Phloeodes diabolicus (formerly Nosoderma diabolicum), common name: diabolical ironclad beetle, is a beetle of the Family Zopheridae. It is native to the California Floristic Province in the states of California and Baja California, where it is believed to eat fungi growing under rotting tree bark.[1] It is flightless and has a lifespan of eight years,[2] compared to the weeks or months long lifespan of typical beetles.[3]
This beetle is noted for its durability. Its thick, densely layered and interlocking elytra, connected to the ventral cuticle by complex lateral support structures, are able to support maximum force of 149 newtons, approximately equal to the force exerted by 15 kilograms or 33.069 lbs.[2]
These inch-long beetles have the potential for extremely long lifespans due to their structure and shape. Many beetles have a rounded body, but the diabolical ironclad is different, having a flat shape and low-to-the-ground profile makes these beetles extremely hard to squash. The compression is not focused on one spot but rather spread across the shell evenly distributing the force over the whole shell. The shell provides many issues for entomologists trying to display their specimen. The beetles cannot be mounted normally using stainless steel pins, but rather they need to drill holes in the shell where they desire to place the pin.[4]
Utilizing a jigsaw-like layering of their joints and appendages provide stability to withstand such extreme forces. This is done by layering multiple different scales of different sizes, ranging from microscopic to visible sizes, providing exceptional mechanical strength. The jigsaw pattern seen is a multilayered exoskeleton, including a waterproof epicuticle, an underlying exocuticle and lastly an internal endocuticle. In each of the cuticles, polysaccharide α-chitin combine with proteins to form fibers within each layer. These fibers are twisted and stacked upon each other creating a "helicoid" arrangement, forming laminated structures. This formation allows for strong, energy-absorbent and tolerant structures. Being energy absorbent, the skeleton is able to deflect, twist and arrest crack propagation between each layer. The lack of flight allows the hardened elytra to be locked in place with the hindwings which aids the structure. Using a compositional analysis it was found that the microstructure of exoskeleton is protein-rich and contains no inorganic structure (common in crustacean exoskeleton), while also containing a thicker endocuticle than other insects.[5]
There are two main areas that allow the skeleton to endure such forces as much as 39,000 times its own body weight. The first is the connection between the two halves of the shell: the interconnections are zipper-like providing additional strength and are stiff and resist bending pressure. The back of the beetle is not interlocked in the same way allowing the bottom halves to slide past each other, providing flexibility to absorb squishing compression. The second area is the puzzle-like design that runs the length of the back connecting the left and right side. Protrusions called blades fit together like jigsaw pieces, glued together by proteins aiding in damage resistance. The connection allows the blades to absorb impacts without snapping. The protection allows the beetle to be almost predator proof, denying most species the ability to break the shell.[6]
The structure of its shell has inspired efforts to design similar materials and joints for use in submillimeter engineering.[7]
Phloeodes diabolicus is a member of the Phloeodes genus under the Zopheridae family, sharing a range with a reproductively isolated sister species Phloeodes plicatus,[8] that encompasses California, portions of southern Oregon, and Baja California. Like other Zopherini these insects are holometabolous and well adapted to wood boring, particularly in the larval stage showing a larger thorax and smaller legs than non-wood boring beetles.[9] These beetles are believed to be non-specific decomposers eating rotten wood from many trees and shrubs and the fungi that grow upon them, however, Phloeodes diabolicus is noted to be found most frequently underneath the bark of decomposing oak trees and believed to prefer white rot fungi as a food source. This species is flightless with a fused shell theorized to have evolved for protection from crushing and evaporative moisture loss over its long lifespan of approximately eight years. This allows it to survive in drier climates and resist predation by birds and lizards without being able to fly away from them.[10]
Other commonalities with the rest of its associated family include a tendency to play dead when threatened and the ability to go long periods without food or water. Unlike other species in its family the waxy secreted coating that normally prevents moisture loss is believed to also serve a function in sexual attraction as in Phloeodes diabolicus this secretion is a masculine secondary sex characteristic.
This species was named for and initially categorized taxonomically by John LeConte. When this species was first identified in the 1800s it was classified as a member of the Nosoderma genus, as were all of what were later re-classified as Phloeodes. The genus Noserus was also described by LeCont during the 1800s but is later made synonymous with Phloeodes in 1999, as it was determined that Phloeodes diabolicus and Phloedes (Noserus) plicatus, both key species in their genera, belong in the same genus. The entire genus was moved from Tenebrionidae to Zopheridae.[11] Multiple other species, such as P. latipennis, that were initially identified by a few specimens were later made synonymous with Phloeodes diabolicus starting in 1936 and continuing until 2006. Individual differences in taxonomic classification continue, including the entire genus Phloeodes being absorbed into Nosoderma (Verodes) but as of 2008 the genus Phloeodes has been restored and Phloedes diabolicus is classified within it.