All about beetles

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Beetles have captured the imagination of people since ancient times. They’ve appeared in paintings, sculptures, gemstones, jewelry, and coins. Ancient Egyptians based amulets on scarab beetles, which were a symbol of the sun. One of their sun gods, Khephri, had a scarab head. You might even have beetle art or jewelry yourself — like, perhaps, the ever-popular ladybug beetle.

We don’t consider them gods, but they’re nearly omnipresent. Just step outside and they’ll be nearby — at our feet, on shrubs, trees and the side of our houses. Indoors, there may be carpet beetles, cellar beetles or larder beetles. Just ask J.B.S. Haldane (1892-1964). A famous population geneticist, he was once asked, regarding his studies of the world, what he had learned about the “Creator.” His reply: “An inordinate fondness for beetles.” He was referring to the fact that beetles comprise 30 percent of all insects and about 30 percent of all other animal species. They’re nearly everywhere. 

Worldwide, there are more than 350,000 known kinds of beetles. North America alone has more than 30,000. Excluding oceans, beetles are found everywhere in the world, including polar regions and deserts. 

Three beetles in the genus Hydrophilus trapped in La Brea Tar Pits, California

Beetles, Hydrophilus sp., trapped in La Brea Tar Pits, Calif. (Didier Descouens / Wiki; cc by-sa 4.0)



Impressive as these figures may be, they’re merely a drop in the bucket, according to scientists, who estimate there are between 5 and 8 million still to be discovered. In fact, new ones are being discovered faster than taxonomists can classify them.

Based on the oldest fossil found so far, beetles date at least to 318 to 299 million years ago (late Carboniferous Period to early Permian Period), long before the age of dinosaurs.

Beetles are in the order Coleoptera (koal-e-OP-ter-uh). The name comes from the Greek koleon, meaning sheath, and pteron, meaning wings — “sheath wing.” This is a reference to the hardened structure of beetles’ forewings.

Physical description
Beetles come in all sizes, ranging from nearly microscopic to alarmingly large! Featherwing beetles (family Otariidae), the smallest species, are only 0.01- to 0.16-inch long (0.3 to 4.0 mm) — about a quarter the size of a grain of rice. Hercules Beetles (family Scarabaeidae), at the other extreme, grow to 6.8 inches (17.5 cm) long (including the horn) — about the size of a dollar bill!

Hercules Beetle, Dynastes hercules ecuatorianus

Hercules Beetle, Dynastes hercules ecuatorianus. (Didier Descouens / Wiki; cc by-sa 4.0) 



Beetles, as with all insects, have an external skeleton. It covers the outside of their body, giving it support and protection. Most beetles have a hard body, although some are soft or leathery. 

Ther’s a great variety among beetles in body shape and color, from boring-drab to brilliantly-bright to highly-patterned. Some are among the most beautiful insects on the planet, wearing spectacular palates of iridescent green-blue-violet or red, gold or copper.

Unidentified Tortoise beetle (Cassidinae) with yellow pronotom, orange stripes, black dots.

Tortoise beetle (subfamily: Cassidinae). (Paulleminh1959 / Pixabay / PD)

Beetle, Chrysochroa fulgidissima, which has metallic colors of red, blue, green.

Chrysochroa fulgidissima (Tamamushi). (Brian Adler / wiki; PD)

Seven-spotted-Lady Beetle, Coccinella septempunctata. is red with a black pronoun and seven spots on elytra.

Seven-spotted-Lady Beetle, Coccinella septempunctata, (Dominik Stodulski / cc by-sa

Beetles have three body sections — head, thorax, and abdomen. It can sometimes be hard to discern all three sections, so just gently turn them on their back to see. 

Head
A beetle’s head contains two eyes, mouthparts and two antennae. The eyes are compound, which means they’re made up of hundreds of individual photoreceptors* that transmit signals via neurons to the brain, which interprets them. Beetles that hunt, such as ground beetles, and those that use their sight for finding mates, such as fireflies, have larger eyes, Beetles that swim on the surface of ponds have divided eyes — the top half sees above the water and the lower half sees below water. This division makes them appear as though they have four eyes.

A beetle’s mouth has no resemblance to that of a human or other mammal and is generally referred to as mouthparts. They have several mouthparts, including hard, and sometimes tooth-like, mandibles (jaws) adapted for biting and chewing.

Most species also have two maxillary palpi and two labial palpi (sing.: PAL-pus). These are finger-like appendages which move food to the beetle’s mouth and can detect smells. Their antennae (an-TEN-ee) are flexible and vary in length and type.

Beetle anatomy (Fiddler beetle, Eupoecila australasiae). (Bugboy5240 / Wiki; CC BY-SA 3.0)



It may be hard to imagine, but beetles have a brain! It doesn’t play a very big role in controlling bodily functions, which are primarily handled by ganglia located in various parts of its body. Instead, it contains clusters of neurons that interpret incoming sensory information.

Here’s something unexpected for us to think about: Beetles may be able to “think,” too, at least in some fashion. Researchers testing ground beetles in a controlled setting found they were capable of learning the best way to avoid aggressive ants by choosing from several options variously presented to them.

Senses
Beetles have tiny, sensitive hairs, called setae, on their body and appendages which can feel the slightest thing that touches them. Their antennae can detect smell, taste and, with some species, vibrations. And, they’re used as feelers, too. Their mouthparts “taste” by detecting the chemical composition of foods they sample — it tells them whether to eat it or not. They hear with a tympanal organ, a thin membrane located behind their head. It “hears” when sound vibrations strike it. 

Thorax
The thorax is located between the head and the abdomen. Three pairs of legs and two pairs of wings are attached to it.

Each leg has a foot with one or two claws at the end. The legs are adapted to the lifestyle of the species. For example, water beetles have long, flat legs for swimming. Beetles that dig have strong front legs that are widened and have spines or horns on them. Flea beetles have strong hind legs for jumping.

Their forewings are distinctive from most other insects because they’re hardened. Called elytra (EL-uh-truh), they’re used as a protective cover for two thin, delicate hind wings. The elytra, in fact, is probably the easiest way for us to identify a beetle as a beetle. That, and the tight, straight line they form down the center of the beetle’s back when it’s not flying.

Common Cockchafer Beetle, Melolontha melolontha, displaying raised elytra and hind wings at take-off.

Common Cockchafer Beetle (Melolontha melolontha), displaying elytra and hind wings at take-off. (Mario Sarto / Wiki; CC)



Common Cockchafer Beetle (Melolontha melolontha), displaying elytra and hind wings at take-off. (Mario Sarto / Wiki; CC)

Elytra provide more than just cover. Their toughness allows some species to crawl into soil and yard debris or bore into trees without damaging their hind wings. What’s more, some species can trap moisture under their elytra, enabling them to live in deserts — they carry their water supply with them. Other species trap air under their elytra and live in water.

Elytra are well-designed as a shield, but they aren’t useful for flight. When their bearers wish to fly, they must raise them up, out of the way (reminiscent of the gull-wing doors on the famous 1970s-era DeLorean cars). This clears the way for the delicate, membranous hind wings, which have been carefully folded underneath, to spread out and take to the air.

Some species fly more than others. Ground beetles, for example, prefer to spend most of their lives on the ground or in water, and there are some of them that have fused elytra and can’t fly, anyway. 

Another noticeable characteristic on the thorax is a shield-like plate, called the scutellum, on the back. It typically forms a small triangle-shape at the base of their folded wings.

Abdomen
The abdomen is made up of a series of segments. Each has a hole called a spiracle through which air can enter and carbon dioxide exit the body. (For more detail about that go here.) It also contains part of the nervous system, digestive system, circulatory system, reproductive system, and the anus, as well as glands that produce pheromones. The color-coded illustration below will help you visualize the beetle’s internal structure.

Color drawing of the internal anatomy of a beetle with an identifying label for each part.

Beetle anatomy. (Bugboy52 40 / Wiki; cc by 2.0)



Reproduction and life cycle
Beetles undergo complete metamorphosis. That means they pass through four life stages in their development: Egg to larva to pupa to adult. The process from egg through adult may take as little as a few weeks or up to several years, depending on the species.

Ladybird beetle larva standing on the green surface of a leaf.

It’s hard to believe this larva is a future ladybug! (© James Benet / iStock)



Hatched “baby” beetles are called larvae (sing.: larva). They have well-defined heads with short antennae. They differ in appearance between species. Some don’t have legs, while others do. But one thing they all have in common is they look very different from how they’ll appear as adults.

For example, the curled, soft, whitish larva of a stag beetle looks nothing like the dark, hard-bodied adult it will become. And, the larvae of ladybird beetles (ladybugs) look somewhat alligator-like (see above).

Larvae are eating machines. Their sole job is to grow and ready themselves for pupation. As they put on size, they periodically outgrow their skin’s ability to stretch any farther, so they shed it in a process called molting. They go through several molts.

Following their last molt, they construct a cocoon around themselves within which they’ll pupate. This is the stage where they’ll transform from larvae into adults. It’s quite magical, comparable to the transformation of a caterpillar into a beautiful butterfly. Pupation may last, depending on the species, from a few days to many months to years. 

Multicolored Asian Lady Beetle, Harmonia axyridis, standing on its larval case.

This Multicolored Asian Lady Beetle, Harmonia axyridis, has just come out of its larval case. Yellow now, but In a couple of hours, it will be orange with black dots. (Judy Gallagher / Flickr; cc by 2.0)



When a beetle breaks out of its cocoon it’s a full-sized adult, ready to eat, mate and complete the last stage of its life, which may range from a few weeks to a year or more.

Habitat
There are beetle species for every environment. Some live on leaves, in or on flowers and in leaf litter. Some live in rotting wood, under loose tree bark, under stones in a rock pile and, at night, under porch lights. Elsewhere, they live in grain silos, caves and the nests of insects, birds and mammals. Some live in water or near water in sand and seashore debris. 

Food sources
Beetles include all sources of food in their diet — plants, trees and animals. Some adults don’t eat at all, but most are plant eaters. They may eat bark, burrow through it to eat the heartwood or into soil for the roots. Some eat nectar or seeds. Others feed on rotting wood, fungi, dead organic matter, carrion or even dung. Some beetles are predators that prey on other insects and invertebrates. Among them are lady beetles, soldier beetles, rove beetle and ground beetles.

Predators and defenses
Beetles are prey for other wildlife. Birds, amphibians, reptiles, other insects, spiders and even other beetles eat them. Mammals, such as shrews and bears — and in some places of the world, humans — eat them, too.

Their first line of defense is camouflage, blending into their surroundings. Another is their hard elytra, which presents a substantial barrier against other insects. Lady beetles and others with a dome-shaped body can even tuck their legs and antennae underneath, as some turtles do. Some ground beetles have glands that spray formic acid, which can burn a predator’s mouth, skin and eyes. Many brightly colored beetles taste bad and predators learn to stay away from them. Some clever beetles mimic the coloration of distasteful ones to discourage predators. Weevils, a type of beetle, play dead!

They’re beneficial
The scales weigh heavily in favor of beetles as beneficial rather than pests. It’s a relative few that cause damage to trees, crops and garden plants. Most are predators that help control insect populations.

Ladybird beetles (ladybugs), lacewing beetles and soldier beetles, for example, prey on such insects as aphids, mealybugs, scales and other plant pests. And, they help pollinate as they hunt for prey.

Ground beetles feed on snails, cutworms, caterpillars, slugs, root maggots and others. Many scarab beetles eat the droppings of other animals, which helps to keep our planet clean. Some species of scavenger beetles feed on mosquito larvae. 

*A compound eye is an organ made up of hundreds, sometimes thousands, of individual photoreceptors. Each photoreceptor, called an ommatidium (oh-muh-TID-ee-um), consists of a lens and cells for distinguishing light and, sometimes, color. Compound eyes create a mosaic which an insect’s brain is able to interpret as one image. Compound eyes are particularly adept at detecting motion. This is one reason it’s hard to sneak up on them.

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