form, destitute of bristles, with a very small head and minute mandibles (Fig. 13, I). In this new form it attacks the Chalicodoma larva, making a very small incision, through which the juices of the host are transferred to the guest without further injury to the grub. It is interesting to note that, if the facts are accurately described and the inferences are correct, there are associated with two types of instinctive behaviour two distinct types of structure. The creature can have no conscious control over its structural development, and there is no ground for assuming that it has any control over its instinctive behaviour.

The specialization of structure and of instinctive behaviour, in accordance with a definite sequence of life-conditions, is even more remarkable in another of the many parasites which Chalicodoma unwittingly labours to nourish. This time it is a fly (Argyromœba), which lays a minute egg on the outside of the cell. From this egg is hatched a slender threadlike worm, barely one-twentieth of an inch in length (Fig. 13, C). It has three pairs of longish bristles near the anterior end, and a single yet longer pair at the hinder extremity. These aid it in creeping over the wall of the cell. Its small head is armed with short, stiff bristles. For many days it wanders over the surface of the cell, inserting its bristly head into each minute cranny and crack. Throughout this long period it has never a bite nor sup. Probably many of them never succeed in finding a crevice by which they can effect an entrance, but those that do manage to wriggle in undergo a change, lose their bristles, and develop a minute suctorial mouth, through which the contents of the larva are absorbed into their swelling bodies (Fig. 13, D). When fully grown they are quite helpless, and unable to get out from the cell in which they are now imprisoned. For months they lie quiescent, but in the succeeding spring they pass into a pupal condition very different from that of most flies. The relatively large head is armed with strong spines; the middle region bears bristles directed backwards; the posterior end has short spines (Fig. 13, E). Fixing itself to the interior of the cell by the latter, it strikes with its armoured head repeated blows on the walls of its prison until a breach is at last made, and sufficiently enlarged to form a suitable exit. Then the pupa-skin bursts, and the imago insect emerges and flies off. At each stage of life there is the closest relation between structure and behaviour, and each is equally adapted to a biological end of which the creature has never had an opportunity of gaining any experience.

Exceedingly multifarious are the ways in which insects thus provide for the future of young they will never see. Antherophagus lives in flowers, and is believed to seize with its mandibles humble bees, which then unwittingly bear the parasitic beetle to the nests in which alone the larvæ have been found. The larvæ of our common oil-beetle (Meloë) are parasitic on the bee, Anthophora. It deposits its ten thousand eggs without observable discrimination; but the active young larva instinctively seizes and attaches itself to any hairy object. Thousands must go astray. They have been found on hairy beetles, flies, and bees of the wrong genus. Some, however, become thus attached to the one suitable species, and are conveyed by the Anthophora to her nest, where they promptly eat the egg she lays. It is not difficult to picture to one’s self how this incompletely evolved instinct might be further perfected by natural selection, through the survival of those females which laid their eggs in the haunts of the bee-host. And such an advance in instinctive behaviour is seen in another and rarer beetle—Sitaris. Her eggs are laid in August near the entrance to a nest of the Anthophora. In September they hatch to form larvæ, which hibernate in groups till the following spring. Then they become active (Fig. 13, A), and attach themselves to hairy objects. Being near the Anthophora nest, there is an increased chance of their fastening upon this bee. The chance is still far from good, for if this were so, we should not find that the Sitaris laid as many as two thousand eggs. Still, on these grounds, we may presume that its chance of survival is about five times as good as that of Meloë, which lays ten thousand eggs. The larva is said generally to attach itself to a male bee, which is hatched earlier than his mate, and to pass on to the female at the nuptial period; but in any case it eventually slips on to the egg that she lays. This forms the food of the larva during the remainder of this stage of its existence. It then moults and assumes a new form, capable of feeding on the honey (Fig. 13, B); and, after further changes, becomes a pupa, and then assumes the imago condition.

In these cases the advantage is wholly on the side of the parasite. But there are cases of close relationship between insects and flowering plants where the instinctive behaviour gives rise to reciprocal benefit. The Yucca is a genus of American Liliaceous plants, with large pale sweet-smelling flowers; and these are dependent for fertilization on the instinctive behaviour of a small straw-coloured moth of the genus Pronuba. Just when the Yucca plant blossoms in the summer, the moths emerge from their chrysalis cases. They mate; and the female then flies to a flower, collects a pellet of pollen from the anthers, proceeds to another flower, pierces the pistil with her sharp ovipositor, lays her eggs among the ovules, and finally darting to the stigma stuffs the pollen pellet into its funnel-shaped extremity (Fig. 14). If the flower be not thus fertilized the ovules do not develop; and if the ovules do not develop the grubs which are hatched from the moth’s eggs die of starvation. There are enough ovules to supply food to the grubs, and leave a balance to continue the race of Yuccas.

Whether the female moth is attracted to the flower by sight or smell, we do not know. And whether the male finds the female, in the case of the Yucca moth, through scent, we are not in a position to state with certainty. It has, however, been shown that in certain moths[33] some odour emitted by the female is the attractive stimulus, affecting sense-organs situated on the antennæ of the male. To females confined in an opaque vessel over the mouth of which gauze was tied, the males came in numbers; but when a clear glass vessel was inverted, and sand was packed round the mouth, so as to prevent the escape of air from the interior, no males came, though the imprisoned females were clearly visible. If the antennæ of the males were either removed or coated with shellac the moths failed to notice the females even when close to them. In what way the intact male is made aware of the direction from which the scent comes, we do not know—possibly by differential stimulation in the antennæ, the moth instinctively turning in the direction of greater stimulation. It will be seen, therefore, that in the case of the behaviour of the Yucca moth—behaviour which is essential to the biological end of reproduction—there is much detail concerning which we are ignorant. But for our present purpose the important point to notice is that the procedure of the female cannot be due to imitation; nor can it be the outcome of individually acquired experience; for the method of procedure is not gradually learnt, but is carried out without apparent hesitation the first and only time the appropriate occasion presents itself. Not only does the moth take no heed of her grubs, but they are so placed that she could not in any case ascertain by observation that only if the ovules are fertilized do her offspring thrive. She cannot possibly know what effect the stuffing of the pollen on to the stigma exercises, or indeed whether it have any effect at all. And yet generation after generation these moths collect the pollen from the anthers and bear it to the stigma. Spence’s words “without knowledge of the end in view” are amply justified in this case, as in other cases of typically instinctive behaviour.

III.—The Instinctive Behaviour of Young Birds

Since it is easy to hatch birds of many species in an incubator, and to rear them under conditions which not only afford facilities for observation but exclude parental influence, their study has special advantages. One can with some approach to accuracy distinguish the instinctive from the acquired factors in their behaviour.[34]

The callow young of such birds as pigeons, jays, and thrushes are hatched in a helpless condition, and require constant and assiduous ministration to their elementary organic needs. Most of their instincts are of the deferred type. But pheasants, plovers, moor-hens, domestic chicks, and ducklings, with many others, are active soon after birth, and exhibit powers of complex co-ordination, with little or no practice of the necessary limb-movements. They walk and balance the body so soon and so well as to show us that this mode of procedure is congenital, and has not to be gradually acquired through the guidance of experience. Young water-birds swim with neat and orderly strokes the very first time they are gently placed in water. Even little chicks a day or two old can swim well. Dr. Thorndike, who draws attention to this fact,[35] appears to accept the view, suggested by Dr. Bashford Dean, that the movements are not those of swimming but only of running. I have carefully watched the action through the glass walls of a tank and compared it with that of a young moor-hen. In the two cases it is quite similar in type, and the type appears to be different from that of running, though it is perhaps hard to distinguish the two. In any case, the hand over hand action is well co-ordinated, and is very different from a mere excited struggle. Chicks twenty-six hours old taken straight from the incubator drawer, before they had taken food, made directly for the side of the tank and tried to scramble out. They gradually sank deeper through the wetting of the down, but could keep afloat for from two to three minutes. I have made observations on chicks of various ages from twenty-four hours to a month, and find in all cases similar results; but with the older birds the flapping of the wings and more vigorous action cause them to get water-logged more rapidly. There is some apparent distress with cries; but less than one might expect under the circumstances. For the purposes of the above illustration Mr. Charles Whymper had before him a sketch I made of the leg-action, and instantaneous photographs of the chicks swimming for which I am indebted to my colleague Mr. George Brebner. I have not observed the behaviour of an adult hen when placed in the water. Dr. Thorndike says, “there is no vigorous instinct to strike out toward the shore,” she “will float about aimlessly for awhile and only very slowly reach the shore.” But Mrs. Foster Wood informs me that she has seen a hen leap into a pond after her brood of ducklings and swim to the other side, a distance of twenty feet.

Diving, in water-birds, is also an instinctive mode of behaviour; and this is obviously a more difficult procedure than swimming, one further removed from reflex action. And careful observations have placed beyond question the fact that flight is also instinctive. A swallow, for example, taken from the nest under conditions which made it practically certain that it had never yet taken wing, exhibited guided flight, and attempted to alight on a suitable ledge. Of course flight is generally a deferred instinct, and is not performed until the wings have reached a suitable state of development. An instinctive response, which may perhaps be regarded as one of its initial stages, is seen in quite young chicks. If placed in a basket, and rapidly lowered therein through a foot or two, the chick will extend its skinny and scarcely feathered wings. But though, from the usual conditions of development, flight in birds is a deferred instinct, yet in exceptional cases it may be connate. The mound-builders (Megapodes) of the Australian region are hatched from large eggs in warm earth or sand, and are not tended by the parents. So well fledged are these birds that they can fly the day they emerge from the egg. Dr. Worcester, while digging in one of their mounds, made an unsuccessful attempt to seize one which was newly hatched; but it flew several rods into thick brush, and this notwithstanding the fact that it had probably never before seen the light of day.