Darwin, Earthworms & Circadian Rhythms: A Fertile Field for Science Fair Experiments

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Darwin & Earthworms

Strange noises, including shouts, whistling, and music, from the playing of a bassoon and a piano, echoed from Down House in Downe, England. However, it was not a festive 19th century social occasion; instead, a careful scientific experiment was being conducted by the ever-curious naturalist Charles Darwin (1809-1882). Cooperative earthworms, Lumbricus terrestris, were being tested to see if they would react to sound. He also investigated their sensitivity to light, vibrations, odors, and touch. For his work, he kept earthworms in soil-filled flowerpots in his library. Darwin suffered from chronic insomnia, a condition that no doubt led to his making so many observations of the nocturnal earthworm.

Intrigued with their everyday behavior, Darwin wondered just how smart earthworms were. After watching earthworms plug up their burrows with petioles, leaves, pebbles, or twigs wedged in subtly different ways, Darwin concluded that they possessed a surprising level of intelligence. After watching them manipulate pine leaves, Darwin commented "In order, therefore, that worms should do their work well, they must drag pine leaves into their burrows by their bases, where the two needles are conjoined. But how they are guided in this work is a perplexing question" (Darwin, 1881). For one experiment, Charles Darwin and his son Francis cut the sharp ends from the pine needles, but found that the earthworms still preferred to drag the needles into their burrows by the bases. "It appeared both to my son and myself as if the worms instantly perceived as soon as they had seized a leaf in the proper manner" (Darwin, 1881). How the eyeless earthworm gains an "overview" of individual leaves so as to be able to maneuver them was not readily understood, but it was thought to depend mainly on the earthworm's tactile sense.

In Charles Darwin's book, The Formation of Vegetable Mould through the Action of Worms, there are measurements and calculations that the Darwins made of the weight of the nightly manure castings produced by earthworms, and consequently how much of an accumulation of this "vegetable mould" would build up over the years. Such detailed observations and careful attempts to numerically quantify various natural processes are found throughout Darwin's writings. The earthworm's relatively simple digestive system, consisting of a mouth, pharynx, esophagus, crop, gizzard, and intestine, enables it to grind vegetable matter and other stray material that it ingests to produce the clay-like manure castings egested from its anus and deposited on the ground's surface. This relentless activity mixes and enriches the topsoil to create fertile humus.

Generally, earthworms live less than three years, but some have been known to live as long as six years in captivity. The age at which earthworms reach sexual maturity varies greatly based on season, environmental temperature, food availability, and moisture content of the soil. (Further information, as well as good photographs and drawings of dissected earthworms, can be found online with a GoogleTM search.)

The reproduction of earthworms is of particular interest. Both male and female reproductive organs are found in the same individual. In general, organisms are classified as diecious (two houses) if male and female reproductive organs are in separate individuals, and monecious (one house) if both types of organs are in one individual, as is the case for earthworms. Even though earthworms are monecious, they cannot self-fertilize. The common earthworm comes to the surface of the ground to mate on damp summer nights. Reproductive readiness is evident by the swollen clitellum, an enlarged region seen partway down the length of the body from segments, or metameres, 31 or 32 to 37. During mating, they position themselves so that the flat, ventral parts of their bodies are together, and their head ends point in opposite directions, which is necessary so that the genital pores match up. …