Earliestlnsects

ARCHAEOGNATHA: THE BRISTLETAILS

Bristletails, or Archaeognatha (= Microcoryphia), are the most primitive of living insects, having persisted since at least the mid-Devonian. These cryptic, somewhat cylindrical insects occur under loose bark or stones (Figure 5.1). Except for a rare few, bristletails are nocturnal and typically hide in crevices during the day. About 500 species are known worldwide and live in diverse habitats, including elevations as high as 4,800 meters (15,749 feet) in the Himalayas. The typical diet of bristletails is composed of algae and lichens, which they glean using their monocondylic mandibles as picks. While they are not predatory, some species will scavenge remains of arthropods and some will even eat their own exuviae. Major references for the Archaeognatha include accounts by Paclt (1956), Sturm (1984, 1994a,b, 1995a,b, 1997, 2003a), Kaplin (1985), Mendes (1990, 2002), Sturm and Bach de Roca (1993), Larink (1997a,b), Bitsch and Nel (1999), and Sturm and Machida (2001).

Particularly noteworthy is the mating behavior of Archaeognatha, which have three principal modes of sperm transfer. Species do not technically copulate, and sperm transfer is indirect even though fertilization is internalized. Males transfer to the female droplets of sperm or sper-matophores. Perhaps as a result, the genitalia of archaeog-naths are simple and males and females differ relatively little in the appendicular structures of the genital segments. Couples remain in close contact during mating and have distinctive courtship behaviors. In many machilids the males use a "carrier-thread" (Sturm, 1952, 1955). The male taps the female with his maxillary palpi and once she becomes receptive he secretes a thread of silk. The silken line is drawn out, to which the male attaches droplets of sperm. The female is prevented from moving forward by the male and while the female is facing away from the thread, the sperm droplets are transferred to her ovipositor and eventually into her gono-pore. In the "firebrat," Petrobius, the sperm droplets are set directly onto the female ovipositor and in Meinertellidae the male creates stalked spermatophores that are retrieved by the female (Sturm and Machida, 2001). The ovipositor is used to lay eggs in deep crevices or in holes actually dug by the ovipositor.

The Archaeognatha consists of only four families and approximately 500 species: Meinertellidae, Machilidae, Trias-somachilidae, and Dasyleptidae (the last two are extinct). The Meinertellidae occur predominantly in the Southern Hemisphere (Sturm, 1984). The Machilidae, considered by many authors to possess more primitive features than the Mein-ertellidae (e.g., Sturm and Machida, 2001), are mostly found in the Northern Hemisphere, although a few machilines can be found below the equator in Africa and Asia.

Together with silverfish (Zygentoma, which are discussed later), the bristletails comprise the only surviving orders of primitively wingless insects and were at one time considered a single group, Thysanura (e.g., Remington, 1954), as well as often being united with the Entognatha into a larger grouping called the Apterygota, neither of which are monophyletic. Defining features of the Archaeognatha are the large compound eyes that meet at the top of the head, and the well-developed ocelli, both presumably adaptations for nocturnal living. Other features include "jumping," which is actually a sudden flexion of the abdomen that propels the insect into the air. Also, the meso- and metapleura consist of a single sclerite with large pleural apodemes. Primitively, the archaeognaths have monocondylic mandibles, the head skeleton is composed of paired anterior and posterior plates, "styli" (exopodites?) are usually on the mid and hind coxae, and, unlike abdominal styli, these lack musculature. They also have long, seven-segmented maxillary palpi (even longer than the legs); a terminal filament; eversible vesicles; and abdominal styli (Figure 4.7). Like the Zygentoma, the integument of archaeognaths is generally covered with scales that sometimes form patterns.

Even though the putative archaeognaths are among the oldest fossil insects on record, the overall record of the order is still sparse enough that it has provided little insight into the internal evolution of the order. Among macerated material taken from the mid-Devonian Gaspe fossil beds of Quebec

Thysanura Representative
5.1. Representative basal, wingless insects: a bristletail (Archaeognatha) and a silverfish (Zygentoma). These are modern species belonging to groups that evolved at least as early as the Devonian, 400 mya. Redrawn from Insects of Australia.

were two fragments of a bristletail, a head and thorax, representing the oldest record of insects in North America (Labandeira et al., 1988). The Gaspe fragments, however, preserve only primitive features of the Archaeognatha, and no more conclusive assignment can be made concerning them.

Several species from the Carboniferous and Permian are placed in the extinct genus Dasyleptus (Dasyleptidae) (Figures 5.2, 5.3). These enigmatic Paleozoic fossils were previously considered to represent an extinct order of mono-condylic ectognaths called Monura, closely related to the Archaeognatha (Sharov, 1957). This position was strongly supported by a reconstruction of Dasyleptus (e.g., Kukalova-Peck, 1987, 1997); however, this intrepretation has been challenged (e.g., Kaplin, 1985; Bitsch and Nel, 1999; Rasnitsyn, 1999), and, instead, individuals of Dasyleptus appear to be typical juvenile silverfish, albeit larger. Rasnitsyn (1999) has recently reviewed the family Dasyleptidae.

The earliest Mesozoic fossils of the order are Triassomachilis uralensis (Triassomachilidae) from the Upper Trias-sic of Russia (Sharov, 1948). Triassomachilis has at times been excluded from the Archaeognatha (e.g., Kukalova-Peck, 1991; Sinitshenkova, 2000c). The fossils are poorly preserved but do exhibit the dorsal, thoracic hump typical of Archaeognatha as well as annulated, abdominal styli and a terminal filament in addition to the long cerci. However, Sharov's renderings of the fossils show relatively small compound eyes that do not meet on the vertex, as well as short maxillary palpi, which are primitive differences from all other Archaeognatha including the much older Dasyleptidae. Paclt (1972) considered the differences of Triassomachilis and other bristletails to be artifacts of preservation misinterpreted by Sharov. The original material should be newly studied to determine the affinities of Triassomachilis and the validity of a family Triassomachilidae.

Dasyleptus
5.2. Dasyleptus brongniarti (Dasyleptidae) from the Permian of Russia was formerly placed in an extinct order, Monura, but is today considered an immature bristletail and a sister group to all other Archaeog-natha. PIN 1197; length 10.5 mm.
5.3. Dasyleptus sharovi from the Early Permian of Elmo, Kansas. MCZ; length 11 mm.

All other fossil bristletails occur in Cretaceous or Tertiary ambers. The earliest is a meinertellid in Early Cretaceous amber from Lebanon, a species typical of the Machiloides group of genera and attesting to the antiquity of this family (Sturm and Machida, 2001). Other Cretaceous amber archaeognaths are known from Burma and New Jersey (Grimaldi et al., 2000a, 2002). Fossil machilids are also known from Baltic amber, while meinertellids are known from Dominican amber (e.g., Silvestri, 1912; Sturm and Machida, 2001). Although described as an archaeognath, the Pliocene fossil Onychomachilis fischeri (Pierce, 1951) is actually a sil-verfish, perhaps near the Nicoletiidae (Sturm and Machida, 2001).

(Figure 4.8). The gonangulum represents a basal differentiation of the second gonocoxa to form a sclerite with three points of articulation - to the ninth abdominal tergum, the first gonapophysis, and the second gonocoxa. Other features of dicondylic insects are the reduced maxillary palpi (primitively five-segmented, but this is also observed within Entog-natha, the Ellipura in particular), and the development of tracheal commissures and connectives in the abdomen (Kristensen, 1981). Thus, despite the great overall similarity of bristletails and silverfish, important but subtle features indicate that silverfish are actually more closely related to winged insects.

ZYGENTOMA: THE SILVERFISH

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