Classification of insects
There are over 24,000 species of insects in Britain and, globally, well over one million species have been described to date. The classification of insects can be complex but it is very important to group and identify insects so that they can be studied reliably. Insects, like all animals, are classified using a hierarchical system of classification. Here is an example using the marmalade hoverfly, Episyrphus balteatus:
Kingdom: Animalia (all animals)
Phylum: Arthropoda (all arthropods)
Class: Insecta (only the insects)
Order: Diptera (only the true flies)
Family: Syrphidae (only the hoverflies)
Genus: Episyrphus (only a sub-set of the hoverflies)
From this hierarchy we derive the scientific name for the marmalade hoverfly – Episyrphus balteatus. This ‘binomial nomenclature’ allows there to be a two-word, universally recognised name for each species, which avoids the confusion that might arise from using a common name in one particular language or from a particular region. Traditionally, the genus and species should be written in italics.
There are also groupings that fit between the traditional ranks of the hierarchy which are often included because they are evolutionarily important. For example, insects in the wider sense constitute the subphylum Hexapoda, which separates the arthropods with six legs from others such as centipedes and spiders. Hexapoda is then divided into two classes: the Entognatha includes primitively wingless hexapods such as springtails, while all the ‘true’ insects are subdivided into five major groups also know as superorders, the Apterygota, Palaeoptera, Polyneoptera, Paraneoptera and Endopterygota. You can explore the world of insect classification from this page and learn about the fascinating groups of insects that can be found in Britain.
More discussion about this classification, with a list of more detailed references, can be found in Peter C. Barnard’s book The Royal Entomological Society Book of British Insects, published by Wiley-Blackwell in 2011, and obtainable from the RES. It also contains detailed information on all the 558 families of British insects.
Wingless primitive insects
The Archaeognatha are active, cylindrical insects up to 18 mm long, with long flagellate antennae, and the abdomen bears a long terminal filament flanked by a pair of cerci, giving a three-tailed appearance. The ectognathous mouthparts are notable for the seven-segmented maxillary palps, which are longer than the legs. Much of the body is covered with flat scales, including the caudal appendages, giving the bristletails a shiny appearance.
Silverfish and firebrats
When the Archaeognatha were recognised as a separate order of insects, the remaining group of silverfish, firebrats and their relatives were sometimes still known as the Thysanura, but to avoid confusion with the older broad grouping they are now generally called the Zygentoma. There are many superficial similarities between the two orders, but the dicondylous mandibles clearly place the Zygentoma close to the higher insects.
Insects lacking the ability to fold their wings over the abdomen
Mayflies or upwing flies
Arguably the most primitive of the winged insects, mayflies are generally recognised by their triangular fore wings held vertically above the body at rest. Adults are a familiar sight by the side of freshwater bodies and they are particularly well known to anglers, who have given names to many of the common species, both in Europe and N. America.
Dragonflies and damselflies
Despite being linked with the Ephemeroptera within the Palaeoptera, the Odonata have a very different appearance from the mayflies. The wings are long, narrow and parallel-sided, and the fore and hind wings are the same length. Details of the wing venation are widely used at the higher level classification.
The higher groups of winged insects, the Neoptera, have the ability to flex their wings so that they can be folded flat over the body. The evolutionary advantage of this was probably to protect the wings from damage while crawling through vegetation or other confined spaces; this led to the development of thicker fore wings as protective covers for the hind wings and abdomen seen in many insect groups and culminating in highly modified elytra in the Coleoptera.
These small predatory insects are the most recently discovered insect order, recognised only in 2001. Fewer than 20 species are known, all in southern and east Africa. Because they hold the distal ends of their tarsi off the ground when walking they have been dubbed heelwalkers; their phylogenetic relationships are not yet clear though they have strong links with the order Grylloblattaria, and some authors place the two groups together in the Notoptera.
Grasshoppers, crickets and bush-crickets
Even in their restricted sense, excluding the Dictyoptera, Mantodea and Phasmida, the Orthoptera still form the largest of the Polyneopteran or ‘orthopteroid’ orders; they are probably most closely related to the Phasmida. Included within the group are the locusts, which are among the most devastating crop pests in many parts of the world; hence the Orthoptera as a whole are relatively well studied, at least by economic entomologists.
The Phasmida, also known as Phasmatodea or Phasmatoptera, are well known for their mimicry of twigs or leaves, and even their eggs resemble plant seeds, sometimes with sculptured surfaces. They are clearly related to the Orthoptera; one group of Tettigoniidae in Australia shows a remarkable similarity to the stick-insects, but they also have many unique morphological and biological characters.
Although the Plecoptera clearly belong in the ‘orthopteroid’ group of orders, they are probably most closely related to the Zoraptera and Embioptera, these three orders sometimes being placed in the superorder Plecopterida. However, they are apparently an early offshoot because their nymphs are always aquatic, unlike the entirely terrestrial nymphs of the other two orders.
The terminal, forceps-like cerci make earwigs easily recognisable to most people, and the common Forficula auricularia is almost cosmopolitan in distribution, at least in the cooler regions of the world. Although Dermaptera clearly belong in the ‘orthopteroid’ group, their exact relationships with the Orthoptera and other orders in this group are not clear, although the Dictyoptera may well be the closest.
Cockroaches, termites and mantids
The exact relationships of the cockroaches, termites and mantids have been debated for some time, and all three have previously been considered as separate orders. The cockroaches (Blattodea, Blattaria, or even Blattoptera) were recognised as being close to the Mantodea , but now the termites (Isoptera) have been shown to be nested within this clade, called the Dictyoptera, and the three groups can be treated as suborders therein.
Also known as Embiidina, the webspinners are a fairly small group with around 450 known species, found mainly in the tropical regions of all continents; they also extend into some temperate parts of the USA and southern Europe. Around a dozen species are found in southern Europe in the families Embiidae and Oligotomidae. The Embioptera, together with their sister group Zoraptera, seem distantly related to Plecoptera, but also share some characteristics with the Phasmida and Orthoptera.
Also known as Grylloblattodea, this is a small and obscure order with fewer than 30 species found only in the western USA and Canada, and parts of China, Korea and Japan. Commonly known as rock crawlers or ice crawlers, they are usually found at high altitudes, and share some characters with the orders Dictyoptera and Phasmida.
An obscure order with only around 35 known species principally in tropical regions, living in rotting wood and feeding mainly on fungal hyphae. They have no common name, and live in small colonies, superficially resembling termites and Psocoptera, but they are probably the sister group of the Embioptera.
At first sight the Paraneoptera might appear to consist of a rather disparate group of insect orders, namely the true bugs, lice, book lice and thrips, but the monophyly of this group is generally accepted on both morphological and molecular characters. The exact relationships between the constituent orders are not as clear, though the Psocoptera and Phthiraptera probably form a monophyletic pair, the superorder Psocodea.
Unfortunately there is an increasing tendency to call any insect a ‘bug’, so the term ‘true bug’ has to be used to signify a hemipteran. Although the affinities of the Hemiptera with other insect orders need further clarification, there is no doubt about the monophyly of the order. The formation of the mouthparts into the elongated rostrum is unique to the Hemiptera and is not found even in other groups with sucking mouthparts.
Sucking and biting lice
Lice are unique among the insects in being ectoparasitic in both the juvenile and adult stages; indeed all stages of the life-cycle are spent on the host’s body. Although they are very common, few people study the group and many people never see them at all unless they study the bird or mammal hosts.
Booklice and barklice
The Pscocoptera have long been regarded as phylogenetically close to the Phthiraptera, but recent research has suggested that the Phthiraptera may even be a subgroup of the Psocoptera, within the wider group of Psocodea. Not surprisingly there is some reluctance to accept this view fully, because it would have implications for the integrity of the Psocoptera.
The thrips are probably most closely related to the Hemiptera, based on certain mouthpart structures and other characters. Their minute size means that they are a little known group, and many people are surprised to learn how many species there are. Some feed on fungi, others on higher plants, and a small number of these can be agricultural or horticultural pests. Because of its ancient Greek origin, the name thrips is both singular and plural: there is no such word as ‘thrip’.
The larvae look very different to the adults, and undergo metamorphosis in a pupa where the wings develop internally.
Adult beetles have a distinctive appearance, with the hard elytra covering most of the body and meeting down the mid-line, which makes them easy to recognise but this apparent uniformity hides an enormous variety of life histories. There are numerous variations in larval forms and feeding habits, and beetles can be found in a wide range of habitats. In turn this makes them important in ecological research and environmental surveys.
The exact relationships of the Diptera with other insect orders are still open to question; undoubtedly they belong in the loosely defined group of Panorpoids, which includes the Lepidoptera, Mecoptera, Siphonaptera and Trichoptera.
Ants, bees and wasps
This group is arguably the largest and most complex of all the orders of insects and they seem to be only distantly related to the other endopterygotes. There is evidence that they are closer to the Panorpid groups rather than the Coleoptera but the picture is far from clear. Although their monophyly is not in doubt, it is hard to give defining characters for the whole group that are not plesiomorphic.
Butterflies and moths
The close relationship between the Lepidoptera and Trichoptera is often cited as one of the classic sister-group relationships at ordinal level in all the insects. The flattened scales on the wings of butterflies and moths are clearly derived from the normal hairs found on caddisfly (and many other) wings; indeed such scales occur sporadically in several other insect groups, even in the Archaeognatha, for example.
Traditionally the Mecoptera have been regarded as closely related to the Diptera and, although this is still accepted, there is increasing evidence that at least one family, the Boreidae, has a closer relationship with the Siphonaptera. If this is true, then it might have implications for the monophyly of the Mecoptera, or else it might mean that the Siphonaptera would be reduced to a subgroup.
In early texts the Megaloptera were included within the Neuroptera, together with the Raphidioptera, but they have long been recognised as a distinct order. The monophyly of this group of three orders, sometimes termed the Neuropterida, is not in question, but the exact relationships between them are yet to be fully resolved. Adult Megaloptera have broad flattened heads with biting mouthparts, and two pairs of very similar wings; all the larvae are freshwater predators.
Since the time of Linnaeus the Neuroptera were often treated as a convenient receptacle for the diverse groups of endopterygote insects that did not fit anywhere else; the modern view is that the group is closely related to the Megaloptera and Raphidioptera . To avoid confusion with the name Neuroptera in its older broad sense, some authors employ the name Planipennia for the modern restricted usage.
Of the three orders in the Neuropterida, the Raphidioptera may be the most basal, leaving the Neuroptera and Megaloptera as sister-groups. They get the common name of snakeflies from their elongate pronotum which is mobile and can be elevated such that the insect resembles a snake about to strike at its prey; the German name of Kamelhalsfliegen, or camel-neck flies, is equally appropriate. Both the adults and larvae are predatory, though adults are also reported as feeding on pollen.
Being obligate ectoparasites on mammals and birds, adult fleas have become highly specialised in their morphology and life histories; as a consequence their affinities with other insect groups have long been debated. Traditionally they have been linked with the Diptera or Mecoptera; recent morphological and molecular studies have suggested a close relationship with the family Boreidae within the Mecoptera.
The Strepsiptera are one of the strangest insect groups, little known to most people, and hardly seen even by entomologists unless they make a special effort to study them. All are obligate endoparasites in other insects and, as so often in parasitic groups, there are many unique morphological and biological adaptations.
Caddisflies or sedge flies
Trichoptera are closely related to the Lepidoptera, forming the group Amphiesmenoptera, and the two orders are often cited as being one of the best examples of a sister-group pair at the ordinal level.