The furcula, or furca, is a tail-like appendage shaped like a fork, found ventrally on the fourth abdominal segment of springtails. Present in most species dwelling the upper soil layers, it is used for jumping when the animal is threatened. While at rest, it is retracted under the abdomen and held there by a structure variously called the retinaculum or hamula, located on the third abdominal segment. When the retinaculum releases the furcula, it springs open and hits the substrate, propelling the springtail into the air. This mechanism is not used for locomotion, but only for escaping from predators or severe stress.
One reason not to use it for locomotion is that its action is extremely unpredictable; when the furcula is released, the organism is sent tumbling through the air on an unpredictable trajectory, and lands practically randomly.
Not surprisingly, some species, such as the intertidal marine inhabitant Anurida maritima do without an organ that might get them into more trouble than it rescues them from. Some other species, such as in the genus Hypogastrura have only a very short furcula. At the same time the device is hard to predict and is versatile. Even a springtail drifting on the surface tension layer of water often can jump successfully. Also, most predators of springtails are small and often have little power of sight, so if the prey leaps in time, the chances are that from the hunter's point of view, it simply vanishes.
- Christian, E., 1978. The jump of the springtails. Naturwissenschaften 65:495-496.
- Imms' General Textbook of Entomology: Volume 1: Structure, Physiology and Development Volume 2: Classification and Biology. Berlin: Springer. 1977. ISBN 0-412-61390-5.
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