The Diplura are a group of blind, mostly unpigmented soil arthropods that, although often included in the class Insecta, are currently recognized as the insects’ closest relatives. Diplurans and primitively wingless insects most conspicuously share the same tagmosis (head followed by a three-segmented thorax with three pairs of walking legs and a segmented abdomen devoid of walking legs), as well as cerci on the ultimate abdominal segment and vestiges of abdominal appendages (styli and coxal vesicles; Fig. 1D) on pregenital segments. Diplurans, however, are clearly distinguished from true insects by the maintenance of multisegmented
FIGURE 1 Morphological characteristics of the Diplura, exemplified by Metajapyx sp. (Japygidae: A, C, E) and Campodea sp.
(Campodeidae: B, D, F), SEM. (A and B) Frontal view of the
head, showing entognathy. (C) Lateral view of pleuron of abdominal segment III, showing three isolated pleurites (asterisks) and abdominal spiracle (sp). (D) Ventral view on the first two abdominal sterna, showing subcoxal appendages (sap) flanking transverse rows of glandular setae on first abdominal segment, as well as styli (arrow) and eversible vesicles (ev) on second abdominal segment, (E) Pincer-shaped cerci, ventral view. (F) Male genital papilla (gp). Abbreviations: ad, admentum; an, antenna; ce, cercus; ev, eversible vesicle; gp, genital papilla; 1p, labial palp; mxp, maxillary palp; pp, processus palpiformis; sap, subcoxal appendage; s1-10, abdominal sternites 1-10; t3, tergite of third abdominal segment .
antennae devoid of a chordotonal sensory organ in the pedicel (and with all antennomeres showing intrinsic muscles except for the ultimate segment) and by the lack of an appendicular ovipositor and of a median unpaired caudal filament. The name-giving character of the Diplura (two-pronged bristletails) accordingly refers to the presence of only one pair of caudal appendages, the cerci (Fig. 2 ).
Diplurans are most frequently found in humid soils of forests and grasslands, often under rocks or rotting wood and leaf litter, but also at depths of about 1 m below the soil surface. Some species are known to inhabit caves and show typical adaptations to troglo-bitic life habits (see Cave Insects). Others are collected in nests of ants and termites, but obligate nidicolous life habits have not yet been observed. The body length of the omnivorous species generally ranges in adults from 3 to 12 mm, whereas predacious members of the subgroup Japygoidea, especially the Chinese Gigasjapyginae and South American Dinjapygidae, may even reach nearly 60 mm in length. The Diplura are distributed worldwide, predominantly in tropical and subtropical regions, and only seem to be entirely lacking in deserts, polar regions, and mountainous regions above 3500 m altitude. Approximately 1000 species are known to date and are
FIGURE 2 Dipluran (Parajapyx sp.).
assigned to 10 families (Fig. 3) ; the majority of species belong to the Japygidae and Campodeidae. Dinjapygidae and Evalljapygidae are known from the New World only, whereas Heterojapygidae are restricted to the Australian region and Madagascar.
FIGURE 3 Consensus tree inferred from 52 morphological characters depicting the relationships among the 10 currently recognized families of the Diplura. Numbers in brackets refer to the number of species described thus far (compiled by Alberto Sandro, Museu Valencia d’Historia Natural; see http://insects.tamu.edu). The traditional classification into three major subgroups is highlighted in gray.
COMPARATIVE ANATOMY AND CLASSIFICATION
Diplurans are commonly considered as primitive hexapods, but their general morphology is characterized by adaptations to living conditions in soil crevices, that is, a prognathous head; absence of eyes and pigmentation; and a soft, elongate, more or less dorso-ventrally flattened body. Yellowish to brownish pigmented areas are shown by the predacious japygoids only and are restricted to hardened regions of the mouthparts, the cerci, and postabdominal segments. Primitive features mainly seem to refer to the internal anatomy and have especially been pointed out for the tracheal and circulatory systems. The excretory system may also show the plesio-morphic state within the ” cerci-bearing hexapods ” (Cercophora, i.e., Diplura and Insecta) in being composed of a circlet of merely 6- 16 short Malpighian papillae (instead of tubules), in addition to paired labial kidneys and the midgut epithelium.
Variable ovary structures have provoked a debate over whether diplurans form a monophyletic group. Current cladistic analyses of both molecular and morphological data, however, unambiguously support dipluran monophyly. Unique traits of the Diplura especially concern aspects of their entognathy and mouthparts (Fig. 1A,B) , monocondylic pivot joints between the proximal podomeres of the walking legs, presence of only 10 abdominal segments, and the formation of a retractile, sexually dimorphic genital papilla between abdominal segments 8 and 9 that bears the gonopores in both sexes
( Fig. 1F ).
The major subgroups within the Diplura are traditionally classified according to the shape of their cerci, which are segmented and usually filiform in the “Rhabdura” (Projapygoidea and Campodeoidea) and unsegmented and pincer-shaped in the “Dicellurata” (Japygoidea; Fig. 1E). An alternative classification unites Projapygoidea with Japygoidea in the “Japygina” and separates the campodeoid diplurans in the “Campodeina.” This latter classification is presently favored by some molecular data albeit with depauperate taxon sampling. Morphology, in contrast, presently fails to unravel the affinities of the projapygoid diplurans (Fig. 3), because they show a peculiar mix of characteristics of both Campodeoidea and Japygoidea. Apart from segmented cerci, characters shared by projapygoids and campodeoids especially concern the presence of a mandibular prostheca, tubular or flattened projections on the labium (processus palpiformes; Fig. 1B), formation of so-called subcoxal appendages on the first abdominal segment (Fig. 1D), as well as an increased concentration of the ventral nerve cord with only seven abdominal ganglia. In contrast, projapygoids and japygoids particularly share a subdivision of the ovary into a variable number of ovarioles, paired spiracles on each of the seven pregenital segments of the abdomen (Fig. 1C), and combed appendages on the maxillary lacinia.
The monophyly of both Japygoidea and Campodeoidea is beyond any doubt, the former being characterized by their predacious life habits and pincer-shaped cerci, the latter by the absence of abdominal spiracles, among other features. Projapygoids are uniquely distinguished from all other diplurans by abdominal spinning glands that open to the exterior via a retractile orifice (fusule) on the terminal segment of the cerci. Certain characters of the mouthparts, components of the trunk endoskeleton, and details of the male genital papilla nevertheless indicate that they form a paraphyletic grade with respect to either Campodeoidea or Japygoidea.
The fossil record of diplurans is poor and contributes few insights into their phylogeny and evolution. Despite a likely Devonian origin, the oldest known occurrence is a Mesozoic japygid from the Lower Cretaceous of Brazil that corresponds in all reconstructed details with extant taxa.
BIOLOGY
Dipluran reproduction and development have not yet been extensively studied thus far but are known in detail for a few species of the Japygidae and Campodeidae. Sperm transfer is indirect by stalked spermatophores filled with bundled spermatozoids, which are randomly deposited on a substrate. Some males ” recycle ” (feed) their spermatophores when ignored by females. These eventually pick up the spermatophores and become internally fertilized. The role of gonopod-like appendages flanking the genital papilla in some male japygids remains unclear. Sexual dimorphism, especially in male antennae and/or cerci, is characteristic for some species but is likewise obscure, since no courtship behavior has ever been observed.
Female campodeids produce clusters of about 10 eggs and suspend them on leaf litter or under stones. The larger egg clusters of japygids are suspended in soil cavities and guarded by the mother. Brood care is continued for the hatchlings. Immobile, non-feeding prelarvae hatch after 1-3 weeks. This pupoid stage takes about 2 days and finishes with the molt of the prelarva. The newly molted immature is fully mobile and readily starts feeding. Development of the immature forms is epimorphic, and molting continues after maturity is achieved. The individual life span is supposed to take about 3 years.
Projapygoids and campodeids are generally omnivorous and feed on both live and decaying vegetation or animals, but predatory activities have also been observed. The cerci of some projapy-gids seem to be similarly involved in capturing prey as in japygids by using the secretions of the cercal spinning gland for fixing prey. Prey capture in japygoids does not necessarily rely on the pincer-shaped cerci. Some specimens were observed to catch their prey with the hook-shaped lacinia of the first maxillae and to involve their cerci, if at all, to maintain the prey. Few herbivorous species are recorded as agricultural threats and are listed as potential pest species (e.g., Octostigma herbivora).
COLLECTING AND SPECIMEN PREPARATION
Soil-inhabiting arthropods are most efficiently extracted from soil samples by Berlese-Tullgren (or Kempson) techniques, but for diplu-rans this method causes severe difficulties with subsequent species identification. Important taxonomic characters are the number and arrangement of trichobothria and further details of the antennae and cerci, which in rhabduran representatives easily break during the extraction. The best method for collecting diplurans is by hand with a wetted hair brush or, when larger, with forceps. For identification purposes, specimens preserved in 75% ethanol need to be mounted on microscope slides for taxonomic studies. Comprehensive determination keys and a survey of papers needed for species identification were provided by Allen (2002).
