Males of Dynastes bear two long horns, one on the head, and the other on the pronotum, forming a "plier"; the pronotal horn has reddish setae on its underside. This pronotal horn is absent in females.[3] Some species have an iridescent colouration to their elytra.[4] Certain species of the genus Dynastes also have the ability to change colour.[5] Specific species have been noted to occur with either black or yellowish to khaki green elytra.[5] This variation in colour is due to a spongy layer below the transparent cuticle;[5] this spongy layer is a network of filamentous strands made up of three-dimensional photonic crystals lying parallel to the cuticle surface.[6] 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.[5] This is due to the change in refraction index allowing us to see the difference in colours.[6] This system is known as a hygrochromic effect.[4] 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.[5] 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.[5] 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.[4] 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).[5] 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.[5]