Can they learn by experience?

Lorus Johnson Milne & Margery J. Milne, A Multitude of Living Things, New York, Dood, Mead & Company, (1945) 1947, pp. 35-43.


A house of leaves is but one of the many ways in which caddis larvae shield their bodies from enemy attack, and maintain a tube to restrict a breathing current and make it more efficient. As is shown so clearly in the fable of the Three Little Pigs, a house of straw is not much protection from a hungry adversary. This is particularly true when the case is no stronger than a single thickness of fallen leaf. Yet such a lightweight cylinder is much easier to shift from place to place. It offers concealment, and some of the largest caddis build no other type. But a more durable covering can be made from the same building blocks of the pieces are installed crosswise, tangent to the central tunnel where the larva hides its vulnerable parts. Some case worms prepare such a shelter from bits of grass, and although the outside of their trailing sheaths is ragged and unkempt, the walls can resist pressure and retain their form. The thicker wall is not all gain, however, since to reach the bottom the caddis larva must either expose more of its body or bend its back sharply to carry the case on a vertical abdomen while creeping along in search of food. Most caddis retain the horizontal position of the shelter, and use abnormally long and strong legs to reach from the doorway and overcome the increased friction of rough case against bottom tangles. But however a tube is constructed, the biggest difficulty is always in getting it started. Adding to something already begun is far easier than making the first step. Most case worms that build these sturdier shelters with vegetable fragments raise themselves as high as possible on their long legs, and with head hung downward, swing their mouths in a horizontal circle while secreting a steady stream of salivary cement. The clear plastic sinks a fraction of an inch to rest in an open loop on the bottom, and build up there into a rather rigid ring. The insect can raise this hoop around its body and use it as the framework for the back door of the new case.
A few case worms are familiar enough to have common names. Fishermen call them “cadbaits” and know how fond fish are of the plump-bodied insects. One is the “log cabin builder” because it uses twigs and sticks in place of grass, straw and leaves to fashion a still bulkier shelter. It is met often in woodland pools left in spring by melting snows. Although the pond is very temporary, the log cabin builders may abound. As they crawl among the fallen leaves, the whole bottom seems a-twitch. Each caddis larva creeps on ahead of its shelter for a fraction of an inch, gets a good grip on some plant fragment, and jerks the case forward until only its head sticks out. Often one case worm follows another around, and steals pieces from the rear end of the neighbor’s house rather than seek out loglets of its own. These are the pirates, too, that sometimes covet another’s shelter so much that they enter the back door and bite their way into ownership. If the dispossessed one shows initiative it vacates promptly enough but makes a quick circuit of its case to enter from the rear and turn the tables. Such a malicious game of tag may keep up for minutes. Then either one tires of the futile repetitions, or the larva in the case finds out how to prevent a recurrence. If the caddis in the tube turns quickly end for end, it can meet jaw with jaw as the opponent reaches the rear doorway and attempts to force an entry. Where many case worms of the same kind live side by side in a small pond, these tragic-comic duels are frequent. The persistent and ingenious keep their homes. Others have to build new ones over and over. Somehow there seem never to be enough shelters to go around; always there is a housing shortage!
Straw and sticks are lighter in weight but afford less protection than an equal quantity of sand and stone. Many case worms are expert masons. Some start their building careers as carpenters, but as they grow larger and the case projects farther into the fast current of the streams where they live, the creatures add stones as ballast. In a short time the whole shelter may be of rock particles, and the vegetable beginning of the case rots off or is cut away altogether. Other caddis use sand, and profit by its abundance and the ease with which a case can be begun. Each has merely to dig a little hole in the bottom, add cement to its rim, lace the head in the cavity and with a rooting movement, lift the sand-encrusted ring onto the neck and shoulders. Generally these tubes are cylinders too, but a few kinds of caddis larvae add such extensive side wings and hoods to their sand cases that the shelter resembles a tortoise shell. It is camouflaged completely as it rests on the gritty bottom, and even when the builder is extended for foraging, the hood conceals him from above.
The young of deep-water caddis remain active through the winter, although the lake above them may be covered with a thick coat of ice. Those in the shallows, however, usually bury themselves at least part way in the bottom. The ever-curious Thoreau found them in the shadiest coves of Walden Pond by a method of observation few naturalists have tried. He noted that the first ice of autumn was particularly hard “ and transparent, and affords the best opportunity that ever offers for examining the bottom where it is shallow; for you can lie at your length on ice only an inch thick, like a skater insect on the surface of the water, and study the bottom at your leisure, only two or three inches distant, like a picture behind glass, and the water is necessarily always smooth then. There are many furrows in the sand where some creature has travelled about…. And, for wrecks, it is strewn with the cases of caddis-worms made of minute grains of white quartz. Perhaps these have creased it, for you find some of their cases in the furrows…”
Caddis show definite preferences for certain building materials. They also make entertaining and instructive pets in small aquaria. Many biologists have tested them by depriving them of the shelters, to learn how much ingenuity the insects have for using unfamiliar substitutes. Can they learn by experience? But as always happens in such experimental studies, investigators are impressed as much by individual differences shown by caddis worms that “ look alike” as by the plain resourcefulness of some. Those that ordinarily use leaves for case construction can be provided with rice paper such as is given goldfish for food. The insects immediately cut neat rectangles and sew them into shelters. But the building material swells in the water and eventually disintegrates, leaving the caddis naked as before. Bond paper houses are much more permanent, but experimenters are usually disappointed when the case worms show no preference for any special color or texture.
In a western thermal spring, Professor C.T. Brues once found jewelled caddis cases. Larvae on a pebbly bottom had carefully selected certain brightly colored bits of opal and used them in great preponderance for their shelters. Yet these were far from being the most abundant minerals there. It was tempting to assume that the creatures responded to the brilliance of the stones. But after some investigation, the professor concluded that vision must really play a part since the opalized pieces were not physically different from the white or slightly opalized bits of quartz. Tactile sense, coupled with an appreciation of weight, would not be sufficient explanation since the opalized fragments were not common except in the cases. This is not usual in case worms, as the professor knew. Later a student supplied similar case makers with beads of several sizes and many hues, and found that shelters constructed from these spheres of glass contained a definite proportion of small and large beads, but the colors were taken at random.
Most case worms deprived of their shelters will use other materials for new homes if they have available none of their normal building substances. A stone mason will prepare a crude tube from sticks or leaves, or a carpenter will use sand if it has nothing else. The smoothness of the product, the time taken to get it together, and the delay before making the first start on the strange substitutes, are all subject to considerable variation from one individual to another. And a caddis larva that has already built one shelter of a new structural material, waits far less time before starting a duplicate, does a far better job on the second case, and requires fewer minutes to get it finished than it showed the first time. But comparing one cadbait to another of the same kind indicates that individual differences are almost as great as among dogs or human beings. They are merely on another scale and involve activities unlike our own. The comforting assumption that these small creatures go through their lives guided only by identical instincts, proves to be as erroneous as the notion that all men are created equal in ability rather than in opportunity. The great danger lies in the temptation for the observer to claim human thought processes for animals with such different designs and viewpoints. After watching caddis worms build their shelters, it is too easy to be enthusiastic. Charles Kingsley carefully put his observations into the adventures of Tom who “watched the caddises eating dead sticks as greedily as you would eat plum-pudding, and building their houses with silk and glue. One would begin with some pebbles; …. Then she found a shell, and stuck it on too, and the poor shell was alive, and did not like at all being taken to build houses with;… and so on, till she was patched all over like an Irishmans’ coat. Then she found a long straw, five times as long as herself, and said ‘Hurrah! My sister has a tail, and I’ll have one too;’…”

For many of the hungry animals in a pond or stream, a caddis case is quite enough protection. But even the bulky, stony shelters of brook forms do not deter trout and other fish. They swallow anything that twitches on the bottom. The caddis worm lands inside, to be literally digested in its home. Experts often puzzle over vials of stomach contents to learn what fish eat, and caddis cases weighing down the swimmer’s stomach are a very common find. The quantities and assortment of such debris show what a large part of a tour’s diet is devoured in spite of its hard and indigestible covering.
Two biologists who had been asked many times to help identify such stomach contents were greatly intrigued with the many types of architecture the caddis used in case construction. One continued to complete a doctoral dissertation on the distribution of adult caddis-flies, while the other finished a similar thesis on the case worms. Community of interest was consolidated when the two specialists joined in marriage. Together they toured the United States and Canada, collecting in all the ponds and streams they reached. The variety seemed endless. In lakes were slender, curving tubes of sand that tapered like miniature elephant tusks. The case worms that built these shelters emerged as the most delicate of caddis-flies, with unbelievable fine feelers extending two or three times as long as their bodies. They crept as larvae through the warm waters of Lake Okeechobee in Florida, the chilly shallows of Georgian Bay in Canada, and the blue depths of Mono Lake in California. Relatives of the log cabin builder in the Rocky Mountains of Colorado used tiny snail shells exclusively as reinforcement in place of twigs or stones. Other kinds cemented sand grains into a spiral shelter that resembled a single snail shell so much that it was easy to understand how one of the early conchologists believed it to be a mollusc and name it Valvata arenia- “the sandy univalve.”
Caddis larvae thrive in the most remarkable places. Some cling to the bottom of the Niagara River, under roaring cataracts of water so pale green with churned-in air that it has scarcely any buoyancy. Yet these caddis emerge by the billions during the summer, and often make a nuisance of themselves by flying to street lights and dying there in windrows. Some natives of Buffalo even have developed a special allergy- a hayfever response to the microscopic hairs from the wings of these local caddis-flies- that has become a medical curiosity. Caddis live in the icy waters of melting glaciers; others inhabit hot springs in our western states, where the liquid is charged with salts and warmer than a human hand can bear.
Our local pond varieties are most successful when the water is not warmer than 65 degrees Fahrenheit. For a long while it was thought that this was due to an intolerance of heat on the part of the insect. But one year a scientist kept some of these insects in an aquarium atop the Harvard Biological Laboratories, in a greenhouse where sunlight and steam heat combined to provide a constant temperature of over 90 degrees. He ran a hose line into the aquarium and blew air under high pressure through a fine-grained cake of diatomaceous earth. The tiny bubbles issued into the water in so great a profusion that great numbers of them dissolved, keeping the water saturated with air. The gas accumulated inside the caddis cases in such quantities as to embarrass the case worms. If they lost hold of the bottom vegetation their shelters zoomed upward to the surface and stayed there because of enclosed air. Only with difficulty did the cadbaits reach a side wall and drag their bubbly cases down it to the food on the aquarium floor. They clung to vegetation glittering with more bubbles, to prevent a repetition of the free vertical ride. But in spite of these inconveniences, and a temperature long considered fatal, the insects flourished. They grew to maturity and emerged in far fewer weeks than would have been possible in the cooler waters of a natural pond. Oxygen made all the difference in their success. It is not present in adequate quantities when a pond gets warm; the caddis need more air than they can get. They suffocate in the heat. Abundance of air may explain the great population of Niagara River caddis too, and the plain liking of many kinds for rapidly running water.
River caddis have spread into the salty waters of the Baltic Sea. One special kind on the coast of New Zealand has become so well adapted to marine life that it makes its case of seaweeds and coral fragments. Fishermen in our northern lake regions have complained with cause of caddis chewing their fishnets under the ice in winter. Sanitary engineers have sought eary ways to eliminate other case worms that invaded sewers and water mains, where they throve in the dark on microscopic animal and plant materials carried to them by the current. These aquatic niches are a most varied extension of the small number of sites in which caterpillars live. Yet the caddis seem to be aquatic relatives of butterflies and moths; the case worm even has a caterpillar shape and man of its habits. But in their watery surroundings they have been able to find living quarters in far more diverse situations than have their leaf-eating, caterpillar kin.