Protura (Proturans)

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Protura

(Proturans)

Class Entognatha

Order Protura

Number of families 4

Evolution and systematics

No fossil proturan is known, but the order is believed to date from the Devonian. Most authors include Protura together with Collembola and Diplura in the class Entognatha within the subphylum Hexapoda, because their mouthparts are enclosed in gnathal pouches formed by the ventral overgrowth of the lateral portions of the head. Some authors consider proturans to be related most closely to collembolans, and join these two orders in a separate class called Ellipura. The order Protura is subdivided into two suborders: Eosentomoidea and Acerentomoidea, each including two families—Eosentomidae and Sinentomidae in the first and Protentomidae and Acerentomidae the second.

Physical characteristics

Proturans are cryptic, pale, and minute wingless hexapods ranging from 0.024 to 0.06 in (0.6–1.5 mm) in length. The head is conical and prognathous, and only the tips of the styliform mouthparts can be seen externally. A pair of "pseudoculi," with an olfactory or chemosensory function, is found on the head. Eyes and antennae are wanting, and the sensory function of the latter is taken over by the forelegs, which are longer than the middle and hind legs and are provided with numerous sensillae. All legs have five segments and end in a single claw; they are longer in proturans living near the soil surface and shorter in those living deeper in the soil. The adult abdomen has 11 segments plus the telson; the first three segments each bear a pair of small, segmented abdominal appendages on the posterior corners, called styli. Cerci are lacking.

Larvae look like adults, except for the number of abdominal segments, which increases from eight in the first larval instar to the definitive 11 of the adult (a phenomenon called anamorphosis). The third instar ("larva II") has nine segments, the fourth instar ("maturus junius") has 11 segments but incomplete chaetotaxy (bristle arrangement) and no genital armature, and the fifth instar ("preimago," present only in Acerentomidae) has complete chaetotaxy but incomplete genital armature; there is no instar with 10 segments. It is unknown whether proturans keep molting during adult life. Only some of the proturans (Eosentomidae) possess a tracheal system for respiration, the remaining groups breathe through the integument.

Distribution

Present in every zoogeographic region, with the exception of the Arctic and Antarctic.

Habitat

Protura are found primarily in leaf litter, rotten logs, soil, and moss in wooded areas but also inhabit agricultural soils, meadows, and turf. Most proturans live close to the soil surface, although some small species can be found as deep as 10 in (25 cm). They prefer moist organic soils that are not too acidic.

Behavior

Proturans move slowly; the forelegs are held forward in front of the head and the middle and hind legs are used for walking. They have been observed to curve the telson over the head and discharge a sticky secretion on enemies. Proturans sometimes form large conspicuous aggregations.

Feeding ecology and diet

Some species have been seen sucking the contents of free hyphae of fungi (subterranean network of fungi consisting of threadlike tubular filaments that feed, grow, and ultimately may produce a mushroom or some other kind of reproductive structure) associated with oak and hornbeam roots, but nothing else is known about the trophic relationships of proturans.

Reproductive biology

Males deposit spermatophores on the ground, and unattended females collect them. Life cycles are mostly unknown. In the known European Acerentomidae, eggs are laid in early spring and adults overwinter; among the Eosentomidae there may be more than one generation per year, because larvae are found throughout the year. Proturans living near the soil surface tend to have one generation per year in temperate zones, whereas those that live deeper in the soil are apt to be reproductively active throughout the year. There are also species that spend the summer near the surface and migrate down into the soil stratum for the winter.

Conservation status

About 500 species have been described, none of which is listed by the IUCN.

Significance to humans

Proturans are so small and cryptic that people rarely notice them. They can be seen in soil samples of Berlese-type funnels. (A Berlese-type funnel is an insect collecting device that consists of a large funnel containing a piece of screen suspended over a container; soil is placed in the funnel, and heat from a light placed above the funnel forces the hidden insects down the funnel into the container.) They are decomposers, helping in the breakdown and recycling of organic nutrients. No proturan is considered a pest.

None known.

Resources

Books

Bernard, E. C., and S. L. Tuxen. "Class and Order Protura." In Immature Insects, edited by F. W. Stehr. Vol. 1. Dubuque, IA: Kendall/Hunt, 1987.

Copeland, T. P., and G. Imadaté. "Insecta: Protura." In Soil Biology Guide, edited by D. Dindal. New York: John Wiley and Sons, 1990.

Imadaté, G. Fauna Japonica Protura (Insecta). Tokyo: Keigaku, 1974.

——. "Protura." In The Insects of Australia: A Textbook for Students and Research Workers, Vol. 1, edited by CSIRO. 2nd edition. Carlton, Australia: Melbourne University Press, 1991.

Janetschek, Heinz. Handbuch der Zoologie eine Naturgeschichte der Staemme des Tierreiches. 2nd edition. Vol. 4, Arthropoda. Half, Insecta. Part 2/3, Special, Protura. Berlin: Walter de Gruyter, 1970.

Nosek, Josef. The European Protura: Their Taxonomy, Ecology and Distribution, with Keys for Determination. Geneva, Switzerland: Museum d'Histoire Naturelle, 1973.

Tuxen, S. L. The Protura: A Revision of the Species of the World with Keys for Determination. Paris: Hermann, 1964.

——. Fauna of New Zealand. Number 9, Protura. New Zealand: Science Information Publishing Centre, 1986.

Wooten, Anthony. Insects of the World. New York: Blanford Press, 1984.

Periodicals

Berlese, A. "Monografia dei Myrientomata." Redia 6 (1909): 1–182.

Ewing, H. E. "The Protura of North America." Annals of the Entomological Society of America 33 (1940): 495–551.

Houston, W. W. K., and P. Greenslade. "Protura, Collembola and Diplura." Zoological Catalogue of Australia 22 (1994): 1–188.

Nosek, J., and D. Keith M. Kevan. "Key and Diagnoses of Proturan Genera of the World." Annotationes zoologicae et botanicae (Bratislava) 122 (1978): 1–54.

Tuxen, S. L. "Monographie der Proturen. 1. Morphologie nebst Bemerkungen über Systematik and Ökologie." Zeitschrift für Morphologie und Oekologie der Tiere 22 (1931): 671–720.

Natalia von Ellenrieder, PhD

Grzimek's Animal Life Encyclopedia