[The following is a research paper I did for the entomology class I took this spring. – Dawn, 06/22/07]
Dung Beetles of Scarabaeidae and the Sacred Scarab
Family Scarabaeidae is split into two main dietary groups. Dung beetles, or “scarabs”, feast on the waste of other animals. Chafer beetles eat plants. While Scarabaeidae dung beetles are not the only coprophagous beetles, they are the most significant group of dung-feeders. These “true” dung beetles belong to Scarabaeidae subfamilies Scarabaeinae and Aphodiinae.
The dung beetles are of singular interest for many reasons. They keep our planet clean of animal wastes, disperse plant seeds, improve soil quality, control pests, and serve as food for a variety of animals. They are also fun to watch. I wish to introduce many characteristics of the dung beetles, and to describe the religious significance of one species of in particular.
Scarabaeidae Subfamilies: History and Size
Dung beetles are a group whose fossils extend back only 40 million years, which makes them fairly new compared to some beetle species going back 245 million years. There are now about 7,000 dung beetle species in the Scarabaeidae family. Several thousand species live in Africa, a continent that also happens to contain many species of large herbivores producing large quantities of dung. By contrast, the United States has fewer than a dozen species which are significant in dung burial.
Physical Characteristics and Flight
Dung beetles are generally black or brown in color, although some are of metallic black, purple, blue, green, bronze or gold, and may have intricate patterns. They range in size from a tiny 1 mm to a whopping 60 mm. Most are flattened dorsoventrally but stout and oval in shape, and capable of remarkable strength. A few of the “dweller” types described below are conversely small and elongate.
A grooved shield is typically present in dung beetles. They have soft mouthparts suited to their diet. Males sometimes have one or two horns, at the head or thorax. Their antennae are segmented, elbowed and end in an oval club which contains three to seven expansible leaves. The antennae segments are layered and fit together like the pages in a book (“lamellate”). These antennae give dung beetles a keen sense of smell; adults locate dung by odor and can begin work on a dung pat within seconds of it dropping to the ground. Their eyes are divided in half for ground vision and polarized light vision.
Dung beetles tend to walk clumsily, but their legs are broad and powerful. Most dung beetles burrow into the ground and have serrated edges on their front legs to help in digging. “Roller”-type beetles, described below, have elongated back legs which are specialized for rolling the dung into a smooth sphere. Desert species additionally have hair on their legs which makes it easier to move across sand.
Most dung beetles are strong fliers—they can fly up to several miles in one flight. As in other beetle species, they have long flying wings folded under thick, hard modified forewings called elytra. Adults zigzag in the air, detect dung by smell from a great distance, then fly upwind along a plume of odor to their food source.
Dung beetle larvae are slug-shaped, cylindrical and curved into a C-shape—this larval shape is referred to as scarabaeiform. They lack abdominal prolegs but have well-developed, functional thoracic legs. Their heads are likewise well-developed, with prognathous mouthparts suited for eating fibrous material.
Geographic Range and Habitat
Dung beetles live on every continent except Antarctica. Various species are adapted to all kinds of habitats including desert, farmland, forest and grassland. Roller-types thrive in tropical areas. Dung beetles do not, however, enjoy extremely cold or dry weather. North American species include Tumblebugs (Canthon spp.) and the Glossy Pillbug (Geotrupes splendidus).
In one study, it was noted that similar species of dung beetle may use different strategies of thermoregulation. This likely is in response to a specific habitat and determines the hours of the day in which the beetle is active.
Dung beetles, unsurprisingly, feed on dung in both the adult and larval stages. They do not need anything else—not even water.
Many species can detect dung immediately after it is dropped due to the methane release of large herbivores. Some species will hitch a ride near the mammal’s tail in anticipation of a deposit. Most species prefer the dung of herbivores, but others are less choosy and will use dung of several other creatures, including other beetles. On the other hand, many are quite species-specific: they will fly many miles in search of dung of just the right animal. For example, Onthophagus caenobita chooses the feces of humans. Zonocopris gibbicolis feeds on the dung of large snails, hitching a ride on snail shells for easy access. Other species hunt through the foliage of rain forest canopies for primate droppings.
Larvae eat more of the dung matter than do the adults. The larvae feed on both fine and course particles which they cut and chew with their strong mouthparts. Adults have mouthparts with membrane-like jaws which instead extract nutrient-laden liquids and fine particles from the dung. Sometimes animals such as dung-feeding maggots are drained of fluid too as they get trapped in the adult’s mandibles. Some species are thieves, stealing the dung balls of others to eat and lay eggs in. If the ball already contains another’s egg, they will likely eat that too.
Dung beetles are categorized into three groups according to their method of collecting and utilizing dung. There are rollers, tunnelers, and dwellers.
Rollers, also known as “tumble bugs”, shape the dung into balls then roll it way from the dung heaps before burying it. They travel backwards, holding the ball between hind legs and pushing along the ground with forelegs. Often, the burrow area is prepared ahead of time by a female while a male collects and rolls dung for her. Some of the larger males can move balls of dung up to 50 times their own weight.
These males make a loose selection of dung material when feeding themselves, but will be very selective when preparing a ball in which the female will lay her egg. If the pair works together in forming the ball, they can make much larger than the male working alone. Then the female rides on the ball as the male rolls it away. The roller strategy is considered to be evolutionarily the most advanced. It decreases the chances of theft by other beetles. It also has seed-dispersing benefits addressed later.
Tunnelers fly until they locate a dung heap. Then they excavate tunnels underneath. They haul bundles of dung down into the underground chambers where they feed and lay eggs. Like rollers, the female does most of the digging work and the male collects the dung. Unlike rollers, the female sorts through the dung to find quality edible and egg-laying material.
Dwellers don’t move the dung at all. They live and breed in the dung heap just as it has fallen from the animal. They engage in little to no digging, and generally do not form brood balls. Most dwellers belong to the subfamily Aphodiidae.
Reproduction and Life Cycle
The life expectancy of most dung beetles is up to three years. As members of the order Coleoptera, they experience complete metamorphosis. They are usually solitary insects except during the mating period. As mentioned above, the male and female beetle pair collects and buries manure that will serve as both incubator and food for their larvae. There are three instar stages before the non-feeding pupal stage. Dweller species simply lay their eggs where the dung pat falls. Rollers and tunnelers show a much greater degree of parental care.
Rollers and tunnelers do something far more interesting. The females of both types have shorter, thicker legs and are usually in charge of digging. They can create vast underground systems of chambers going into the earth 18 inches or more. Males sometimes help haul the excess soil out of the burrow. Each chamber is intended for one dung ball or heap, into which the female will lay one egg.
The roller-type female accepts the spherical dung offered by the male and further fashions it into a pear shape. She deposits the egg into the pear’s neck. She coats the pear with an antiseptic mixture of dung, saliva and feces to create a hard case. A female of a lesser maternal species covers the ball with soil and more dung to protect it, then moves on to the next dung ball. In some species, however, she stays with the offspring to protect it, cleaning it and removing its feces, fungi and bacteria.
The male will leave to find further partners.
After about a week the egg hatches. The larva feeds on the fecal matter surrounding it, which is full of roughage. It eats about half of the dung ball and provides structural integrity to the ball by continually sealing it with its own excrement. If the ball is damaged in some way, the larva will die.
If the weather is right, the larva will pupate in another three or four weeks. The new adult emerges after eating its way out of its maternal ball, and digging a tunnel to crawl out through. The first order of business then is the search of fresh manure. The adult will breed two weeks later. A complete generation will take about six weeks under ideal environmental conditions.
Seasonal changes affect dung beetle activity. For example, in Queensland, Australia, dung beetles are most active in the warmer months, especially after rain. As winter approaches, breeding activity decreases as the beetles prepare to spend the winter months underground. Many adults die, while others make it through by becoming far less active or even achieving diapause. Diapause ends when the beetle senses a warm enough temperature or adequate rainfall. Soil moisture level is critical for many species.
Some larvae survive unfavorable conditions by stopping development for several months. For example, Onthophagus gazella larvae are triggered to pupate with the first good spring rain, and the second good rain leads to the emergence of the new adults. In other species the young adult may emerge from the dung ball but remain in the soil waiting for rain.
On the other hand, there are species which prefer a more arid climate. An Australian import to Texas called Euoniticellus intermedius is active during dry weather when native species are not. The new adults need not wait underground during hot summer months.
Benefits from Dung Beetles
There are so many great things about dung beetles that it’s hard to decide where to begin. They keep the planet free from enormous piles of waste, they enhance soil quality, they disperse seeds, they allow more palatable forage material to grow for livestock, they destroy parasites, and they are edible.
Several ranchers and researchers have documented the amazing speed with which dung beetles, especially the rollers and tunnelers, clean up after other animals. One researcher observed a 1.5 Kg heap of elephant dung deposited on the African savannah. It attracted 16,000 dung beetles of many species. Within two hours, they had eaten and/or buried the dung completely. One U.S. rancher says that when rainfall is ideal and cattle density is at peak levels, dung beetles bury a ton of wet manure per acre per day and remove 90% of the surface material. By another estimate, a single dung beetle can bury 250 times its own weight in a day.
The effect of all this digging and burying of manure is incredible soil enhancement. Nutrient recycling is aided when dung is exposed to soil microbes, plant roots and earthworms. Plants in particular make use of the of manure’s nitrogen content, most of which would be lost to the atmosphere if left on the surface. The tunneling behavior helps aerate the soil, increasing its capacity to absorb and hold water. This reduces the detrimental effects of both drought and flood years.
According to one study done in the tropics, dung beetles are essential to biodiversity there. They team up with primates to disperse seeds. This is especially of interest to scientists attempting to regenerate rain forest that has been clear-cut, used, and abandoned. Monkeys are the primary seed dispersers, eating fruit from one tree, swinging among others, and then defecating far away from the original tree. Dung beetles of the roller and tunneler types are the secondary dispersers. They swarm the fresh pile of excrement and take the undigested seeds in the dung even further away. The seeds in the fruit need to get as far away as possible from the parent tree in order to avoid any diseases the parent has. As the seeds are buried by the beetles, they escape being eaten by ground-living, seed-eating mammals which might destroy them. They are safe to germinate and become seedlings on the forest floor.
Dung beetles are edible by many species. People eat them, as do toads, pigs, foxes, and various birds such as owls. Burrowing owls have been observed littering their nests and surrounding areas with the dung of other animals. In 2004, researchers reported their findings of a several-year study in which they found that a primary element to the success of these birds is that the dung attracts dung beetles, one of their food sources. The dung and other bits of rubbish may serve other purposes such as insulation, but importantly, the owls that leave dung outside their burrows catch and eat more beetles than when the dung is not there—up to ten times more.
Where cattle is being raised, dung beetles make grazing more pleasant for them. Most cattle will not graze near manure dropped by their own species. The forage is still edible, but they will avoid it because of the dung pile. This can make up to ten percent per acre per year unavailable for grazing. Dung beetles clean up so that land use is much more effective.
More important still, dung breeds pests to cows and humans alike, such as horn flies (Haematobia irritans) and face flies (Musca autumnalis.) One pat of manure can generate 60 to 80 horn fly adults if not controlled. These flies decrease in numbers significantly where dung beetles feed. The beetles not only deny food for maggots, they often destroy fly eggs while feeding. Gastrointestinal parasites of livestock also take cover in manure. The eggs are first passed off in the feces of hosts, then develop into larvae and become infective. They migrate onto the forage material to be eaten by new hosts. Dung beetles remove a critical step of the parasite’s life cycle: the incubator of eggs.
In Australia, there are about 44 native species of dung beetles. These are adapted to the small, dry dung pellets of marsupials. When cattle ranching began there, these native species ignored cattle dung. For about 180 years, Australia had no effective method of manure removal. Earlier this century, there were 20 million cattle in Australia, each dropping 12 pads per day on average. Each cow pad can produce 2,000 flies, so every day in total about 480 thousand million new flies emerged as adults.
This had several damaging consequences. Forage plants were choked off. The populations of pestiferous flies exploded, attacking people, cattle and other animals. The bush fly was especially horrible to live with in summer months. So in 1967 the Australian government released dung beetles from Europe and Asia. They tried 41 species from all kinds of habitats, and by 1991 22 species had established healthy populations. Multiple species are important in accomplishing more uniform distribution and burial of dung, dependable activity 24 hours a day, four seasons per year, and beetles who would attend to the manure of multiple species of herbivores.
This strategy has been hugely successful. It has prevented the accumulation of manure along with its attendant pests. It has improved the fertility of the soil, and it has made forage material more palatable to livestock.
Relationship to Other Animals
Where dung beetles go, others follow, sometimes literally. One rancher observed that with the introduction of these beetles to his pastures, earthworms began appearing where none had lived before, eventually up to 30 earthworms per cubic foot according to core soil samples. At several sites on his ranch, harvester ants have built their mounts out of the earthworms’ castings.
Some smaller organisms have adapted to take rides around on the shells of dung beetles. These phoretic creatures find greater opportunities for dispersal with this hitch-riding method, so that their progeny may have more suitable sites for development. Some Scarabaeus species carry simultaneously the tiny fly Ceroptera rufitarsis and the coprophilous mite Macrocheles saceri.
Morphology and Evidence of Rainforest Evolution
Scientists in Australia are studying the intricate surfaces of the genitals of native dung beetles in order to learn more about the ecological history of that continent’s rain forests. Using an invention known as a “phalloblaster” (created at Australia’s Commonwealth Scientific and Industrial Research Organisation) to evert the hidden, inverted phallus of male beetles, entomologists have been able to identify closely-related beetles. The researchers can then infer evolutionary relationships between different varieties going back for millions of years.
The location and variety of species gives clues about the creation of geographic obstacles such as river valleys, which would separate populations and give rise to new species. By mapping out which species split off at what times, the scientists can speculate which parts of the forests are more ancient than others, and therefore are more vital to preserve.
Navigation by Moon
In 2003, researchers in Sweden made a surprising discovery. Since the 1950s we have known that insects such as ants and bees can navigate by the polarization patterns given by the sun. Marie Dacke and colleagues of the University of Lund in Sweden found that nocturnal dung beetles can navigate at night by sensing the polarization patterns from sunlight reflected by the moon. The beetles can sense these moonlight patterns, scattered by the atmosphere, which vary across the sky depending on the position of the moon.
The research team discovered that otherwise the beetles cannot find their ways through a dangerous terrain. On moonless or cloudy nights, the beetles become confused and appear to crawl around aimlessly. By contrast, the presence of these lunar patterns help the beetles navigate in a straight line toward a destination. Says Dacke, “By rolling in a straight line, it not only gets away spending as little energy as possible, but it is also guaranteed not to return (accidentally, to the dung pile).” Dacke and her team suspect that other creatures in the animal kingdom will be identified in the future who also navigate by the moon.
A number of cultures have held the miniscule dung beetle in high regard. Shamanistic cultures in the early Old World made ornaments which resembles scarabs and other beetles, and sometimes revered the dung beetle as creator. Similarly, some South American tribes believed that a large scarab named Aksak modeled man and woman from clay.
The most famous worshippers of the scarab were in ancient Egypt. The first scarab worshipped was probably the bright metallic Kheper aegyptorum. It was observed that the beetle pushed its ball across the ground like the sun rolled across the sky, and buried its ball in the ground like the sun disappeared into the ground each night. In this was, it was associated with the solar gods Ra and Kheper.
The Egyptians also observed that beetles seemingly appeared without help from the ground—adults were digging their way out after maturing underground. Thus the beetles were understood to be self-created or even resurrected. Perhaps this early culture felt that humans could achieve rebirth by imitating what they believed to happen after the scarab went underground with its ball of dung. The process of mummification mimics the pupa stage of the scarab: a temporary state of being until the protected dead body could regenerate itself. Jars and coffins containing mummified scarabs were often placed in tombs, and sometimes scarab amulets replaced the deceased’s heart to ensure that the truthful heart would not incriminate its owner in the next world.
Scarab amulets became popular ornaments for the living Egyptians too. They symbolized the light and warmth of the sun. Typically, the scarab image was carved on top and the bottom contained engravings of hieroglyphics or geometric patterns. The scarab inspired his own hieroglyphic symbol which was used to represent many concepts: the name of the creator-god Kheper, the Sacred Scarab itself, and the word kheper, meaning “become(s)” or “manifestation(s) of”. Colossal scarab statues graced many Egyptian temples such as the temple of Amun in Karnak.
The Egyptian climate changed over time, and K. aegyptorum moved south to be replaced by the black Scarabaeus sacer in religious significance. This is the biggest and now the most famous of all dung beetles. S. sacer has a range ringing the Mediterranean from northern Africa to southern Europe. It has a broad, flat head notched with six angular teeth arranged in a semi-circle. This is used as a digging tool and as a rake for scraping together fecal matter. Its forelegs are flat and bow-shaped, edged with additional strong teeth for scraping. Its four hind legs are long and slender, adept at fashioning its glob of dung into a smooth sphere. Its dung balls range in size from that of a walnut to that of a clenched human fist.
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