The furcula, or furca It is a forked, tail-like appendage. It is present in most species of

springtail Springtails (Collembola) form the largest of the three lineages of modern hexapods that are no longer considered insects (the other two are the Protura and Diplura). Although the three orders are sometimes grouped together in a class called Ento ...

s, and in them it is attached ventrally to the fourth

abdominal The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the torso ...

segment. The organ most often is present in species of ''

Collembola Springtails (Collembola) form the largest of the three lineages of modern hexapods that are no longer considered insects (the other two are the Protura and Diplura). Although the three orders are sometimes grouped together in a class called Ento ...

'' that lives in the upper soil layers where it is used for jumping to avoid predators. While at rest, it is retracted under the abdomen and held there by a structure variously called the

retinaculum A retinaculum (plural ''retinacula'') is a band of thickened deep fascia around tendons that holds them in place. It is not part of any muscle. Its function is mostly to stabilize a tendon. The term retinaculum is New Latin, derived from the Lati ...

or hamula, which in turn is located beneath the third abdominal segment. When the furcula escapes from retinaculum, it swings downwards and hits the substrate, propelling the springtail into the air. The animal does not use this mechanism for ordinary locomotion, but only for escaping from

predator Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...

s or severe stress. One reason not to use the furcula for general locomotion other than to escape threats, is that its action is very unpredictable; when the furcula is released, the springtail is sent tumbling through the air on a practically arbitrary trajectory, and lands almost randomly. That may have advantages in escaping some forms of attack, but is not of much use in adopting any particular route. Although the action of the furcula is hard to predict, it is versatile. Even a springtail drifting on the surface tension of a layer of water often can jump successfully. Furthermore the furcula is effective in environments typical of Collembola; most predators of springtails are small and many 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. Some species of Collembola, for instance in the

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

Hypogastrura ''Hypogastrura'' is a genus of springtails and allies in the family Hypogastruridae. There are at least 150 described species in the genus. Their name means ‘lack of a stomach tail’. See also * List of Hypogastrura species References ...

'', have only a very short furcula. Some other species have no furcula at all; examples include the intertidal marine species '' Anurida maritima'' and some

riparian A riparian zone or riparian area is the interface between land and a river or stream. Riparian is also the proper nomenclature for one of the terrestrial biomes of the Earth. Plant habitats and communities along the river margins and banks a ...

species that live on the brink of flowing fresh water. The absence of a furcula in such species commonly is explained as an adaptation to a way of life in which a jumping or flying organ might get the animals into trouble more often than it rescues them. Its loss presumably is analogous to the loss of the ability to fly, such as is common among birds and insects inhabiting oceanic islands. The furca allows for distant jumps relative to the body length. ''Entomobrya dorsalis'' which has a length of less than 2 mm long, uses their furca to jump up to 16 cm. ''Sminthurus viridis'' has a jump where the acceleration peaks were measured at 970 m / s², which corresponds to an acceleration of phenomenal 98.9 g (compared to 3.0 g for an astronaut during a space shuttle launch). During the jumps some species can perform several tumbles making their orientation unpredictable at the time of landing, after which they have the ability to recover very quickly. The body of ''Lepidocyrtus paradoxus'' (Entomobryomorph) remains relatively vertical with its head remaining at the top once during its jump, while the ''Hypogastrura socialis'' (Poduromorph) jumps forward with its head down and their body rotating on an axis. In aquatic species the mucron is often wider, more flattened in shape like a paddle, which allows it during jumps to be supported on the liquid without breaking the surface tension.{{cite web , url=https://collemboles.fr/en/morphology-and-physiology/63-the-furca-jumping-organ-of-springtails.html , title=Springtails - the furca - Jumping organ of springtails

References

Springtail anatomy