Ptiliidae contains 75 of the smallest insects in Britain, all 1mm or less in length, and includes the smallest British beetle, the ironically-named Nephanes titan (Newman). The common names comes from the distinctive structure of the wings, which often protrude from beneath the elytra when at rest.  Most species are brown or black, and many species are found in decaying organic matter, particularly compost heaps, dung heaps, grass cuttings and rotting wood, where conditions are favourable for the fungal growth that both adults and larvae graze on.

There are three subfamilies on the 2012 British list: Nossidiinae, Ptiliinae and Acrotrichinae.  Nossidiinae has just one British representative, Nossidium pillosellum (Marsham), which is found mainly in rotting elm stumps (Ulmus spp.).

Subfamily Ptiliinae includes 44 species.  Several are known in Britain from very few specimens, including Ptiliolum sahlbergi Flach, Ptiliolum caledonicum (Sharp), Oligella intermedia Besuchet, and Ptilium caesicum Erichson, while Euryptilium gillmeisteri Flach is a relatively new arrival.  Some species, notably Ptilium myrmecophilum (Allibert) and Ptenidium formicetorum Kraatz are associated with ant nests, but most occur in decaying organic matter of various kinds.

Acrotrichinae contains the remaining 30 species, also found mainly in leaf litter, dung, compost and fungi.  Nephanes titan is a member of this subfamily, and is common in old dung heaps.

Thirty-one small (1-3mm) species in Britain, of various body shapes: Hydraena and Octhebius are similar to Helophoridae in general appearance (though without the striate pronotum or extended palps), while Limnebius are more smoothly rounded, akin to Tachyporus species (Staphylinidae).  All crawlers rather than swimmers, they are nevertheless found in and near waterbodies, including in brackish water (particularly Ochthebius).  Many of the aquatic species occur in silt and vegetation at the edges of waterbodies (especially Limnebius species), although several occur in fast-flowing water (particularly Hydraena).  Males of Limnebius and Hydraena have spectacular genitalia.

The 2012 British list has 52 species of Histeridae (1-10mm long), in seven subfamilies.  The family have truncate elytra, leaving the final two abdominal segments exposed, and characteristic geniculate (elbowed) antennae with terminal clubs.  Generally strong-flying, quadrate species, most are carnivorous and are often found on carrion, where they usually arrive late and feed on other scavenging species.  When disturbed, they fold their appendages into grooves on the underside and play dead, often for a long time.

Subfamily Abraeinae contains nine small (1-3mm) species, mostly associated with wood in various states of decay.  An exception is Halacritus punctum Aubé, which is only found under seaweed just above the high tide mark.  Teretrius fabricii Mazur, a predator of Lyctus spp. (Bostrichidae), has not been recorded in Britain since 1936 and may well be extinct here.

Subfamily Saprininae contains 16 2-7mm species.  Most of the Saprinus and Hypocaccus spp. are found in carrion and dung.  Myrmetes paykulli Kanaar is widespread but scarce and found in association with the wood ant Formica rufa L., while Saprinus virescens (Paykull) is a predator, probably of Phaedon and Gastrophysa larvae (Chrysomelidae).

Subfamily Dendrophilinae has 7 British representatives.  Several can be found beneath tree bark, but additionally one Dendrophilus species, D. punctatus (Herbst) is found in bird nests and occasionally in association with the wood ant F. rufa, where it can be joined by Dendrophilus pygmaeus (L.).  Carcinops pumilio (Erichson) is another carrion species (although it is also found in other decaying organic matter, particularly bat droppings and vegetation), while Kissister minimus (Aubé) is usually found amongst grass roots in sandy areas.  Dendrophilus xaveri Marsuel is of Japanese origin, and rarely, briefly established in grain and flour warehouses in port cities, particularly Bristol, London and Liverpool.

Subfamily Onthophilinae includes just two species, Onthophilus striatus (Forster) and O. punctatus (Muller), which can be separated from other Histeridae by the presence of longitudinal keels on the elytra.  Onthophilus punctatus is very localised in England where it is associated with subterranean mammal nests and runs, but O. striatus is common and widespread in dung and leaf litter.

Subfamily Tribalinae has only one British representative, the black 2-3mm length beetle Epierus comptus, found only under beech (Fagus sylvatica L.) bark in Wiltshire and Hampshire.

Subfamily Histerinae includes 16 medium-sized (3-11mm) British species, mostly rounded, and shining black, rarely with red patterning.  Many are common in dung, carrion and rotting fungi, but Hister quadrinotatus Scriba and H. illigeri Duftschmidt are thought to be extinct in Britain.  Margarinotus marginatus (Erichson) is a specialist of mole nests and is thus likely to be under-recorded. Hister unicolor Linnaeus and Margarinotus ventralis (Marseul) are most likely to be encountered in dung on pasture, whereas Margarinotus brunneus (Fabricius) is most typically found in carrion and Margarinotus merdarius (Hoffmann) in garden grass and compost heaps or in birds' nests in tree cavities. The rarely-encountered and usually strikingly red and black Hister quadrimaculatus Linnaeus, is restricted to the English coast in Hampshire and Kent.

Subfamily Haeterinae has only one British species, Haeterius ferrugineus (Olivier), a rare 1-2mm reddish beetle associated with ant nests, particularly those of Formica sanguinea Lat. in southern England.

One species in the British Isles, the 4-6mm Sphaerites glabratus Fabricius.  Shining black, with brassy metallic reflections, S. glabratus is found in coniferous forests, mainly in decaying fungi but also at sap runs and in dung or carrion.  The pygidium extends beyond the truncated ends of the striate elytra.

70 British species, 1.5-38mm long, split between aquatic and terrestrial lifestyles. In general, members of the subfamily Hydrophilinae (but also Coelostoma orbiculare (Fabricius) and some of the genus Cercyon) are aquatic, but the subfamily Sphaeridiinae are terrestrial, in dung and other organic detritus.  Most of the Cercyon spp. and all Sphaeridium are associated with dung, while still more Cercyon species occur in wet debris such as rotting seaweed.  The maxillary palps of many species are elongate, in some cases longer than the antennae, which have instead been modified for use as breathing apparatus.  The aquatic species are generally poor swimmers, moving more by walking underwater than by actively diving or swimming.

There is only one British species, Spercheus emarginatus (Schaller), formerly widespread in southeastern England but which is probably now extinct in the British Isles.  The beetle is 6mm long, largely medium brown to a paler yellow-brown, and the front of the head is deeply notched. It is usually found in ditches and stagnant water around the roots of aquatic plants, particularly reeds.  The aquatic larvae have obvious protuberant warts on the sides of the abdomen.

One British genus, Hydrochus, containing 7 species of crawling aquatic beetles.  Generally sluggish, they play dead when disturbed and are correspondingly difficult to find in the field.  Adults are elongate, with heavily-pitted elytra and pronotum, and are most often found in plant litter in well-vegetated ponds and bogs.

One British species, Georissus crenulatus (Rossi), which is a local species found on, and usually caked in, the drying mud and silt of river banks and coasts.  Somewhat resembling a miniature bloody-nosed beetle (Timarcha tenebricosa (Fabricius)), G. crenulatus is 1-2.2mm long, all black, with clubbed antennae and striate elytra.  Elytra have punctuate stria, with raised or tuberculate intervals.

One British genus, Helophorus, with 20 species.  Small (2-7mm) elongate beetles, with the body outline constricted between the elytra and pronotum, they have long palps, similar in size to their antennae and 7 longitudinal grooves on the coarsely-granulate pronotum.  The genus is characteristic of impermanent ponds, often ephemeral pools on grassland, although a few species are associated with dry ground.  Dissection of males is very useful for reliable ID.

A large family (362 species on the 2012 edition of the British list) which includes some of the largest and showiest British beetles, reaching 30mm long.  With such a large group – there are more carabids on the British list than there are breeding bird species - carabids can be found almost anywhere, but riparian and coastal habitats seem particularly species-rich.

Traditionally sub-divided into many subfamilies, the 2012 British list recognises only four: Cincindelidae (five species of tiger beetle), Brachininae (two species of bombardier beetles), Omophroninae (one species), and Carabinae (the ‘traditional’ ground beetles, 354 species).

The one British species of Omophroninae is Omophron limbatum (Fabricius), a semi-aquatic species found burrowing in sand around flooded sand-pits.  Rather more rounded than most carabids, the species is a straw-yellow colour with darker patterning, reminiscent of a 14-spot ladybird (Propylea quattuordecimpunctata (L.)).

The two species of Brachininae, the bombardier beetle Brachinus crepitans (L.) and the streaked bombardier beetle B. sclopeta Fabricius, both of which are famous for their ability to expel a 100°C jet of chemicals from the anus to deter predators.  This is produced when hydroquinone and hydrogen peroxide from separate glands are mixed, and the resulting explosion can be heard as an audible crackle.  Both species are 6-10mm long and have an orange head and thorax, and blue-green elytra, B. sclopeta with an orange streak down the elytral suture.  Brachinus crepitans is found mainly on calcareous grasslands, arable field margins and chalk quarries, with most modern records from coastal areas, while B. sclopeta has always been rare and is now only found on a handful of brownfield sites in London.

The Cincindelidae were previously raised to family level, but have been regarded as a subfamily within Carabidae since 1974.  The five British species are relatively large (8-19mm) and brightly-coloured.  Diurnal predators, they can be found running at speed in open, dry situations primarily in spring and early summer.  Larvae lurk at the bottom of vertical tunnels which act as pitfall traps for small invertebrates.

Carabinae is by far the largest subfamily, and includes both the largest and most frequently encountered species, such as Carabus violaceus L., Carabus problematicus Herbst, Nebria brevicollis (Fabricius), and Pterostichus madidus (Fabricius).  Most are ground-dwelling predators or scavengers, but others burrow in the soil (tribes Broscinae and Scaritinae), forage on tree branches (various Dromiini), or in the intertidal zone (Aepus marinus (Strom) & Aepus robinii (Laboulbene)).  Most are largely nocturnal, but several large-eyed species are diurnal (Elaphrus and Notiophilus spp. in particular)

Many species have pronounced habitat preferences, and coastal habitats, upland moorland, woodland and riparian shingle all have distinct faunas.  Several come to light, particularly Ophonus spp.