Collembola

(invertebrate zoology) The springtails, an order of primitive insects in the subclass Apterygota having six abdominal segments.

(Springtails)

Class: Entognatha

Order: Collembola

Number of families: 20

Evolution and systematics

Rhyniella praecursor, a springtail from the Devonian (about 400 million years ago [mya]), represents the oldest known fossil of terrestrial animals. Springtails constitute an extremely ancient group that branched off very early during the evolution of the line that led to the more advanced hexapods, but their exact position within Arthropoda has not been resolved. Springtails traditionally were considered members of the sub-class Apterygota within the class Insecta. Some authors now join springtails with proturans and diplurans, with which they share mouthparts enclosed in a pouch formed by elongated lateral portions of the head, in a taxon called Entognatha within the Hexapoda. Other authors include them together with Proturans in a taxon called Ellipura within the Hexapoda, and still others exclude them from the Hexapoda in a separate class (Collembola) and suggest that the hexapod characteristics (development of a head with five segments, thorax with three locomotory segments, and reduction of appendages on remaining body segments) evolved independently at least four times.

Collembola are classified into three suborders, Arthropleona, Neelipleona, and Symphypleona. Arthropleona comprises species that are longer than they are wide and in which the divisions between the thoracic and abdominal segments are easily visible. In contrast, the Neelipleona and Symphypleona are spherical, with poorly defined intersegmental boundaries on the body. Neelipleona differs in that the body is formed largely by expansion of the thoracic rather than the abdominal segments, as is the case in Symphypleona.

Physical characteristics

Springtails are small, wingless arthropods, usually 0.04–0.12 in (1–3 mm) long. They range from a little more than 0.008 in (0.2 mm), the smallest hexapods in the world (some species of Neelidae), to 0.4 in (10 mm) for some members of the tropical subfamily Uchidanurinae. Species living in caves or deep in leaf litter or soil tend to be white or gray, whereas species living in more open environments often are clothed with colored scales. Compound eyes are absent or reduced to a cluster of not more than eight ommatidia, and antennae are four-segmented (segments sometimes are subdivided, giving the appearance of more than four segments). The name Collembola, derived from the Greek coll, meaning "glue," and embol, meaning a "wedge," refers to the collophore, a peg-shaped structure on the venter of the first abdominal segment. The collophore once was thought to function as an adhesive organ, but it is involved mainly in maintaining the water balance, absorbing moisture from the environment. The common name springtails refers to a forked jumping organ, the furcula, found on the underside of the fourth abdominal segment. The furcula is retracted against the venter of the abdomen and held there by a structure on the third abdominal segment, the retinaculum. The genital opening is found on the fifth abdominal segment. There are only six abdominal segments (not 11 as in higher hexapods), and caudal appendages, or cerci, are lacking.

Distribution

Springtails occur worldwide, including the frozen Antarctic, where they reach the most southerly distribution of any hexapod (84°47′ south latitude) and survive at temperatures lower than 76°F ( 60°C).

Habitat

Springtails are most abundant in warm, damp places, and many live in leaf litter and soil. Some live in caves or in the

Behavior

The "jumping" behavior characteristic of springtails occurs when the retinaculum releases the furcula, causing it to snap down against the substrate and flip the organism some distance (up to 8 in, 20 cm) through the air. This device, present in all but a few genera, seems to be an effective adaptation to avoid predation. Some Collembola use the collophore not only for water balance but also for adhering to smooth surfaces. Others extend a pair of eversible vesicles from the collophore's tip, move their front legs across the tip, and use the vesicles for grooming themselves.

Feeding ecology and diet

Springtails are primarily soil or litter dwellers, and most species feed on the fungi and bacteria found in rotting organic matter. Many arboreal and soil species also feed on algae, and some eat other plant materials. A few other species are carnivorous, feeding on nematodes and other springtails and their eggs. Mouthparts are styliform for sucking or have molar plates for grinding.

Reproductive biology

Like other non-insect hexapods, these insects continue to molt after they reach sexual maturity; unlike other hexapods, reproductive activity occurs only during alternate instars. Each reproductive stage is followed by a molt, a short period of feeding, and another molt. Reproduction usually is bisexual, although there are some parthenogenetic species. In many species males deposit a stalked spermatophore on the substrate for the females to find. Spermatophores can be deposited anywhere. Alternatively, the males may wait to find a receptive female and then deposit spermatophores nearby, or they may use the third pair of legs to transfer a drop of sperm to the female's genital opening. In some species competing males eat one another's spermatophores before setting up their own in the same place. Many symphypleonids display elaborate courtship behavior: the male dances and butts heads with the female to entice her to take up a spermatophore deposited on the substrate. In Sminthurides aquaticus, the male holds a potential mate with specialized antennal claspers and is carried by her until she becomes sexually receptive.

Eggs are deposited singly by some species or laid in large masses by several females. Some female sminthurids cover their eggs with a mixture of eaten soil and fecal material, which protects them from dehydration and fungal attack. Springtails lack metamorphosis; the eggs hatch into young that are similar in appearance but smaller than the adults and lacking reproductive organs. They usually molt four to five times before reaching sexual maturity and continue to molt periodically (up to 50 times) throughout the rest of their adult lives. Some species have many generations in a single year (multivoltine), particularly in the tropics, whereas many others have only one (univoltine); some species in the Antarctic may take up to four years per generation.

Conservation status

There are more than 6,000 species worldwide, plus an estimated 25% unknown species. No springtails are listed by the IUCN. This is a highly adaptable and resistant group. Endemic species within endangered areas could be candidates for conservation programs.

Significance to humans

Springtails are part of the community of decomposers that break down and recycle organic waste, and, in this respect, they play a significant role in energy flow for many ecosystems. Most people see springtails when they lift stones in a garden or turn over compost. Swarms on snow are called "snow fleas."

Springtails are considered pests in houses, but they are harmless uninvited guests. A few species feeding on living plants constitute pests: Bourletiella hortensis (the garden springtail) may damage seedlings in early spring, Sminthurus viridis (the lucerne flea) is a pest of legume pastures, and Hypogastrura armata is a frequent pest of commercial mushrooms.

Species accounts

Resources

Christiansen, Kenneth A., and Peter F. Bellinger. The Collembola of North America North of the Rio Grande: A Taxonomic Analysis. Grinnell, IA: Grinnell College, 1981.

Coleman, David C., and D. A. Crossley. Fundamentals of Soil Ecology. San Diego: Acedemic Press, 1996.

Maynard, E. A. A Monograph of the Collembola or Springtail Insects of New York State. Ithaca, NY: Comstock Publishing Co, Inc., 1951.

Hopkin, S. P. Biology of the Springtails (Insecta: Collembola). Oxford, U.K.: Oxford University Press, 1997.

Lubbock, J. B. Monograph of the Collembola and Thysanura. London: Ray Society, 1873.

Salmon, J. T. An Index to the Collembola. Vol. 1. Bulletin no. 7. Wellington, New Zealand: Royal Society of New Zealand, 1964.

Handschin, E. Die Tierwelt Deutschlands und der angrenzenden Meeresteile. Vol. 16: Urinsekten oder Aperygota (Protura, Collembola, Diplura und Thysanura), edited by F. Dahl. Jena, Germany: [n.p.], 1929.

Bellinger, P. F., K. A. Christiansen, and F. Janssens. "Checklist of the Collembola, 1996–2003." (11 Feb. 2003). "Collembola." Feb. 1997 (11 Feb. 2003). .

[Article by: Natalia von Ellenrieder, PhD]

An order of primitive insects, commonly called springtails, belonging to the subclass Apterygota. These tiny insects do not undergo a metamorphosis. They have six abdominal segments, some of which may be ankylosed to give an apparently smaller number (see illustration). Appended to the fourth abdominal segment in most of the species is a spring, or furcula. This consists of a basal piece and two apical parallel structures which end in hooks. Another pair of structures, united at the base, occur ventrally on the third abdominal segment. These bear teeth which engage with the furcula and hold it beneath the body. When released, the spring flies backward, catapulting the insect.

A collembolan, Entomobrya cubensis. (After J. W. Folsom, Proc. U.S. Nat. Mus., 72(6), plate 6, 1927)

Collembola live in humid places, often in leaf mold. Some species are active at cool temperatures, hence another common name, snowflea, is used. The earliest known insect fossil, Rhyniella praecursor, is thought to belong to this order. See also Insecta.