Box 9.3. Archaeognatha (bristletails) and Zygentoma (Thysanura; silverfish)
The Archaeognatha and Zygentoma represent the surviving remnants of a wider radiation of primitively flightless insects. These two apterygote orders superficially are similar, but differ in pleural structures and quite fundamentally in their mouthpart morphology. The thoracic segments are subequal and unfused, with poorly developed pleura. The abdomen is 11-segmented, with styles and often protrusible vesicles on some segments; it bears a long, multisegmented caudal appendix dorsalis, located mediodorsally on the tergum of segment 11, forming an epiproct extension lying between the paired cerci and dorsal to the genitalia. In females the gonapophyses of segments 8 and 9 form an ovipositor. Fertilization is indirect, by transfer of a spermatophore or sperm droplets. Development is ametabolous and molting continues for life. For phylogenetic considerations see section 7.4.1.
The bristletails are primitively wingless insects, with some 500 species in two extant families. They are moderate sized, 6–25 mm long, elongate, and cylindrical. The head is hypognathous, and bears large compound eyes that are in contact dorsally; three ocelli are present; the antennae are multisegmented. The mouth- parts are partially retracted into the head, and include elongate, monocondylar (single-articulated) mandibles, and elongate, seven-segmented maxillary palps. The thorax is humped, and the legs have large coxae each bearing a style and the tarsi are two- or three- segmented. The abdomen continues the thoracic contour; segments 2–9 bear ventral muscle-containing styles (representing limbs), whereas segments 1–7 have one or two pairs of protrusible vesicles medial to the styles (fully developed only in mature individuals). The paired multisegmented cerci are shorter than the median caudal appendage (as shown here for Petrobius maritima, after Lubbock 1873).
Fertilization is indirect, with sperm droplets attached to silken lines produced from the male gonapophyses, or stalked spermatophores are deposited on the ground, or more rarely sperm are deposited on the female’s ovipositor. Bristletails often are active nocturnally, feeding on litter, detritus, algae, lichens and mosses, and sheltering beneath bark or in litter during the day. They can run fast and jump, using the arched thorax and flexed abdomen to spring considerable distances.
Zygentoma (Thysanura; silverfish)
Silverfish are primitively wingless insects, with some 400 species in five extant families. Their bodies are moderately sized (5–30 mm long) and dorsoventrally flattened, often with silvery scales. The head is hypognathous to slightly prognathous; compound eyes are absent or reduced to isolated ommatidia, and there may be one to three ocelli present; the antennae are multisegmented. The mouthparts are mandibulate, and include dicondylar (double-articulated) mandibles, and five-segmented maxillary palps. The legs have large coxae and two- to five-segmented tarsi. The abdomen continues the taper of the thorax, with segments 7–9 at least, but sometimes 2–9, bearing ventral muscle- containing styles; mature individuals may have a pair of protrusible vesicles medial to the styles on segments 2–7, although these are often reduced or absent. The paired elongate multisegmented cerci are nearly as long as the median caudal appendage (as shown here for Lepisma saccharina, after Lubbock 1873).
Fertilization is indirect, via flask-shaped spermatophores that females pick up from the substrate. Many silverfish live in litter or under bark; some are subterranean or are cavernicolous, but some species can tolerate low humidity and high temperatures of arid areas; for example, there are desert-living lepismatid silverfish in the sand dunes of the Namib Desert in south-western Africa, where they are important detritivores. Some other zygentoman species live in mammal burrows, a few are commensals in nests of ants and termites, and several species are familiar synanthropic insects, living in human dwellings. These include L. saccharina, Ctenolepisma longicauda (silverfishes), and Lepismodes inquilinus (= Thermobia domestica) (the firebrat), which eat materials such as paper, cotton, and plant debris, using their own cellulase to digest the cellulose.