Coleoptera - beetles
Introduction
With over 350,000 described species, beetles (order Coleoptera) represent the largest group of organisms on Earth. Many are familiar to us like ladybugs, Japanese beetles, fireflies and dung beetles. There is no doubt that there are many more types living around you since they are very successful in most terrestrial, freshwater aquatic and coastal habitats. Most species of beetles are benign and go unnoticed by people by living in obscure places. There are many species, though, that impact our lives directly by being pests of our agricultural commodities or living spaces.
The feeding habits of beetles are extremely varied and, being holometabolous, adults and larvae may feed on different resources. Larval food diversity includes (but is not restricted to) the following: dead wood (including timber) at various stages of decay; live plants, including their roots, leaves, shoots, stems/trunks, flowers and seeds; dung and rotting vegetable or animal matter at various levels of decay; fungi in various stages and habitats; food stores of other animals including humans; and insects, arthropods and other animals, either by actively hunting these prey or by ambushing them. Some are also parasitic on other insects. Adult beetles also feed on a variety of things including many types of plants, animals, fungi and some harvested commodities.
Other than the many foods they eat, some beetles also have interesting life histories. Many come in bizarre forms and brilliant colors. Some have large horns or mandibles used as ornaments or weapons for combat (usually male vs. male). The smallest beetles in the world are among the smallest multicellular animals (0.3mm - smaller than the period on this page), while the largest beetles are giants of the insect world, reaching 7 inches long and weighing more than a large mouse! Many produce interesting chemicals, such as foul-smelling defensive odors (for example some darkling beetles; Tenebrionidae), tiny explosions (bombardier beetles; Carabidae), irritating oils (blister beetles; Meloidae) or bioluminescent light (fireflies, glow worms and some click beetles). Some are good parents, building nests, providing food and/or defending their young. Others have strange lifecycles where even the young can produce offspring! Unfortunately there are too many beetle species to describe all of their many different biologies, feeding habits and lifestyles on this page. More information can be found in the family sections, and further information is accessible through the references and links.
What do beetles look like?
Almost all beetles have elytra (sing. elytron), front wings that are hardened or leathery without any signs of a branching wing venation (see above right). Only rarely do beetles lack wings, though many groups have shortened elytra (which may or may not hide the hind wings). Their hind wings are membranous like other insects, but fold or roll under the elytra to protect them from being damaged. Because of this many beetles are also extremely hardened, one of the reasons beetles are thought to be so successful. Beetles come in a variety of shapes, textures and colors - the different forms of adults and larvae are almost unimaginable. Adults and most larvae have strong biting mouthparts (mandibles) used to feed on different diets (see above). Some adults have their mandibles at the end of a long rostrum (such as weevils; Curculionidae), while a few species have mandibles but use a long "tongue" to drink nectar from flowers (some Meloidae). The antennae of adults come in many forms, from long and straight to elbowed or clubbed (see right), and are often useful for identification. The legs of adult beetles are usually adapted to their particular environment, thus they are sometimes modified. Most beetles have running or crawling legs, but many have rake-like forelegs used to dig, flattened or hairy legs for swimming, or enlarged hind legs for jumping. The tarsi are important for identification, because beetles have a variable number of tarsal segments on each leg. A tarsal formula is often given in the form of "front leg # - middle leg # - hind leg #", so scarab beetles have a 5-5-5 formula, while darkling beetles have a 5-5-4 formula, and so on. The relative shape of a beetle - whether it is flat, hardened, cylindrical, dome-shaped or elongate - can also be diagnostic for different types of beetles.
Life cycle
Beetles go through the normal egg, larva, pupa and adult stages of other holometabolous insects [1]. The number of eggs laid, larval molts (instars) and the duration of those periods, as well as duration of the pupal stage and adult lifespan, all vary with species and other factors such as temperature and food requirements. Beetles lay their eggs in soil, rotting wood, under the bark of living trees, on leaves and other parts of plants and fungi. Some build nests for their young, like dung and carrion beetles. Most larvae are free living in the environment, but some groups are parasitic on other insects (such as rove beetles that parasitize filth fly pupae). A few families (such as Meloidae and Rhipiphoridae) go through what is called hypermetamorphosis, where larvae drastically change shape throughout their molts, usually being highly mobile crawling types in the beginning. This almost always occurs in parasitic larvae that need to find and grab onto an adult host (sometimes a bee at a flower for example) to hitch a ride back to their nest; many also actively seek out nests and hideouts. Once there the larva starts to transform into a more grub-like feeding stage, where it will eat the food stores, eggs, larvae and/or pupae of the host. Beetle pupae are usually exarate (appendages free from body) and without functional mouthparts. They generally look like the adults but pale yellow or white and less defined; a few groups are known to pupate in silk cocoons.
How would you take a good diagnostic picture of a beetle?
The range of beetle sizes means that various types of equipment may be needed to photograph the diagnostic features of adults and larvae. Larger and/or colorful specimens can usually be identified to family, genus and sometimes species in whole-body photos (particularly by specialists). Important features to photograph on adult beetles include the antennae, tarsi of all legs (see tarsal formula above), underside, and the details of the head, pronotum, and elytra. For species identifications, it may be necessary to dissect out the genitalia of males and compare the shape to other known species. Larval beetles are sometimes difficult to identify from photos unless they are distinct or belong to a commonly encountered group (e.g., ladybugs, Coccinellidae). To positively identify larvae often requires closeup examination of the head (including the mandibles, antennae and underside), tail end (which is usually modified and diagnostic) and other body parts like legs and spiracles.
If you cannot get an image of an adult or larva, collecting them to give to a specialist may be necessary. Beetle adults can be caught and frozen, or killed and stored in alcohol (high-grade isopropyl, but preferably 140-190 proof ethanol); larvae are best killed and preserved in alcohol. If none of these options is available and an ID is necessary, capturing the beetle (for example in a bag or container) without damaging it too badly is the best option. After caught it can be taken to be identified whether it lives or dies naturally, both of which are good enough for ID. Lastly, be careful when capturing large adults and larvae that appear to have strong mandibles - some can give painful bites and break the skin.
How do beetles cause damage?
Beetles can cause damage in many ways, mostly related to field crops, stored products and trees/timber. Economic damage can be caused by larvae, adults or both, but often it is the feeding of the larvae that causes the most damage. Many larvae feed externally on their host(s), devouring leaves (e.g., many Chrysomelidae; right), roots (e.g., some Scarabaeidae) and other plant parts. Some beetles, like leaf-miners (e.g. some Chrysomelidae and Buprestidae), are small and feed in the thin space between leaf surfaces. Their mines can leave distinct marks and cause leaf loss. Certain bark beetles (Scolytinae) and metallic wood borers (Buprestidae), as well as others, burrow just below the surface of the bark on trees - they can end up killing twigs or even entire trees depending on the level of infestation (left). Still others (such as many Cerambycidae and Buprestidae) can bore into the stems of plants, particularly the wood of trees (living or dead). These pests can also infest the wood used in structures we build (homes, fences, boats, etc.). A few species make galls in plants and some also girdle stems (adults; right), causing branch loss. Lastly, many beetle species are important because they live in and feed on grains and other materials that we keep in storage facilities and our homes (cereals and other dried goods), reducing products even after harvest. Some beetles even feed on our clothing and other fabrics/materials.
In addition to direct physical damage, many beetles can cause infections of plants through their feeding. For example, some cucumber beetles (Chrysomelidae: Diabrotica and others) can transmit bacterial wilts to vegetables through their saliva. In some cases, like in bark and ambrosia beetles (Scolytinae), the insects purposefully infect the plant (trees in this case) with the fungi on which they feed. Often the pathogens weaken the tree causing it to become susceptible to other beetle attacks. In all, many types of pathogens can be transmitted by beetles, including viruses, bacteria, fungi and nematode worms.
How to identify immature beetles
Beetle larvae are extremely variable in form, and many names are given to the different types (see pictures). Their shape is usually related to where they live and how they feed or move. Some larvae have legs while others lack them (termed apodous). Many that live under ground or in wood are grub-like, having soft, pale bodies and dark, hardened heads bearing strong mandibles. Others are more robust and hardened all over. As far as shapes, beetle larvae can be elongate (and with enlarged heads like Cerambycidae and Buprestidae), flattened, cylindrical (like many Tenebrionidae), c-shaped (like Scarabaeidae, Anobiidae and many Curculionidae), have various plates, spines and growths (some Chrysomelidae and plant-feeding Coccinellidae), or be covered in hairs like a pipe cleaner (like Dermestidae). Some are brightly colored (like many predatory Coccinellidae) or even covered in a wax coating (other Coccinellidae). Larvae of some species change form throughout their life (see hypermetamorphosis above under Life Cycle).
Characteristics common to all beetle larvae are hard to define, but generally beetles will have a well developed head, with an inverted Y-shaped line on the face (frons and up the vertex). They usually have strong mandibles that strike each other in a horizontal plane (>-<). Most have well-developed legs (3 pairs on their thorax), but some like weevils (Curculionidae), jewel beetles (Buprestidae), and many longhorn beetles (Cerambycidae) are legless. They almost never have false legs (prolegs) like the ones in caterpillars (Lepidoptera) and some primitive wasps (Hymenoptera). Lastly, beetle larvae usually have darkened spiracles along most of their body and a modified tip of the abdomen (sometimes having spines, plates, bumps or hairs).
Common Families of Human Importance
There are many families of beetles that are common and affect our lives either positively (beneficials) or negatively (pests). Since this page is just an introduction, please follow these links for specific information about these families:
Coleoptera Families 1: scarabs, jewel beetles, click beetles, and relatives
Coleoptera Families 2: powder-post beetles, sap beetles, ladybugs, and relatives
Coleoptera Families 3: leaf beetles, longhorn beetles, weevils, and relatives
References & Links
- Arnett, R. H., Jr. and M. C. Thomas (eds.). American Beetles, Volume 1: Archostemata, Myxophaga, Adephaga, Staphyliniformia. CRC Press; Boca Raton, FL. ix-443 p.
- Arnett, R. H., Jr., M. C. Thomas, P. E. Skelley and J. H. Frank. (eds.). 2002. American Beetles. Volume 2. Polyphaga: Scarabaeoidea through Curculionoidea. CRC Press, Boca Raton, FL. xiv + 861 pp.
- White, R. E. 1983. A field guide to the beetles of North America. The Peterson Field Guide Series. Houghton Mifflin Company, New York, 368 pp.
- Triplehorn, C. A., and N. F. Johnson (eds). 2005. Borror and Delong’s introduction to the study of insects. 7th Edition. Brooks/Cole Publishing, Kentucky, U.S.A. 868 pp.
- Grimaldi, D. and M. S. Engel. 2005. Evolution of the Insects. Cambridge University Press, New York. 755 pp.
