1 BEETLES HAVE BODY ARMOUR
Aristotle is credited with coining the term Coleoptera, the name we still use today for the beetle insect order. From the Greek words koleos (‘sheath’) and pteron (‘wing’), it sums up beetles’ most successful innovation – wing cases, or elytra. These are toughened and often ridged or corrugated. In addition, they are curved to fit neatly together, snug over the abdomen, protecting the delicate membranous hind wings furled beneath. But with the flick of a few muscles, the elytra hinge forwards, large hind wings unflap and the beetle is airborne.
The first beetloids evolved from an ancestor they shared with the lacewings, and the earliest beetle fossils, which date from about 300 million years ago, had more flattened, wing-like, ill-fitting elytra. These primordial wing-cases show a distinctive net-like tracery of ridges and struts. Today, members of the primitive beetle family Cupedidae still have this archaic elytral reinforcement.
The other major armoured component of a beetle’s body is the broad shield behind the head. Called the thorax by the uninitiated, this is really just the upper plate of the first segment of the thorax, yet it dominates the beetle’s forebody. Entomologists refer to it as the pronotum. Its domed, flattened or flanged form combines with the elytra to give beetles their sleek outline. For millions of years, this is what has enabled them to hide in tight crannies or under loose bark or logs, or to push into root thatch, soil, carrion, dung or between the gills of mushrooms.
3 BEETLES LEAD A DOUBLE LIFE
If we know little about beetles, we know even less about their larvae – yet it is the grubs that do the eating and growing. At the end of its ‘larvahood’, a beetle maggot undergoes a magical transformation, in which most of its internal organs are dissolved and reassembled to create the complete, finished, winged adult. This process takes place in the pupa, or chrysalis, and is called holometabolism (complete metamorphosis). It is in stark contrast to the gradually increasing nymph growth of insects such as grasshoppers, leaf bugs, cockroaches and earwigs, which show hemimetabolism (incomplete metamorphosis).
This split between a sedentary, self-feeding embryo stage and an active adult stage that disperses by flight is another of the beetles’ secrets of success. Because these insects have divided their life histories into two halves, the adult beetles often live according to different seasonal time frames, and in completely different habitats or ecological niches to their larvae. They thus neatly avoid the often-crushing pressure of competing with their own offspring. For instance, the larvae of the vine weevil, Otiorhynchus
sulcatus, are subterranean rootfeeders, whereas the adults live in trees, nibbling leaf margins into distinctive stamp-edge perforations.
The fat oil beetl eetles that waddle about Britain’sBrit chalk downs, grassy banks nks and sandy undercliffs are anotherer exampleex of beetles with dramatically split personalities.per During their lumbering perambulations, ns, the adults might chew a buttercup leaf or two,, butbu their larvae live in a parallel universe. The many y thousands of oil beetle eggs hatch into tiny agile larvae called triungulins, which race up plant stems and flowers to cling to flying insects. A few lucky ones will attach to a female solitary bee and get taken back to her small nest burrow in the warm soil, where they transform into a second larval stage – legless blobs that start devouring bee grubs and nectar and pollen stores.
4 BEETLES COME IN ALL SIZES
At an implausible 0.3mm long, the Bolivian feather-wing beetle, Scydosella musawasensis, could scurry around quite happily within this letter ‘O’. It is smaller than many single-celled animals. At the other end of the spectrum, the huge Brazilian longhorn, Titanus giganteus, may reach nearly 17cm and will only just sit in the fully extended palm of your hand; if goaded, it can break a pencil in its jaws. This gives a size differential of at least 130 million times. Other giants include the shorter but bulkiier goliath beetles ( Goliathus s) of Africa, at up to 11cm, and thee elephant beetles ( Megasoma) of Souuth America, which reach nearly 14cm.. But unlike the more fammiliar mammalian giants, suchs as whales, whose liife in the oceans has allowwed them to evolve supermaassive size, aquatic beetles are much smaller than their terrestrial counterparts. The biggest water beetle ever found – the ducal, Megadytes ducalis, of Brazil – is only 5cm long. This is for complex reasons, to do with the need for diffusional gas-exchange gradients across small-bodied organisms that lack central lungs, heart and haemoglobin-based blood systems to transport oxygen.
Most beetles, though, are at the diminutive end of the scale. The average across all known species is a fraction under 7mm. This does mean that some people have been put off studying the Coleoptera, in favour of bigger, gaudier
insects.
5 BEETLES LIVE VIRTUALLY ANYWHERE
Encyclopedias invariably make the claim that insects occur from seashore to mountain-top. This, in fact, does not apply to all insects – it does to beetles, though. A tiny flat-bodied ground beetle ( Aepus) inhabits silt-filled cracks in coastal rocks well below the highwater mark, which get covered by the incoming tide twice a day. Near the top of the world, North America’s mountain bark beetle, Dendroctonus ponderosae, munches pines as high as the tree-line.
Deep underground, some cave beetles have evolved tiny eyes or none at all, as well as long spindly legs and antennae to feel where they are going, and have become flightless (there’s nowhere to fly to). They eke out a living scavenging on the bits in bat guano, and anything else dead that they find in the darkness. In the karst limestone caves of southern Europe, Leptodirus hochenwartii incubates giant eggs inside its body that hatch into already fully grown non-feeding larvae, which pupate quickly and emerge as adults.
Water beetles live in fresh water, but they remain air breathers. Lacking gills, they visit the surface often to replenish the oxygen carried in a thin air bubble known as a plastron. For beetles, obtaining oxygen from the air, by sucking it into fine tubes all along the body, is more efficient than absorbing it from water using gills. Having made the leap to a terrestrial existence from a marine one over half a billion years ago, all insects had to commit to non-saline environments.
6 BEETLES EAT ALMOST ANYTH HING
Beetles have chewing mouthparts s, and so do their larvae. These are two tough, approximately triangular jaws, which are able to exert powerful biting or gr rinding pressure. There is virtually nothin ng that beetles do not feed on.
As well as scavenging in leaf litter, beetles eat plants from the roots to the shoots. They also tackle mammal dung, carrion, other insects and even each other. They graze mould in compost bins and are among the most important com mposting recyclers when it comes to dead wood and fungal deca ay. Some steal food from spiders’ webs. A few species invade ourr homes to infest our stored food and eat our carpets.
Beetles took advantage of the explosion in plant diversityit with the evolution of flowering plants 120–100 million years ago, and now are the second most speciose plant-feeding insects after moth caterpillars. Many of the over 135,000 specialist plant-feeders in the important beetle lineage Phytophaga are extremely host-specific. They will feed only on a single plant species – and often on just one part of that plant. In this way they have carved out tiny specialist niches, allowing diversification rather than competition with each other.
This does mean that some beetles have become serious garden or crop pests. Notorious examples include the lily beetle, Liliocerislilii; the Colorado beetle,
Leptinotarsa decemlineata, which devours potatoes; and the western corn root-worm, Diabroticavirgifera, which attacks maize. However, these ‘bad guys’ are the exceptions.