Bulletin of the East African Natural History Society, 25:59-63

The Spider and the Flies: Observations From an Urban Ecology

©1995 by Charles M. Nelson
Department of History
University of Nairobi

Interactions between fruit flies and the household hunting spider are used to show that aspects of the urban ecology can be interesting and enlightening. With what remains of the African wilderness all around us, we pay too little attention to the urban ecology which is quickly transforming all of eastern Africa.

KEY WORDS: urban ecology, fruit fly, hunting spider, territoriality

Urban ecology? In Kenya? I'm sure that most members of the Natural History Society like to think of East Africa as a besieged wilderness where heroic creatures, with the help of friendly humans, are making what could be their last stand. Urban ecology may be an apt concern in New York, London or Tokyo, but in Nairobi it is overshadowed by a hinterland which is close enough and romantic enough to draw everyone's attention almost all of the time. Well, not quite everyone all the time. Sometimes my attention wanders back to Nairobi and I am amazed and fascinated by what is transpiring under my very nose.

As an archaeologist, I have a somewhat different view of man's place in nature. Perhaps I can illustrate this point by reference to a popular film which many of you may have seen: Time Bandits. Plot: God's helpers, who are of course midgets, get fed up, steal his map of The Universe, decide to become bandits, and fall into the clutches of the Devil, who needs the map to get out of Hell. Enter the little kid and God to save the day. After all, what can you expect of midgets with a mere 5,000 years of experience. On the way the midgets visit a number of times and places. What I found attractive about all these places was that none of them were new and shiny in their own day. For example, in ancient Crete, the frescoes were chipped and the hem of the royal robe was frayed just a bit. There were occasional dents in the gold plate and dust in the corners; steps were worn and the architectural lay-out was somewhat out of date for the manner in which the space was actually being used.

Like every place else in the last 10,000 years, ancient Crete was having a hard time keeping up with its human occupants. In fact, the people were having a hard time keeping up with the changes they, themselves, were creating. Archaeologists call this displacement cultural lag and it is a feature of all known cultures. It is also a special case of a more general problem which we might call ecological lag which occurs when one part of an ecological system changes much faster than the connected bits. And the biggest and most disconnected symbol of this problem in East Africa is NAIROBI. Wowsers! Just pause and think about that for a moment.................

At first glance Nairobi seems completely out of place in the African savanna, an artificial construct which makes less sense than a cubist painting floating in an impressionist pond. But actually, it is the savanna which is out of place, the savanna which much change, the savanna which is being de-created by the growth of a place which produces its own ecological climate. Regardless of what we desire, Nairobi is the future. If you want to know what the African countryside will be like in 100 years, study Nairobi. It is the ecological rules of the city which will permeate and transform the surrounding savanna. To survive, the plants and animals of this countryside will have to find places for themselves in the new ecology. Indeed, this is already happening. Have you noticed that Ibis, over the last three years, have learned to live off of our scattered refuse; they are finding their place in the new order of things.

If you can suspend for a time your horror and rejection of the future, the creation of this urban ecology is a fascinating process. The stories of its creatures, their personalities and lives, are just as compelling as any from the savanna. And anyone can observe these lives. You don't have to be an ecologist or biologist; you only need open your senses to what you are seeing every day. The observations below provide an example which I hope you will find both amusing and revealing. It comes from the ecology of my kitchen and yours. It concerns creatures who live in our homes with us because of the niches we have created inadvertently for their exploitation. It concerns the relationship between the small hunting spiders who stalk our world and the fruit files we provide for sport and, sometimes, food.

The spider in our little drama is the common hunting spider; not the large long-bodied kind, but the small blocky sort between 3 and 7 mm long. These little guys are big in front and small in back, with short but springy legs which allow them to hop several centimeters. They do not live in webs, but patrol territories, looking for insects smaller then they in order to eat. I know this because I have observed very regular patterns in their movement and traced these movements back to territorial centres. Typically, the centre of a territory is a large stationary object which provides concealment and access to surfaces frequented by small insects. Windows, cabinets and large pieces of furniture which stand close to walls are favored territorial centres.

From such a centre, the household hunting spider may forage for up to three meters, a distance which usually brings it into the territory of a neighbor. Foraging patterns are quite regular. For example, a spider who once occupied the window next to the computer table at which I am now writing, once had a foraging routine which coincided with my late afternoon writing schedule. She would appear from beneath the corner of the table, cross to the printer, search its back and top, hop to the plug array on the surge suppresser, then to the wall where the calendar and light fixture would be searched, and finally across the wall and behind the drapes to the window. Sometimes this transect would be varied. From the corner, she would proceed to my keyboard, then the computer and slide scanner, and across the front of the table to my tea mug. Then it was back under the table to reappear on the wall near the drapes. This happened every day at the same time for more than a month.

Household hunting spiders are diurnal, but they do not like changes of light and shadow which suggest the movement of large creatures in their environment. They appear to avoid times and places where such changes in the hunting light occur. Hence you may be living in the same space with these fellows but not know it. You can treat the occasional hunting spider you observe as an intruder. This is, of course, self delusion, a charming behavioral characteristic of humans which promotes the development of the local urban ecology.

Hunting is opportunistic. The opportunities in my flat are commonly mosquitoes and fruit files. The success rate for hunting mosquitoes is very high. In fact, I have never seen a spider miss, but my sample size is too small to rule out occasional misses. On the other hand, the success rate for hunting fruit flies is very low, probably less than one percent. Nonetheless, spiders never pass up the chance to stalk a fruit fly. This raises some interesting questions. First, if predation on mosquitoes is so successful, what keeps the spiders from destroying this component of their food supply. There are two things at work here. First, the spiders do not generally recognize pray unless it moves. Therefore, they must see the mosquito land or they are unlikely to recognize it. Second, their territory is much larger than their radius of effective observation, which is only about 25 cm. For this reason, most mosquitoes that land in the hunting zone of the spider pass through its territory without being observed.

The household hunting spider stalks its pray and then jumps on it from a distance of about 5 cm. Since fruit flies spend most of their time on vertical surfaces, this poses a certain technical problem for the spider. If it simply jumped off the vertical surface, the spider would continue over the fly and down to the floor below. It is the spider's solution to this problem which raises our second question. To hunt on a vertical surface, the spider approaches the fly from above, anchors a thread of silk which she spins from her abdomen, and then launches herself outward from the wall at an angle and with enough force to over-jump the fly by several times the distance which actually separates them. At the same time, she spins a taught line of silk and stops spinning when the thread is precisely the right length to translate the arc of the jump inward to the very point on the wall where the fly sits. This process actually accelerates the spider downward unto its pray while it blocks the most direct flight path from the surface of the wall. Yet, the fly almost always wins this contest. Why, then, does the spider persist. The silk belaying line is pure protein. Even though the spider retrieves the line by eating it, it still takes quite a bit of energy for her to digest and respin the line. If you have to do this one hundred or more times to catch a fruit fly, how valuable to you can that fly be as food? I don't know and have no practical way of measuring the energy dynamics, but merely asking myself that question made me think along other lines.

One such train of thought was the value of practice. Could the spider be practicing to maintain the skill necessary to successfully hunt less difficult pray such as the mosquito. I have a magnifying glass which is 14 cm. in diameter. It provided a clear and greatly magnified view of the hunt. Even so, the actual leap of the spider and the evasion of the fly occurred so quickly that I could not see what was happening or get a feeling for how much leeway the fly might have within the escape envelope defined by the spider's motion. However, by timing the approach of the spider and the length of time she allowed before the leap, I was able to predict to within about one second when the spider would go into action. And by turning on the fluorescent light in the kitchen just before the spider struck, I was able to increase the spider's success rate by several fold even though I could see no difference at all in the actual event. An increase in the light level just a second before the spider struck decreased the escape reaction of the flies just enough so that the spider would succeed about 3 times in 20 tries. From this I concluded that the spider must the missing the flies by such a narrow margin that any improvement would result in greater success. Practice, then, seems a reasonable hypotheses to examine further.

Fruit flies enter the house with fruit. They also love bread and the plastic bags in which bread is kept, and sisal twine from which fruit has been hung. Fruit flies are calm and in no hurry. When in the air you can herd them by manipulating the personal flying space they like to maintain. You can even get them to land in a particular area if you are patient. This is how I created a sufficient number of fruit-fly/spider encounters to complete the observations above. As has been reported, fruit flies are also incredibly agile, but this did not prepare me for a particular type of behavior in which they appear to be playing with the spiders.

Fruit flies vary considerably in their behavior toward other creatures. When attacked by a hunting spider one fly will head for the ceiling where no further attack is possible, another for the opposite side of the room; but some will observe the spider from the air and then settle again well within its 25 cm. tracking range. The spider will immediately resume stalking the fly. In a typical stalk, the spider moves directly at the fly in a straight line. The distance is covered in a series of short sprints. The spider runs forward between 5 and 10 cm. and then remains absolutely still for about 30 seconds before sprinting forward again. Three or four such sprints take the spider within striking distance. There is another pause and then the leap, as described above. Certain fruit flies might land within the tracking radius of a spider as many as four times, inviting attack after attack. Considering the narrow margin of safety enjoyed by the fly in these encounters, this behavior seemed rather curious. But it did not prepare me for the following observation.

One afternoon while preparing lunch, I was watching another hunt unfold, but this time, when the spider jumped, the fruit fly did a quick pirouette in the air and bopped the spider on the head. The spider did a quick 180 with forelegs raking the air, but to no avail. This happened so quickly that at first I thought my eyes had deceived me. Nut no, close observation showed that some flies routinely did this to spiders. Some settled back near the spider and repeated the performance two or three times in succession. When thought about at a human scale, this encounter is as dramatic as any between a lion and its pray. Yet, at the same time, it is comical - serious violence turned into a parody of itself. But why? Why should fruit flies play such a dangerous game with deadly serious hunting spiders?

Now, as an archaeologist, I am also trained as an anthropologist. One of the analytical tools which anthropologists make much of is participant observation, so it is not surprising that my first thought was to play spider in order to see just how interspecific this behavioral pattern might be. So, I set my forefinger down on the wall about 25 cm. from a fly, waited, and then sprinted it forward about 8 cm. I was all spidery predator, intent on my quarry, fully engaged. When it came to the leap, I was deadly serious: I would land on that fruit fly - splat - and nothing would stop me. Alas, I am not faster than the average hunting spider. Fly after fly eluded me with ease. But after about two dozen tries, one of the flies, bless its faceted little eyes, did a neat aerial somersault and bopped be on the fingernail. Joy! Elation! I had made a successful debut into fly society. But what did it mean? Bopping was not reserved for spiders alone. What else got bopped and what provoked it?

Lunging, I thought. But what else lunges at fruit flies? Other fruit flies, of course. I had, in fact, already observed this without thinking about it. Fruit flies in the same air space sometimes lunged towards one another. Looking more, if found that the object of a lunge would sometimes return the gesture with a quick counter lunge, but the sequence of lunge and counter-lunge was so quick that I could not tell how close the flies came to one another. Time for more participant observation. I flew; I lunged - direct lunges, diagonal darts, the works! And then it happened. A fruit fly looped, rolled and bopped the side of my finger. Aha! So contact was being made. This suggests defense of territory might lie at the root of apparent spider baiting. If so, the spiders are not being treated so much as predators as they are competitors for the same wall space. Could it be that the flies are practicing on the spiders, creatures nearly as fast as themselves, so they may better cope with their fellow flies? Male baboons who cooperate to control the dominance hierarchy of their troop are known to tree leopards on occasion. The leopards want no chancy encounters and sensibly retire, but what do the baboons want? Are they protecting the troop, asserting territory and dominance within the troop, or practicing the skills of cooperation and testing the limits of its effectiveness? Thinking about fruit flies and spiders leads outward, away from the urban ecology they inhabit and back into the savanna which gave rise to the species which will all to soon transform or destroy it. Therefore, do not scorn the ecology and natural history of urban environments. Open your mind; follow your curiosity. You may find the city is far more interesting than you ever suspected.

The observations presented in this paper were made during my tenure at the University of Nairobi as a Fulbright Teaching Fellow. I would like to thank the Council for the International Exchange of Scholars and the United States Information Service for their support through this programme, and to assure the American tax payer that their money was really well spent because it also allowed me to make numerous contributions to graduate and undergraduate teaching, curriculum development, and the scholarly pursuit of history and archaeology. Thank you.