About the odd relationship between the tarantula and the giant wasp Pepsis. Although the tarantula can easily kill Pepsis, one species permits the wasp to sting it and lay an egg in its body
SPIDER AND WASP are the tarantula Cyrtopholis portoricae (top) and the digger wasp Pepsis marginata (bottom). The tarantula is shown in an attitude of defense. The wasps of the genus Pepsis are either a deep blue or blue with rust-colored wings. The largest species of the genus have a wingspread of about four inches.
For existence, every species of ani mal must have a regular source of food, and if it happens to live on other animals, its survival may be very delicately balanced. The hunter cannot exist without the hunted; if the latter should perish from the earth, the former would, too. When the hunted also prey on some of the hunters, the matter may become complicated.
This is nowhere better illustrated than in the insect world. Think of the complexity of a situation such as the following: There is a certain wasp, Pimpla inquisitor, whose larvae feed on the larvae of the tussock moth. Pimpla larvae in turn serve as food for the larvae of a second wasp, and the latter in their turn nourish still a third wasp. What subtle balance between fertility and mortality must exist in the case of each of these four species to prevent the extinction of all of them! An excess of mortality over fertility in a single member of the group would ultimately wipe out all four.
This is not a unique case. The two great orders of insects, Hymenoptera and Diptera, are full of such examples of interrelationship. And the spiders (which are not insects but members of a separate order of arthropods) also are killers and victims of insects.
The picture is complicated by the fact that those species which are carnivorous in the larval stage have to be provided with animal food by a vegetarian mother. The survival of the young de pends on the mother’s correct choice of a food which she does not eat herself.
In the feeding and safeguarding of their progeny the insects and spiders exhibit some interesting analogies to reasoning and some crass examples of blind instinct. The case I propose to de scribe here is that of the tarantula spiders and their arch -enemy, the digger wasps of the genus Pepsis. It is a classic example of what looks like intelligence pitted against instinct-a strange situation in which the victim, though fully able to defend itself, submits unwitting ly to its destruction.
MOST tarantulas live in the Tropics, but several species occur in the temperate zone and a few are common in the southern U. S. Some varieties are large and have powerful fangs with which they can inflict a deep wound. These formidable looking spiders do not, however, attack man; you can hold one in your hand, if you are gentle, without being bitten. Their bite is dangerous only to insects and small mammals such as mice; for a man it is no worse than a hornet’s sting.
Tarantulas customarily live in deep cylindrical burrows, from which they emerge at dusk and into which they re tire at dawn. Mature males wander about after dark in search of females and occasionally stray into houses. After mating, the male dies in a few weeks, but a female lives much longer and can mate several years in succession. In a Paris museum is a tropical specimen which is said to have been living in captivity for 25 years.
NEST OF THE MUD DAUBER WASP illustrates an intricate predatory relationship. A single cell of the nest, enlarged 10 times, contains one pupa of a secondary predator and five smaller pupae of a tertiary predator.
A fertilized female tarantula lays from 200 to 400 eggs at a time; thus it is possible for a single tarantula to produce several thousand young. She takes no care of them beyond weaving a cocoon of silk to enclose the eggs. After they hatch, the young walk away, find convenient places in which to dig their bur rows and spend the rest of their lives in solitude. Tarantulas feed mostly on in sects and millipedes. Once their appetite is appeased, they digest the food for several days before eating again. Their sight is poor, being limited to sensing a change in the intensity of light and to the perception of moving objects. They apparently have little or no sense of hearing, for a hungry tarantula will pay no attention to a loudly chirping cricket placed in its cage unless the insect hap pens to touch one of its legs.
But all spiders, and especially hairy ones, have an extremely delicate sense of touch. Laboratory experiments prove that tarantulas can distinguish three types of touch: pressure against the body wall, stroking of the body hair and riffling of certain very fine hairs on the legs called trichobothria. Pressure against the body, by a finger or the end of a pencil, causes the tarantula to move off slowly for a short distance. The touch excites no defensive response un less the approach is from above where the spider can see the motion, in which case it rises on its hind legs, lifts its front legs, opens its fangs and holds this threatening posture as long as the object continues to move. When the mo tion stops, the spider drops back to the ground, remains quiet for a few seconds and then moves slowly away.
The entire body of a tarantula, especially its legs, is thickly clothed with hair. Some of it is short and woolly, some long and stiff. Touching this body hair produces one of two distinct reactions. When the spider is hungry, it responds with an immediate and swift attack. At the touch of’ a cricket’s antennae the tarantula seizes the insect so swiftly that a motion picture taken at the rate of 64 frames per second shows only the result and not the process of capture. But when the spider is not hungry, the stimulation of its hairs merely causes it to shake the touched limb. An insect can walk under its hairy belly unharmed.
The trichobothria, very fine hairs growing from disklike membranes on the legs, were once thought to be the spider’s hearing organs, but we now know that they have nothing to do with sound. They are sensitive only to air movement. A light breeze makes them vibrate slowly without disturbing the common hair. When one blows gently on the trichobothria, the tarantula reacts with a quick jerk of its four front legs.
If the front and hind legs are stimulated at the same time, the spider makes a sudden jump. This reaction is quite in dependent of the state of its appetite.
These three tactile responses-to pressure on the body wall, to moving of the common hair and to flexing of the trichobothria-are so different from one another that there is no possibility of confusing them. They serve the tarantu la adequately for most of its needs and enable it to avoid most annoyances and dangers. But they fail the spider completely when it meets its deadly enemy, the digger wasp Pepsis.
These solitary wasps are beautiful and formidable creatures. Most species are either a deep shiny blue all over, or deep blue with rusty wings. The larg est have a wingspan of about four inches. They live on nectar. When excited, they give off a pungent odor-a warning that they are ready to attack. The sting is much worse than that of a bee or com mon wasp, and the pain and swelling last longer. In the adult stage the wasp lives only a few months. The female produces but a few eggs, one at a time at intervals of two or three days. For each egg the mother must provide one adult tarantula, alive but paralyzed. The tarantula must be of the correct species to nourish the larva. The mother wasp attaches the egg to the paralyzed spi der’s abdomen. Upon hatching from the egg, the larva is many hundreds of times smaller than its living but helpless vic tim. It eats no other food and drinks no water. By the time it has finished its single gargantuan meal and become ready for wasphood, nothing remains of the tarantula but its indigestible chitin ous skeleton.
The mother wasp goes tarantula hunting when the egg in her ovary is almost ready to be laid. Flying low over the ground late on a sunny afternoon, the wasp looks for its victim or for the mouth of a tarantula burrow, a round hole edged by a bit of silk. The sex of the spider makes no difference, but the mother is highly discriminating as to species. Each species of Pepsis requires a certain species of tarantula, and the wasp will not attack the wrong species. In a cage with a tarantula which is not its normal prey the wasp avoids the spider, and is usually killed by it in the night.
Yet when a wasp finds the correct species, it is the other way about. To identify the species the wasp apparently must explore the spider with her antennae. The tarantula shows an amazing tolerance to this exploration. The wasp crawls under it and walks over it without evoking any hostile response. The molestation is so great and so persistent that the tarantula often rises on all eight legs, as if it were on stilts. It may stand this way for several minutes. Meanwhile the wasp, having satisfied it self that the victim is of the right species, moves off a few inches to dig the spider’s grave. Working vigorously with legs and jaws, it excavates a hole 8 to 10 inches deep with a diameter slightly larger than the spider’s girth. Now and again the wasp pops out of the hole to make sure that the spider is still there.
DEATH OF THE SPIDER is shown in these drawings. In the first draw. ing the wasp digs a grave, occasionally looking out. The spider stands with its legs extended after raising its body so the wasp could pass under it. In the second drawing the wasp stings the spider, which falls on its back. In the third the wasp licks a drop of blood from the wound. In the final drawing the spider lies in its grave with the egg of the wasp on its abdomen.
When the grave is finished, the wasp returns to the tarantula to complete her ghastly enterprise. First she feels it all over once more with her antennae. Then her behavior becomes more aggressive. She bends her abdomen, protruding her sting, and searches for the soft mem brane at the point where the spider’s leg joins its body-the only spot where she can penetrate the horny skeleton. From time to time, as the exasperated spider slowly shifts ground, the wasp turns on her back and slides along with the aid of her wings, trying to get under the tarantula for a shot at the vital spot. During all this maneuvering, which can last for several minutes, the tarantula makes no move to save itself. Finally the wasp corners it against some obstruction and grasps one of its legs in her powerful jaws. Now at last the harassed spider tries a desperate but vain defense. The two contestants roll over and over on the ground. It is a terrifying sight and the outcome is always the same. The wasp finally manages to thrust her sting into the soft spot and holds it there for a few seconds while she pumps in the poison. Almost immediately the tarantula falls paralyzed on its back. Its legs stop twitching; its heart stops beating. Yet it is not dead, as is shown by the fact that if taken from the wasp it can be restored to some sensitivity by being kept in a moist chamber for several months.
After paralyzing the tarantula, the wasp cleans herself by dragging her body along the ground and rubbing her feet, sucks the drop of blood oozing from the wound in the spider’s abdomen, then grabs a leg of the flabby, helpless animal in her jaws and drags it down to the bot tom of the grave. She stays there for many minutes, sometimes for several hours, and what she does all that time in the dark we do not know. Eventually she lays her egg and attaches it to the side of the spider’s abdomen with a sticky secretion. Then she emerges, fills the grave with soil carried bit by bit in her jaws, and finally tramples the ground all around to hide any trace of the grave from prowlers. Then she flies away, leav ing her descendant safely started in life.
IN ALL THIS the behavior of the wasp evidently is qualitatively different from that of the spider. The wasp acts like an intelligent animal. This is not to say that instinct plays no part or that she reasons as man does. But her actions are to the point; they are not automatic and can be modified to fit the situation. We do not know for certain how she identifies the tarantula-probably it is by some olfactory or chemo-tactile sense- but she does it purposefully and does not blindly tackle a wrong species.
On the other hand, the tarantula’s behavior shows only confusion. Evidently the wasp’s pawing gives it no pleasure, for it tries to move away. That the wasp is not simulating sexual stimulation is certain, because male and female tarantulas react in the same way to its advances. That the spider is not anesthetized by some odorless secretion is easily shown by blowing lightly at the tarantula and making it jump suddenly. What, then, makes the tarantula behave as stupidly as it does?
No clear, simple answer is available. Possibly the stimulation by the wasp’s antennae is masked by a heavier pres sure on the spider’s body, so that it re acts as when prodded by’ a pencil. But the explanation may be much more com plex. Initiative in attack is not in the nature of tarantulas; most species fight only when cornered so that escape is im possible. Their inherited patterns of behavior apparently prompt them to avoid problems rather than attack them. For example, spiders always weave their webs in three dimensions, and when a spider finds that there is insufficient space to attach certain threads in the third dimension, it leaves the place and seeks another, instead of finishing the web in a single plane. This urge to es cape seems to arise under all circum stances, in all phases of life and to take the place of reasoning. For a spider to change the pattern of its web is as im possible as for an inexperienced man to build a bridge across a chasm obstructing his way.
In a way the instinctive urge to escape is not only easier but often more efficient than reasoning. The tarantula does ex exactly what is most. efficient in all cases except in an encounter with a ruthless and determined attacker dependent for the existence of her own species on kill ing as many tarantulas as she can lay eggs. Perhaps in this case the spider follows its usual pattern of trying to es cape, instead of seizing and killing the wasp, because it is not aware of its dan ger. In any case, the survival of the tarantula species as a whole is protected by the fact that the spider is much more fertile than the wasp.
Alexander Petrunkevitch is emeritus professor of zoology at Yale University.