|Male Hercules beetle, Dynastes hercules.|
The Hercules beetles (Dynastes hercules, Dynastinae) is a rhinoceros beetle native to the rainforests of Central America, South America, Lesser Antilles, and the Andes. They are large members of the Scarabaeidae family with some reaching sizes upwards of 15 cm. Dynastes is a highly sexually dimorphic genus, with only males exhibiting the characteristic horn. An important note as well is that only major males have large horn variation across the different species, with the minor or satellite males showing little variation. Anecdotal reports exist denoting Scarabaeidae the strongest animals in the world. Reports also suggest the Hercules beetle is able to carry up to 850 times its body mass but actual measurements on a much smaller (and relatively stronger: see square-cube law) species of rhinoceros beetle shows a carrying capacity only up to 100 times their body mass, at which point they can barely move.
Hercules beetles are distributed in two climatically differing areas. There are five North American species and ten species residing in the Neotropical regions of the world. Three species of White Hercules beetles occur in Nearctic areas and two in the Central American Neotropical region. None of the taxa for white Hercules beetles overlap in their distribution. Nine species of giant Hercules beetles can be found in South America, and one giant species is located in Central America. Several of the Giant Hercules beetles taxa overlap in their distribution.
Most species of Coleoptera have an iridescent colouration to their elytra. Certain species of the genus Dynastes also have the ability to change colour. Specific species have been noted to occur with either black or yellowish to khaki green elytra. This variation in colour is due to a spongy layer below the transparent cuticle; this spongy layer is a network of filamentous strands made up of three-dimensional photonic crystals lying parallel to the cuticle surface. When the cuticle is filled with gas this layer can show through, presenting the yellow to khaki green colour, but when filled with fluid the cuticle appears black. This is due to the change in refraction index allowing us to see the difference in colours. This system is known as a hygrochromic effect. Female beetles can change colour but not as completely as males, which is not yet explained as the mechanisms for the colour change is still not completely understood. What is known is that changes in humidity affect the levels of moisture in the cuticle which leads to a change in colour in most cases. Since the change is due to humidity it is a reversible process, however, it has been observed that after multiple colour changes or high stress the beetles will maintain some dark spots on their cuticle. Some hypotheses for why this colour change occurs at all are the ability to blend with surroundings depending on the time of day (black for nighttime and yellow for daytime) to best avoid their main predator, the tropical screech owl (Megascops choliba). Another theory has to do with thermoregulation in the sense that a black beetle heats up faster than yellow and then once they have warmed up theoretically there will be less moisture in the cuticle which leads to changing to a colour which does not heat as quickly so they won't overheat.
The larval stage of the Hercules beetle will last one to two years, with the larva growing up to 4.5 inches (11 cm) in length and weighing more than 100 grams. Much of the life of the larva is spent tunneling through rotting wood. After the larval period, transformation into a pupa, and moulting, the beetle then emerges as an adult. Adults of most species can live from two to ten months and some can even live one or two years. eastern Hercules beetles, D. tityus, can live six to twenty-three months in captivity with a hibernation period. The western Hercules beetles, D. grantii, tend to have a shorter adult lifespan in the wild (two to four months), but in captivity they live for about the same amount of time as the eastern species. It has also been noted that captive longevity is possible without a hibernation period.
The larval stage of the Hercules beetle will feed on rotting wood during this two year stage. The adult Hercules beetle feeds on fresh and rotting fruit. They have been observed feeding on peaches, pear, apple, and grapes within captivity.
Although there are numerous species under the genus Dynastes, they are able to produce viable offspring with one another. This has been observed in captivity, but it is unclear if wild beetles will engage in acts of hybridization. Certain species such as D. grantii and D. hyllus are believed to be sister species, while D. tityus is thought to be a sister taxon to the Central American white Hercules lineage. The intermediate species that bridges the white Hercules with the giant Hercules is thought to be D. maya.
- Huang, J. Parapatric genetic introgression and phenotypic assimiation: testing conditions for introgression between Hercules beetles (Dynastes, Dynastinae). Molecular Ecology, vol. 25, 2016, 13p.
- Huang, J., Knowles, L. The species versus subspecies conundrum: quantitative delimitation from integrating multiple data types within a single bayesian approachin Hercules beetles. Systematic Biology, vol. 65, 2016, 15p.
- Kram, R. Inexpensive load carrying by rhinoceros beetles. The Journal of Experimental Biology,vol. 199, 1996, 4p.
- Rassart, M., Colomer, J. F., Tabarrant, T., Vigneron, J. P. Diffractive hygrochromic effect in the cuticle of the hercules beetle Dynastes hercules. New Journal Of Physics, vol. 10, 2008, 14p.
- Hinton, H. E., Jarman, G. M. Physiological colour change in the elytra of the Hercules beetle, Dynastes hercules. Journal of Insect Physiology, vol. 19, 1973, 16p.
- Jae Hyun, K., Jun Hyuk, M., Seung-Yop, L., Jungyul, P. Biologically inspirsed humidity sensor based on three-dimensional photonic crystals. Applied Physics Letters, vol. 97, 2010, 3p.
- Krell, F., Krell, V. Longevity of the Western Hercules beetle, D. grantii Horn (Coleoptera: Scarabaeidae: Dynastinae). The Coleopterists Bulletin, vol. 69, 2015, 1p.
- Catálogo electrónico de los organismos presentes en Colombia (Spanish)
- Gilbert Lachaume: The Beetles of the World, volume 5, Dynastini 1, 1985, Sciences Nat, Venette. 
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- Family Scarabaeidae - Dynastes hercules
- The Breeding/Rearing of Dynastes hercules hercules
- Photos of Dynastes hercules ecuatorianus
- Photos of Dynastes hercules hercules
- Photos of Dynastes hercules lichyi
- Photos of Dynastes hercules occidentalis
- Clemson University Arthropod Collection
- Rhinoceros beetle gallery with many subspecies of Hercules beetle.