The insects are deservedly recognised as one of the most successful groups of organisms on the planet. Thanks in no small part to their unlocking the ability of flight, insects can be seen today in almost every part of the planet above sea level. But not all insects, of course, are flighted; many remain firmly on the ground. A large proportion of these are the descendents of flighted ancestors that returned to a terrestrial existence but there are also some whose ancestors never took to the skies. For most people, the most familiar of these original land-huggers are likely to be the silverfish of the family Lepismatidae.
Silverfish are long-bodied insects with a covering of reflective scales—hence the 'silver' part of their name. The 'fish' part probably refers to the manner of their movement; speaking from my own experience collecting them, these buggers move fast, slipping along the ground like a silver minnow. There are over 250 known species of Lepismatidae (Mendes 2002); probably many more remain to be described. They comprise over half the known species of the insect order Zygentoma (sometimes referred to as the Thysanura though most current entomologists tend to avoid that name due to its previous history referring to a now-obsolete grouping of the Zygentoma with the superficially similar Archaeognatha); the other families in the order are commonly subterranean and less commonly encountered by the average person. The highest diversity of silverfish occurs in tropical and subtropical parts of the world, particularly in arid or semi-arid regions. Adaptations of the rectal epithelium allow silverfish to absorb moisture straight from the atmosphere (or, to put it another way, they drink through their butt), making them ideally suited to tolerating the dryness of deserts. They are also suited to tolerating the relatively dry habitats offered by the interiors of human houses and several species have become our associates (in cooler parts of the world, these synathropic species are often the only lepismatids around). These include the common silverfish Lepisma saccharina and the giant silverfish Ctenolepisma longicaudata. The firebrat Thermobia domestica is a colourfully patterned human associate that likes it particularly warm; it is usually restricted to places like the backs of stoves or alongside hot-water cylinders where it can find the heat it craves. Being detritivores (that is, they feed on dust), human-associated silverfish are usually quite innocuous though they may cause problems if their numbers get too high or if they get into stored foodstuffs.
In areas where they are native, silverfish may be quite diverse. Watson & Irish (1998) conducted a study of an area of the Namib Desert that was home to eight different species of silverfish. They found a tendency for the species to differ in their preferred microhabitat within the area: some were restricted to the upper parts of the sand dunes dominating the region, others were restricted to the rocky hollows separating the dunes. Those found in rocky lower zones resembled the familiar human-associated species (indeed, they included members of the same genus as the giant silverfish, Ctenolepisma) in being elongate and slender. In contrast, those species found higher in the dunes themselves were shorter and more flattened with well-developed spines covering the legs. These features allowed the dune silverfish to effectively 'swim' through the sand, using the spines on the legs to dig about and their flattened form to slip between grains.
Mendes, L. F. 2002. Taxonomy of Zygentoma and Microcoryphia: historical overview, present status and goals for the new millennium. Pedobiologia 46: 225–233.
Watson, R. T., & J. Irish. 1998. An introduction to the Lepismatidae (Thysanura: Insecta) of the Namib Desert sand dunes. Madoqua 15 (4): 285–293.