APTERYGOTE

broadly, any of several types of small, primitive, wingless insects that belong to the subclass Apterygota (class Insecta) and are distinct from members of the subclass Pterygota, which contains the winged insects. Recent taxonomic revisions have raised three groups formerly classed among the apterygotes to the status of classes, parallel to the class Insecta. Thus the strict sense of apterygote no longer includes them but refers only to a subclass of Insecta that includes the silverfish, firebrats, and bristletails (order Thysanura) and the closely related Microcoryphia. The reclassified groups no longer considered strictly apterygotes are the Collembola, commonly known as springtails, the Diplura, or forktails, and the Protura. The true apterygotes, which are thought to be the evolutionary progenitors of winged insects, include the orders Thysanura, Microcoryphia, and the extinct Monura. These two groups are distinguished from one another by the positioning of their mouth apparatus: externally (ectognathous) in the Thysanura, Microcoryphia, and Monura and internally (entognathous) in the Collembola, Diplura, and Protura. Other taxonomically significant distinctions are based on the position of eyes, the number of body segments, and the structure of legs and antennae. All of these organisms may be referred to simply as hexapods. The extant hexapods range in size from the Protura, which attain a maximum length of 2 mm (0.08 inch), to some Diplura that may reach 50 mm; most are under 10 mm. Most are yellow or white in colour, and all but Protura and Collembola have single or double tail filaments. Collembola are the most various and widely distributed, with over 3,500 known species. Antennae and optical structures also differ among these varieties. All of the hexapods inhabit soil, leaf litter, and other forms of organic debris. Since the epidermal layers of the hexapods are thin and allow free passage of air and water vapour, the presence of moisture is essential to their survival. Hexapods generally feed on fungi, decayed vegetation, algae, and spores, but silverfish also consume carrion and even paper products. Apterygotes are preyed upon by certain species of flies, beetles, and ants, and occasionally by fish and reptiles. The bodies of all hexapods are composed of three sections (except those of the extinct Monuran group, which contained 14 segments): the abdomen, thorax, and head. The thorax is in turn divided into three segments, each bearing one pair of legs that terminate in claws. Most breathe through their outer layer (cutaneous respiration), though some varieties have developed breathing systems with tracheae and thoracic orifices (spiracles). On the abdomen of the Collembola, appendages that were once limbs have developed into a springlike leaping mechanism called a furcula and a tenaculum that controls the furcula. The Diplura, Microcoryphia, and Thysanura have sensory appendages (cerci) on the abdomen that are also modified limbs. The sensory apparatus of the hexapod head varies widely: Diplura have none at all, while Collembola frequently have simple eyes called ocelli in addition to their other sensory organs, and Microcoryphia have both ocelli and compound eyes. Sexual reproduction is characteristic of the hexapods, although eggs can sometimes develop without fertilization (a process known as parthenogenesis). Typically, sperm is dropped in packets at random by males and retrieved by females, which retain the packets until they are needed for fertilization or until they are lost during a succeeding molt. Apterygote young, sometimes called nymphs, molt periodically with little physical change. Molting sometimes continues throughout their life cycle. Collembola may go through up to 50 instars (stages between molts) with little actual growth taking place between them. Microcoryphia generally molt six times during a life span, while the total for Thysanura is often over 40. Populations of Collembola, the most common apterygote, are highly sensitive to the condition of microscopic vegetative growth in the soil. Often existing in densities of 100,000 or more per cubic metre of soil, Collembola are susceptible to some insecticides and are normally unaffected by herbicides. Different species of Collembola and members of the same species at different stages of growth tend to inhabit different strata of the soil. Like most soil-dwelling insects, apterygotes may serve a valuable function in soil aeration and enriching processes. Dipluran (Japygidae) broadly, any of the primitive wingless insects of the subclass Apterygota (class Insecta), distinct from the subclass Pterygota or winged insects. Used in this sense, the term apterygotes commonly includes four groups of primitive insects: proturans, collembolans, diplurans (see photograph), and thysanurans. The taxonomic status of these groups, however, remains unsettled. A typical apterygote is wingless and has six legs. Although the presence of six legs was once an important feature in the identification of true insects, three groups commonly known as apterygotes (i.e., proturans, collembolans, diplurans) are now considered by some entomologists to be offshoots from the main insectan stem of evolution and have been given independent taxonomic status as classes equivalent to the class Insecta. The term apterygotes, therefore, has been redefined to include only those groups thought to be ancestors of pterygotesi.e., the thysanurans (e.g., silverfish, firebrats, and bristletails), together with the Microcoryphia (a group closely related to the Thysanura) and the extinct Monura. For completeness of discussion, however, and because of the similarities of these primitive hexapods, the proturans, collembolans, and diplurans, as well as the thysanurans, microcoryphians, and monurans, are included in this article. Additional reading K. Christiansen, Bionomics of Collembola, A. Rev. Ent., 9:147178 (1964), a concise review of the early literature; A.D. Imms, Imm's General Textbook of Entomology, 10th ed. rev. by O.W. Richards and R.G. Davies, 2 vol. (1977), the standard textbook; S.M. Manton, Introduction to Classification of Arthropoda, in R.C. Moore (ed.), Treatise on Invertebrate Paleontology, pt. R, Arthropoda 4 (1969), a brief coverage of the subject; Z. Massoud, Monographie des Neanuridae, Collemboles Poduromorphes pices buccales modifies, Biologie de l'Amrique Australe, 3:7399 (1967), an important revision and classification of Collembola with sucking mouthparts; J. Paclt, Biologie der primr flgellosen Insekten (1956), an extensive review of earlier literature and research; J.T. Salmon, An Index to the Collembola, 3 vol. (196465); A.G. Sharov, Basic Arthropodan Stock: With Special Reference to Insects (1966), an excellent work that is based on personal paleontological research and emphasizes phylogenetic relationships from worms to winged insects; S.L. Tuxen, Protura (1964, reissued 1986), a taxonomic coverage of this group. Keith Arthur John Wise