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Aug. 23, 2011

The Diverse Life Histories and Ecology of Scarab Beetles

by Kara Rogers

Click to enlarge images

Hercules beetle Dynastes hercules, a member of the Rhinoceros beetle family. The green beetle on the right is a Jade headed buffalo beetle Eudicella smithii.

Scarab beetles are fierce-looking insects. Their legs are adorned with claws, they have stout bodies, and some -- like rhinoceros beetles -- feature prominent, intimidating horns. The approximately 30,000 known species of scarab beetle constitute about one-eighth of all coleopteran (beetle and weevil) species recorded to date and are extraordinarily diverse when it comes to size and color and to behaviors such as feeding and reproduction. This translates into remarkably diverse life histories and ecology among scarab beetles.

Many species of scarab beetles, in fact, have very specialized relationships with their environments. Adult male and female Phanaeus difformis dung beetles, for example, work as a team to locate animal dung, shape the dung into so-called brood balls, and dig tunnels where the balls can be stored. The female then lays an egg in each brood ball, which serves as a ready-to-eat food source for the larva when it hatches about a week later. If the dung ball dries out or does not contain sufficient nutrients, the larva will die. If conditions are just right, however, within a little over a month the larva has matured into an adult, eaten its way out of its brood ball, and gone in search of fresh dung and a mate, beginning the cycle again. Hence, the survival of dung beetles is dependent on fresh dung, which they will travel great distances to locate.

A rhinoceros beetle. Illustration by Kara Rogers.

Compared with P. difformis, larvae of the eastern hercules beetle (Dynastes tityus), a type of rhinoceros beetle with a notably menacing appearance, have a much longer generation time. This species, which is one of the largest beetle species found in the United States, also has a very different relationship with its environment. In the summer, females lay eggs in decaying wood on hardwood trees. The larvae that hatch from the eggs typically remain in the wood, feeding on decaying matter for more than a year and a half before becoming pupae. The pupa stage is spent inside a hollow cell that the larva constructs for itself by using saliva to glue together tiny pieces of wood and feces. Within about 6 to 8 weeks, the pupa emerges as an adult.

The entire life cycle of the eastern hercules beetle may last two to three years, and while larvae feed almost exclusively on decaying wood, adults may feed on sap and decaying leaves. Although eastern hercules beetles can damage trees by ovipositing (laying) eggs in the same sites year after year, the breakdown of dead wood by the larvae releases nutrients to be returned to the soil. Hence the larvae play an important ecological role in hardwood forests.

Other scarab beetles include Japanese beetles (Popillia japonica), June beetles (Phyllophaga), and flower chafers (subfamily Cetoniinae). Some of the world’s most massive insects, measuring up to 11 cm in length and weighing as many as 3.5 oz., are African goliath beetles (Goliathus), which belong to the flower chafer subfamily. Found in tropical and subtropical Africa, goliath beetles differ from most other scarab beetles in that they live buried in the soil as larvae and spend a much longer time—sometimes as many as five months—in the pupa stage. In addition, their life cycles are intimately associated with Africa’s wet and dry seasons. For example, pupation takes place at the end of the wet season, allowing the larva to cocoon itself below ground in a sandy cell that hardens with the onset of the dry season. By the time heavy rains return, the pupa has metamorphosed into an adult goliath beetle, which breaks free from its rain-softened cell and emerges from the soil.
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Kara Rogers is the senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is also a member of the National Association of Science Writers and a contributor to the Britannica Blog, where she runs a series called Science Up Front. She holds a Ph.D. in Pharmacology/Toxicology, but enjoys reading and writing about all things science. You can follow her on Twitter at @karaerogers.

This post also appears on the Britannica Blog.

About Kara Rogers

Kara is a freelance science writer and senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is the author of Out of Nature: Why Drugs From Plants Matter to the Future of Humanity (University of Arizona Press, 2012).

The views expressed are those of the author and are not necessarily those of Science Friday.

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