Insects, Their Ways and Means of Living/Chapter IV
CHAPTER IV
WAYS AND MEANS OF LIVING
In our human society each individual must obtain the things necessary for existence; the manner by which he acquires them, whether by one trade or another, by this means or by that, does not physically matter so long as he provides himself and his family with food, clothing, and shelter. Exactly so it is with all forms of life. The physical demands of living matter make certain things necessary for the maintenance of life in that matter, but nature has no law specifying that any necessity shall be acquired in a certain manner. Life itself is a circumscribed thing, but it has complete freedom of choice in the ways and means of living.
It is useless to attempt to make a definition of what living matter is, or of how it differs from non-living matter, for all definitions have failed to distinguish animate from non-animate substance. But we all know that living things are distinguishable from ordinary non-living things by the fact that they make some kind of response to changes in the contact between themselves and their environment. The "environment," of course, must be broadly interpreted. Biologically, it includes all things and forces that in any way touch upon living matter. Not only has every plant and animal as a whole its environment, but every part of it has an environment. The cells of an animal's stomach, for example, have their environment in the blood and lymph on one side, the contents of the stomach on the other; in the energy of the nerves distributed to them; and in the effects of heat and cold that penetrate them.
The environmental conditions of the life of cells in a complex animal are too complicated for an elemental study; the elements of life and its basic necessities are better understood in a simple organism, or in a one-celled animal; but for purposes of description, it is most convenient to speak of the properties of mere protoplasm. All the vital needs of the most highly organized animal are present in any part of the protoplasmic substance of which it is composed.
Fig. 62. Diagram showing the relation of the germ cells (GCls) and the body cells (BCls) in successive generations
A fertilized germ cell of generation A forms the germ cells and body cells of B, a fertilized germ cell of B forms the germ cells and body cells of C, and so on. The offspring C of B derives nothing from the body cells of the parent B, but both offspring C and parent B have a common origin in a germ cell of A
Protoplasm is a chemical substance, or group of substances, the structure of which is very comp]ex but is maintained so long as there is no disturbance in the environment. Let some least thing happen, however, such as a change in the temperature, in the strength of the light, in the weight of pressure, or in the chemical composition of the surrounding medium, and the protoplasmic molecules, in the presence of oxygen, are likely to have the balance of their constituent particles upset, whereupon they partly decompose by the union of their less stable elements with oxygen to form simpler and more permanent compounds. The decomposition of the protoplasmic substances, like all processes of decomposition, liberates a certain amount of energy that had been stored in the making of the molecule, and this energy may manifest itself in various ways. If it takes the form of a change of shape in the protoplasmic mass, or movement, we say the mass exhibits signs of life. The state of being alive, however, is more truly shown if the act can be repeated, for the essential property of living matter is its power of reverting to its former chemical composition, and its ability thus gained of again reacting to another change in the environment. In restoring its lost elements, it must get these elements anew from the environment, for it can not take them back from the substances that have been lost.
Here, expressed in its lowest terms, is the riddle of the physical basis of life and of the incentive to evolution in the forms of life. Not that these mysteries are any more easily understood for being thus analyzed, but they are more nearly comprehended. Being alive is maintaining the power of repeating an action; it involves sensitivity to stimuli, the constant presence of free oxygen, elimination of waste, and a supply of substances from which carbon, hydrogen, nitrogen, and oxygen, or other necessary elements, are readily available for replacement purposes. Evolution results from the continual effort of living matter to perform its life processes in a more efficient manner, and the different groups of living things are the result of the different methods that life has tried and found advantageous for accomplishing its ends. Living organisms are machines that have become more and more complex in structure, but always for doing the same things.
If animals may be compared with machines in their physical mechanism, they are like them, too, in the fact that they wear out and are at last beyond repair. But here the simile ends, for when your car will no longer run, you must go to the dealer and order a new one. Nature provides continuous service by a much better scheme, for each organism is responsible for its own successor. This phase of life, the replacement of individuals, opens another subject involving ways and means, and it, likewise, can be understood best in its simpler manifestations.
The facts of reproduction in animals are not well expressed by our name for them. Instead of "reproduction," it would be truer to say "repeated production," for individuals do not literally reproduce themselves. Generations are serially related, not each to the preceding; they follow one another as do the buds along the twig of a tree,
Fig. 63. The external structure of an insect
The body of a grasshopper dissected showing the head (H), the thorax (Th), and the abdomen (Ab). The head carries the eyes (E), the antennae (Ant), and the mouth parts, which include the labrum (Lm), the mandibles (Md), the maxillae (Mx), and the labium (Lb). The thorax consists of three segments (1, 2, 3), the first separate and carrying the first legs (L1), the other two combined and carrying the wings (W2, W3), and the second and third legs (L2, L3). The abdomen consists of a series of segments; that of the grasshopper has a large tympanal organ (Tm), probably an ear, on each side of its base. The end of the abdomen carries the external organs of reproduction and egg-laying
and buds on the same twig are identical or nearly so, not because one produces the next, but because all are the result of the same generative forces in the twig. If the spaces of the twig between the buds were shortened until one bud became contiguous with the one before, or became enveloped by it, a relation would be established between the two buds similar to that which exists between successive generations of life forms. The so-called parent generation, in other words, contains the germs of the succeeding generation, but it does not produce them. Each generation is simply the custodian of the germ cells entrusted to it, and the "offspring" resembles the parent, not because it is a chip off the parental block, but because both parent and offspring are developed from the same line of germ cells.
Fig. 64. The leg of a young grasshopper, showing the typical segmentation of an insect's leg
The leg is supported on a pleural plate (Pl) in the lateral wall of its segment. The basal segment of the free part of the leg is the coxa (Cx), then comes a small trochanter (Tr), next a long femur (F) separated by the knee bend from the tibia (Tb), and lastly the foot, consisting of a sub-segmented tarsus (Tar), and a pair of terminal claws (Cl) with an adhesive lobe between them
Parents create the conditions under which the germ cells will develop; they nourish and protect them during the period of their development; and, when each generation has served the purpose of its existence, it sooner or later dies. But the individuals produced from its germ cells do the same for another set of germ cells produced simultaneously with themselves, and so on as long as the species persists.
To express the facts of succession in each specific form of animal, then, we should analyze each generation into germ cells and an accompanying mass of protective cells which forms a body, or soma, the so-called parent. Both the body, or somatic, cells and the germ cells are formed from a single primary cell, which, of course, is usually produced by the union of two incomplete germ cells, a spermatozoon and an egg. The primary germ cell divides, the daughter cells divide, the cells of this division again divide, and the division continues indefinitely until a mass of cells is produced. At a very early stage of division, however, two groups of cells are set apart, one representing the germ cells, the other the somatic cells. The former refrain from further development at this time; the latter proceed to build up the body of the parent. The relation of the somatic cells to the germ cells may be represented diagrammatically as in Figure 62, except that the usual dual parentage and the union of germ cells is not expressed. The sexual form of reproduction is not necessary with all lower animals, nor with all generations of plants; in some insects the eggs can develop without fertilization.
The fully-developed mass of somatic cells, whose real function is that of a servant to the germ cells, has assumed such an importance, as public servants are prone to do, that we ordinarily think of it, the body, the active sentient animal, as the essential thing. This attitude on our part is natural, for we, ourselves, are highly organized masses of somatic cells. From a cosmic standpoint, however, no creature is important. Species of animals and plants exist because they have found ways and means of living that have allowed them to survive, but the physical universe cares nothing about them—the sunshine is not made for them, the winds are not tempered to suit their convenience. Life must accept what it finds and make the best of it, and the question of how best to further its own welfare is the problem that confronts every species.
The sciences of anatomy and physiology are a study of the methods by which the soma, or body, has contrived to meet the requirements imposed upon it by the unchanging laws of the physical universe. The methods adopted are as numerous as the species of plants and animals that have existed since life began. A treatise on entomology, therefore, is an account of the ways and means of living that insects have adopted and perfected in their somatic organization. Before discussing insects in particular, however, we must understand a little more fully the principal conditions of living that nature places on all forms of life.
Fig. 65. Legs of a honeybee, showing special modifications
A, outer surface of a hind leg, with a pollen basket on the tibia (Tb) loaded with pollen. B, a fore leg, showing the antenna cleaner (a) between the tibia and the tarsus, and the long, hairy basal segment of the tarsus (1 Tar), which is used as a brush for cleaning the body
As we have seen, life is a series of chemical reactions in a particular kind of matter that can carry on these reactions. A "reaction" is an action; and every act of living matter involves a breaking down of some of the substances in the protoplasm, the discharging of the waste materials, and the acquisition of new materials to replace those lost. The reaction is inherent in the physical or chemical properties of protoplasmic compounds and depends upon the substances with which the protoplasm is surrounded. It is the function of the creature's mechanism to see that the conditions surrounding its living cells are right for the continuance of the cell reactions. Each cell must be provided with the means of eliminating waste material and of restoring its lost material, since it can not utilize that which it has discarded.
With the conditions of living granted, however, protoplasm is still only potentially alive, for there is yet required a stimulus to set it into activity. The stimulus for life activities comes from changes in the physical forms of energy that surround or infringe upon the potentially living substance; for, "live" matter, like all other matter, is subject to the law of inertia, which decrees that it must remain at rest until motion is imparted to it by other
Fig. 66. The head and mouth parts of a grasshopper
A, facial view of the head, showing the positions of the antennae (Ant), the large compound eyes (E), the simple eyes, or ocelli (O), the broad front lip, or labrum (Lm) suspended from the cranium by the clypeus (Clp), and the bases of the mandibles (Md, Md) closed behind the labrum
B, the mouth parts separated from the head in relative positions, seen from in front: Hphy, hypopharynx, or tongue, attached to base of labium; Lb, labium; Lm, labrum; Md, mandibles; Mx, maxillae
motion. A very small degree of stimulating energy, however, may result in the release of a great quantity of stored energy.
The food of all living matter must contain carbon, hydrogen, nitrogen, and oxygen. The mechanism of plants enables them to take these elements from compounds dissolved in the water of the soil. Animals must get them from other living things, or from the products of living things. Therefore, animals principally have developed the power of movement; they have acquired grasping organs of some sort, a mouth, and an alimentary canal for holding the food when once obtained.
In the insects, the locomotory function is subserved by the legs and by the wings. Since all these organs, the three pairs of legs and the two pairs of wings, are carried by the thorax (Fig. 63, Th), this region of the body is distinctly the locomotor center of the insect. The legs (Fig. 64) are adapted, by modifications of structure in different species, for walking, running, leaping, digging, climbing, swimming, and for many varieties of each of these ways of progression, fitting each species for its particular mode of living and of obtaining its food. The wings of insects are important accessions to their locomotory equipment, since they greatly increase their means of getting about, and thereby extend their range of feeding. The legs, furthermore, are often modified in special ways to perform some function accessory to feeding. The honeybee, as is well known, has pollen-collecting brushes on its front legs (Fig. 65 B), and pollen-carrying baskets on its hind legs (A). The mantis, which captures other insects and eats them alive, has its front legs made over into those efficient organs for grasping its prey and for holding the struggling victim which have already been described (Fig. 46).
The principal organs by which insects obtain and manipulate their food consist of a set of appendages situated on the head in the neighborhood of the mouth, which, in their essential structure, are of the nature of the legs, for insects have no jaws comparable with those of vertebrate animals. The mouth appendages, or mouth parts as they are called, are very different in form in the various groups of insects that have different feeding habits, but in all cases they consist of the same fundamental pieces. Most important is a pair of jawlike appendages, known as the mandibles (Fig. 66 B, Md), placed at the sides of the mouth (A, Md), where they swing sidewise and close upon each other below the mouth. Behind the mandibles is a pair of maxillae (B, Mx) of more complicated form, fitted rather for holding the food than for crushing it. Following the maxillae is a large under lip, or labium (Lb), having the
Fig. 67. Lengthwise section of a grasshopper, showing the general location of the principal internal organs, except the respiratory tracheal system and the organs of reproduction
An, anus; Ant, antenna; Br, brain; Cr, crop; Ht, heart; Int, intestine; Mal, Malpighian tubules; Mth, mouth; Oe, oesophagus; SoeGng, subnesophageal ganglion; Vent, stomach (ventriculus); VNC, ventral nerve cord; W, wings
structure of two maxillae united by their inner margins. A broad flap hangs downward before the mouth to form an upper lip, or labrum (Lm). Between the mouth appendages and attached to the front of the labium there is a large median lobe of the lower head wall behind the mouth, known as the hypopharynx (Hphy).
Insects feed, some on solid foods, others on liquids, and their mouth parts are modified accordingly. So it comes about that, according to their feeding habits, insects may be separated into two groups, which, like the fox and the stork, could not feed either at the table of the other. Those insects, such as the grasshoppers, the crickets, the beetles, and the caterpillars, that bite off pieces of food tissue and chew them, have the mandibles and the other mouth parts of the type described above. Insects that partake only of liquids, as do the plant lice, the cicadas, the moths, the butterflies, the mosquitoes and other flies, have the mouth parts fitted for sucking, or for piercing and sucking. Some of the sucking types of mouth parts will be described in other chapters (Figs. 121, 163, 183), but it will be seen that all are merely adaptations of form based on the ordinary biting type of mouth appendages. The fossil records of the history of insects show that the sucking insects are the more recent products of evolution, since all the earlier kinds of insects, the cockroaches and their kin, have typical biting mouth parts.
The principal thing to observe concerning the organs of feeding, in a study of the physiological aspect of anatomy, is that they serve in all cases to pass the natural food materials from the outside of the animal into the alimentary canal, and to give them whatever crushing or mastication is necessary. It is within the alimentary canal, therefore, that the next steps toward the final nutrition of the animal take place.
The alimentary canal of most insects is a simple tube (Fig. 68), extending either straight through the body, or
Fig. 68. The alimentary canal of a grasshopper
AInt, anterior intestine; An, anus; Cr, crop; GC, gastric caeca, pouches of the stomach; Hphy, hypopharynx (tongue); Lb, base of labium; Mal, Malpighian tubules; MInt, mid-intestine; Mth, mouth; Oe, oesophagus; Rect, hind intestine (rectum); SlGl, salivary glands opening by their united ducts at base of hypopharynx; Vent, ventriculus (stomach)
making only a few turns or loops in its course. It consists of three principal parts, of which the middle part is the true stomach, or ventriculus (Vent) as it is called by insect anatomists. The first part of the tube includes a pharynx immediately behind the mouth, followed by a narrower, tubular oesophagus (Oe), after which comes a sac-like enlargement, or crop (Cr), in which the food is temporarily stored, and finally an antecharnber to the stomach, named the proventriculus. The third part of the alimentary canal, connecting the stomach with the anal opening, is the intestine, usually composed of a narrow anterior part, and a wide posterior part, or rectum (Rect). Muscle layers surrounding the entire alimentary tube cause the food to be swallowed and to be passed along from one section to the next toward the rear exit.
With the taking of the food into the alimentary canal, the matter of nutrition is by no mans accomplished, for the animal is still confronted with the problem of getting the nutrient materials into the inside of its body, where alone they can be used. The alimentary tube has no openings anywhere along its course into the body cavity. Whatever food substances the tissues of the animal receive, therefore, must be taken through the walls of the tube in which they are inclosed, and this transposition is accomplished by dissolving them in a liquid. Most of the nutrient materials in the raw food matter, however, are not soluble in ordinary liquids; they must be changed chemically into a form that will dissolve. The process of getting the nutrient parts of the raw foodstuff into solution constitutes digestion.
The digestive liquids in insects are furnished mostly by the stomach walls or the walls of tubular glands that open into the stomach, but the secretion of a pair of large glands, called the salivary glands (Fig. 68, SlGl), which open between the mouth parts, perhaps has in some cases a digestive action on the food as it is taken into the mouth.
Digestion is a purely chemical process, but it must be a rapid one. Consequently the digestive juices contain not only substances that will transform the food materials into soluble compounds, but other substances that will speed up these reactions, for otherwise the animal would starve on a full stomach by reason of the slowness of its gastric service. The quickening substances of the digestive fluids are called enzymes, and each kind of enzyme acts on only one class of food material. An animal's practical digestive powers, therefore, depend entirely upon the specific enzymes its digestive liquids contain. Lacking this or that enzyme it can not digest the things that depend upon it, and usually its instincts are correlated with its enzymes so that it does not fill its stomach with food it can not digest. A few analyses of the digestive liquids of insects have been made, enough to show that their digestive processes depend upon the presence of the same enzymes as those of other animals, including man.
The grosser digestive substances, in cooperation with the enzymes, soon change all the parts of the food materials in the stomach that the animal needs for its sustenance into soluble compounds which are dissolved in the liquid part of the digestive secretions. Thus is produced a rich, nutrient juice within the alimentary canal which can be absorbed through the walls of the stomach and intestine and can so enter the closed cavity of the body. The next problem is that of distribution, for still the food materials must reach the individual cells of the tissues that compose the animal.
The insect's way of feeding, of digesting its food, and of absorbing it is not essentially different from that of the higher animals, including ourselves, for alimentation is a very old and fundamental function of all animals. Its means of distributing the digested food within its body, however, is quite different from that of vertebrates. The absorbed pabulum, instead of being received into a set of lymphatic vessels and from these sent into blood-filled tubes to be pumped to all parts of the organism, goes directly from the alimentary walls into the general body cavity, which is filled with a liquid that bathes the inner surfaces of all the body tissues. This body liquid is called the "blood" of the insect, but it is a colorless or slightly yellow-tinted lymph. It is kept in motion, however, by a pulsating vessel, or heart, lying in the dorsal part of the body; and by this means the food, now dissolved in the body liquid, is carried into the spaces between the various organs, where the cells of the latter can have access to it.
Fig. 69. Diagram of the typical structure of an insect's heart and supporting diaphragm, with the course of the circulating blood marked by arrows
Ao, aorta, or anterior tubular part of the heart without lateral openings; Dph, membranous diaphragm; Ht, anterior three chambers of the heart, which usually extends to the posterior end of the body; Mcl, muscles of diaphragm, the fibers spreading from the body wall to the heart; Ost, ostium, or one of the lateral openings into the heart chambers
The heart of the insect is a slender tube suspended along the midline of the back close to the dorsal wall of the body (Fig. 67, Ht). It has intake apertures along its sides (Fig. 69, Ost), and its anterior end opens into the body cavity. It pulsates forward, by means of muscle fibers in its walls, thereby sucking the blood in through the lateral openings and discharging it by way of the front exit. An imperfect circulation of the blood is thus established through the spaces between the organs of the body cavity, sufficient for the purposes of so small an animal as an insect.
The final act of nutrition comes now when the blood, charged with the nutrient materials absorbed from the digested food in the alimentary canal, brings these materials into contact with the inner tissues. The tissue cells, by the inherent power of all living matter (which depends on the laws of osmosis and on chemical affinity), take for themselves whatever they need from the menu offered by the blood, and with this matter they build up their own substance. It is evident, therefore, that the blood must contain a sufficient quantity and variety of dietary elements to satisfy all possible cell appetites; that the stomach's walls and their associated glands must furnish the enzymes appropriate for making the necessary elements available from the raw food matter in the stomach; and, finally, that it must be a part of the instincts of each animal species to consume such native foodstuffs of its environment as will supply every variety of nourishing elements that the cells demand.
As we have seen, the demand for food comes from the loss of materials that are decomposed in the tissues during cell activity. Better stated, perhaps, the chemical breakdown within the cell is the cause of the cell activity, or is the cell activity itself. The way in which the activity is expressed does not matter; whether by the contraction of a muscle cell, the secretion of a gland cell, the generation of nerve energy by a nerve cell, or just the minimum activity that maintains life, the result is the same always—the loss of certain substances. But, as with most chemical reduction processes, the protoplasmic activity depends upon the presence of available oxygen; for the decomposition of the unstable substances of the protoplasm is the result of the affinity of some of their elements for oxygen. Consequently, when the stimulus for action comes over a nerve from a nerve center, a sudden reorganization takes place between these protoplasmic elements and the oxygen atoms which results in the formation of water, carbon dioxide, and various stable nitrogenous compounds.
The substances discarded as a result of the cell activities are waste products, and must be eliminated from the organism for their presence would clog the further activity of the cells or would be poisonous to them. The animal, therefore, must have, in addition to its mechanisms for bringing food and oxygen to the cells, a means for the removal of wastes.
The supplying of oxygen and the removing of carbon dioxide and some of the excess water are accomplished by respiration. Respiration is primarily the exchange of gases between the cells of the body and the outside air. If an animal is sufficiently small and soft-skinned, the gas exchange can be made directly by diffusion through the skin. Larger animals, however, must have a device for conveying air into the body where the tissues will have closer access to it. It will be evident, then, that there is not necessarily only one way of accomplishing the purposes of respiration.
Vertebrate animals inhale air into a sac or pair of sacs, called the lungs, through the very thin walls of which the oxygen and carbon dioxide can go into and out of the blood respectively. The blood contains a special oxygen carrier in the red matter, hemoglobin, of its red corpuscles, by means of which the oxygen taken in from the air is transported to the tissues. The carbon dioxide is carried from the tissues partly by the hemoglobin, and partly dissolved in the blood liquid.
Insects have no lungs, nor have they hemoglobin in their blood, which, as we have seen, is merely the liquid that fills the spaces of the body cavity between the organs. Insects have adopted and perfected a method of getting air distributed through their bodies quite different from that of the vertebrates. They have a system of air tubes, called tracheae (Fig. 70), opening from the exterior by small breathing pores, or spiracles (Sp), along the sides of the body, and branching minutely within the body to all parts of the tissues. By this means the air is conveyed directly to the parts where respiration takes place. There are usually in insects ten pairs of spiracles, two on the sides of the thorax, and eight on the abdomen. The spiracles communicate with a pair of large tracheal trunks lying along the sides of the body (Fig. 70), and from these trunks are given off branches into each body segment and into the head, which go to the alimentary canal, the heart, the nervous system, the muscles, and to all the other organs, where they break up into finer branches that terminate in minute end tubes going practically to every cell of the body.
Many insects breathe by regular movements of expansion and contraction of the under surface of the abdomen, but experimenters have not yet agreed as to whether the air goes in and out of the same spiracles or whether it enters one set and is expelled through another. It is probable that the fresh air goes into the smaller tracheal branches principally by gas diffusion, for some insects make no perceptible respiratory movements.
Fig. 70. Respiratory system of a caterpillar.
The external breathing apertures, or spiracles (Sp, Sp), along the sides of the body open into lateral tracheal trunks (a, a), which are connected crosswise by transverse tubes (b, b) and give off minutely branching tracheae into all parts of the head (H) and body
The actual exchange of oxygen from the air and carbon dioxide from the tissues takes place through the thin walls of the minute end tubes of the tracheae. Since these tubes lie in immediate contact with the cell surfaces the gases do not have to go far in order to reach their destinations, and the insect has little need of an oxygen carrier in its blood—its whole body, practically, is a lung. And yet some investigations have made it appear likely that the insect blood does contain an oxygen carrier that functions in a manner similar to that of the hemoglobin of vertebrate blood, though the importance of oxygen transportation in insect physiology has not been determined. In any case, the tracheal method of respiration must be a very efficient one; for, considering the activity of insects, especially the rate at which the wing muscles act during flight, the consumption of oxygen must at times be pretty high.
The activity of insects depends very much, as every one knows, upon the temperature. We have all observed how the house flies disappear upon the first cold snap in the fall and then surprise us by showing up again when the weather turns warm, just after we have taken down the screens. All insects depend largely upon external warmth for the heat necessary to maintain cellular activity. While their movements produce heat, they have no means of conserving this heat in their bodies, as have "warm-blooded" animals. That insects radiate heat, however, is very evident from the high temperature that bees can maintain in their hives during winter by motion of the wings. All insects exhale much water vapor from their spiracles, another evidence of the production of heat in their bodies.
The solid matter thrown off from the cells in activity is discharged into the blood. These waste materials, which are mostly compounds of nitrogen in the form of salts, must then be removed from the blood, for their accumulation in the body would be injurious to the tissues. In vertebrate animals, the nitrogenous wastes are eliminated by the kidneys. Insects have a set of tubes, comparable with the kidneys in function, which open into the intestine at the junction of the latter with the stomach (Fig. 68, Mal), and which are named, after their discoverer, the Malpighian tubules. These tubes extend through the principal spaces of the body cavity, where they are looped and tangled like threads about the other organs and are continually bathed in the blood. The cells of the tube walls pick out the nitrogenous wastes from the blood and discharge them into the intestine, whence they are passed to the exterior with the undigested food refuse.
We thus see that the inside of an insect is not an unorganized mass of pulp, as believed by those people whose education in such matters comes principally from under-foot. The physical unity of all forms of life makes it necessary that every creature must perform the same vital functions. The insects have, in many respects, adopted their own ways of accomplishing these functions, but, as already pointed out, the means of doing a thing does not count with nature so long as the end results are attained. The essential conditions are the supply of necessities and the removal of wastes.
The body of a complex animal may be likened to a great factory, in which the individual workers are represented by the cells, and groups of workers by the organs. That the factory may accomplish its purpose, the activities of each worker must be coordinated with those of all the other workers by orders from a directing office. Just so, the activities of the cells and organs of the animal must be controlled and coordinated; and the directing office of the animal organization is the central nervous system. The work of almost every cell in the body is ordered and controlled by a "nerve impulse" sent to it over a nerve fiber from a nerve center.
The inner structure of the nervous tissues and the working mechanism of the nerve centers are essentially alike in all animals, but the form and arrangement of the nerve tissue masses and the distribution of the nerve fibers may differ much according to the plan of the general body organization. The insects, instead of following the vertebrate plan of having the central nerve cord along the back inclosed in a bony sheath, have found it just as well for their purposes to have the principal nerve cord lying free in the lower part of the body (Fig. 67, VNC). In the head there is a brain (Figs. 67, 72, Br) situated above the oesophagus (Fig. 67, Oe), but it is connected by a pair of cords with another nerve mass below the pharynx in the lower part of the head (SoeGug). From this nerve mass another pair of nerve cords goes to a third nerve mass 
Fig. 71. The nervous system of the head of a grasshopper, as seen by removal of the facial wall
AntNv, antennal nerve; 1Br, 2Br, 3Br, the three parts of the brain; CoeCon, circumoesophageal connectives; 3Com, suboesophageal commissure of the third lobes of the brain; FrGng, frontal ganglion; FrCon, frontal ganglion connective with the brain; LbNv, labial nerve; LmNv, labral nerve; MdNv mandibular nerve; MxNv, maxillary nerve; O, simple eye; OpL, optic lobe connected with the brain; RNv, recurrent nerve; SoeGng, suboesophageal ganglion
lying against the lower wall of the first body segment (Fig. 72, Gng 1), which is likewise connected with a fourth mass in the second segment, and so on. The central nervous system of the insect thus consists of a series of small nerve masses united by double nerve cords. The nerve masses are known as ganglia (Gng), and the uniting cords are called the connectives (Fig. 71, Con). Typically there is a ganglion for each of the first eleven body segments, besides the brain and the lower ganglion of the head.
The brain of an insect (Fig. 71) has a highly complex internal structure, but it is a less important controlling center than is the brain of a vertebrate animal. The other ganglia have much independence of function, each giving the stimuli for movements of its own segment. For this reason, the head of an insect may be cut off and the rest of the creature may still be able to walk and to do various other things until it dies of starvation. Similarly, with some species, the abdomen may be severed and the insect will still eat, though the food runs out of the cut end of the alimentary canal. The detached abdomen may lay eggs, if properly stimulated. Though the insect thus appears to be largely a creature of automatic regulations, acts are not initiated without the brain, and full coordination of the functions is possible only when the entire nervous system is intact.
The active elements of the nerve centers are nerve cells; the nerve fibers are merely conducting threads extended from the cells. If the nerve force that stimulates the other kinds of cells into activity comes from nerve cells, the question then arises as to whence comes the primary stimulus that activates the nerve cells. We must discard the old idea that nerve cells act automatically; being matter, they are subject to the laws of matter—they are inert until compelled to act. The stimulus of the nerve cells comes from something outside of them, either from the environmental forces of the external world or from substances formed by other cells within the body.
Nothing is known definitely of the internal stimuli of insects, but there can be no doubt that substances are formed by the physiological activities of the insect tissues, similar to the hormones, or secretions of the ductless glands of other animals, that control action in other organs either directly or through the nervous system. Thus, some internal condition must prompt the insect to feed when its stomach is empty, and the entrance of food into its pharynx must stimulate the alimentary glands to prepare the digestive juices. Probably a secretion from the reproductive organs of the female, when the eggs are ripe in the ovaries, gives the stimulus for mating, and later sets into motion the reflexes that govern the laying of the eggs. The caterpillar spins its cocoon at the proper time for doing so; the 
Fig. 72. The general nervous system of a grasshopper, as seen from above
Ant, antenna; Ao, aorta; Br, brain; Cer, cercus; E,compound eye; Gng1, ganglion of prothorax; Gng2, ganglion of mesothorax; Gng3+I+II+III, compound ganglion of metathorax, comprising the ganglia belonging to the metathorax and the first three abdominal segments; GngIV–GngVIII ganglia of the fourth to eighth abdominal segments; O, ocelli; Proc, proctodeum, or posterior part of alimentary canal; Sa, suranal plate; SegII–X, second to tenth segments of abdomen; SoeGng, suboesophageal ganglion; Stom, stomodeum, or anterior part of alimentary canal
stimulus, most likely, comes from the products of physiological changes beginning to take place in the body that will soon result in the transformation of the caterpillar into a chrysalis, a stage when the insect needs the protection of a cocoon. These activities of insects we call instincts, but the term is simply a cover for our ignorance of the processes that cause them.
External stimuli are things of the outer environment that affect the living organism. They include matter, electromagnetic energy, and gravity; but the known stimuli do not comprise all the activities of matter or of the "ether." The common stimuli are: pressure of solids, liquids, and gases; humidity; chemical qualities (odors and tastes); sound, heat, light, and gravity. Most of these things stimulate the nerve centers indirectly through nerves connected with the skin or with specialized parts of the skin called sense organs. An animal can respond, therefore, only to those stimuli, or to the degrees of a particular stimulus, to which it is sensitive. If, for example, an animal has no receptive apparatus for sound waves, it will not be affected by sound; if it is not sensitized to certain wave lengths of light, the corresponding colors will not stimulate it. There are few kinds of natural activities in the environment that animals do not perceive; but even our own perceptive powers fall far short of registering all the degrees of any activity that are known to exist and which the physicist can measure.
Insects respond to most of the kinds of stimuli that we perceive by out senses; but if we say that they see, hear, smell, taste, or touch we make the implication that insects have consciousness. It is most likely that their reactions to external stimuli are for the most part performed unconsciously, and that their behavior under the effect of a stimulus is an automatic action entirely comparable to out reflex actions. Behavioristic acts that result from reflexes the biologist calls tropisms. Coordinated groups of tropisms constitute an instinct, though, as we have seen, an instinct may depend also on internal stimuli. It can not be said that consciousness does not play a small part in determining the activities of some insects, especially of those species in which memory, i.e., stored impressions, appears to give a power of choice between different conditions presented. The subject of insect psychology, however, is too intricate to be discussed here.
The phases of life thus far described, the complexity of physical organization, the response to stimuli, the phenomena of consciousness from their lowest to their highest manifestations, all pertain to the soma. Yet, somehow, the plan of the edifice is carried along in the germ cells, and by them the whole somatic structure is rebuilt with but little change of detail from generation to generation. This phase of life activity is still a mystery to us, for no attempted explanation seems adequate to account for the organizing power resident in the germ cells that accomplishes the familiar facts of repeated
Fig. 73. Diagrams of the internal organs of reproduction in insects
A, the female organs, comprising a pair of ovaries (Ov), each composed of a group of egg tubules (ov), a pair of oviducts (DOv),and a median outlet tube, or vagina (Vg), with usually a pair of colleterial glands (ClGl) discharging into the vagina, and a sperm receptacle, or spermatheca (Spm), opening from the upper surface of the latter
B, the male organs, comprising a pair of testes (Tes) composed of spermatic tubules, a pair of sperm ducts, or vasa deferentia (VD), a pair of sperm vesicles (VS), and an outlet tube, or ductus ejaculatorius (DE), with usually a pair of mucous glands (MGl) discharging into the ducts of the sperm vesicles
development which we call reproduction. When we can explain the repetition of buds along the twig, we may have a key to the secret of the germ cells—and possibly to that of organic evolution.
The organs that house the germ cells in the mature insect consist of a pair of ovaries in the female (Fig. 73 A, Ov) in which the eggs mature, and of a pair of testes in the male (B, Tes) in which the spermatozoa reach their complete growth. Appropriate ducts connect the ovaries or the testes with the exterior near the rear end of the body. The female usually has a sac connected with the egg duct (A, Spm) in which the sperm, received at mating, are stored until the eggs are ready to be laid, when they are
Fig. 74. The ovipositor of a long-horned grasshopper, a member of the katydid family, showing the typical structure of the egg-laying organ of female insects A, the ovipositor (Ovp) in natural condition, projecting from near the posterior end of the body
B, the parts of the ovipositor separated, showing the six component pieces, two arising from the eighth abdominal segment (VIII), and four from the ninth (IX). An, anus; Cer, cerci; IX, ninth abdominal segment; Ovp, ovipositor; VgO, vaginal opening; VIII, eighth abdominal segment; X, tenth abdominal segment
extruded upon the latter and bring about fertilization. The egg cells ordinarily are all alike, but the spermatozoa are of two kinds; and according to the kind of sperm received by any particular egg, the future individual will be male or female.
The germ cells accompanying each new soma undergo a series of transformations within the parent body before they themselves are capable of accomplishing their purpose. They multiply enormously. With some animals, only a few of them ever produce new members of the race; but with insects, whose motto is "safety in numbers," each species produces every season a great abundance of new individuals, to the end that the many forces arrayed against them may not bring about their extermination.
The world seems full of forces opposed to organized life. But the truth is, all organization is an opposition to established forces. The reason that the forms of life now existing have held their places in nature is that they have found and perfected ways and means of opposing, for a time, the forces that tend to the dissipation of energy. Life is a revolt against inertia. Those species that have died out are extinct, either because they came to the end of their resources, or because they became so inflexibly adapted to a certain kind of life that they were unable to meet the emergency of a change in the conditions that made this life possible. Efficiency in the ordinary means of living, rather than specialization for a particular way of living, appears to be the best guarantee of continued existence.