# Phloeodes diabolicus

The diabolical ironclad beetle (Phloeodes diabolicus) is a beetle in the Phloeodes genus. 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. It is flightless and has a remarkably long adult lifespan of eight years, compared to the weeks or months of most adult beetle lifespans.
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 a maximum force of 149 newtons, approximately equal to the force exerted by 15 kilograms or 33.069 lbs.

## Taxonomy

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. 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.

## Ecology and behavior

Phloeodes diabolicus shares a range with Phloeodes plicatus, a reproductively isolated sister species. Their range 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. 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.
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.

## Shell structure

The flattened shape and low-to-the-ground profile of the diabolical ironclad beetle, in addition to its tough exoskeleton, makes it extremely hard to crush. The structure of the procuticle allows for focused compressive forces to be distributed evenly across the beetle's body. Because of the exoskeleton's toughness, collectors find it extremely difficult to pin specimens. The beetles cannot be mounted normally using stainless steel pins; rather, a hole must be drilled in the shell for the pin to be inserted.
A jigsaw-like layering of multiple scales of different sizes, ranging from microscopic to visible sizes, provides exceptional mechanical strength to the beetle's appendages. In the beetle's procuticle, polysaccharide α-chitin combines with proteins to form fibers within each layer. These fibers are twisted and stacked upon each other, creating a "helicoid" arrangement and forming laminated structures. This formation allows for a strong, energy-absorbent and tolerant exoskeleton. The exoskeleton is able to deflect, twist, and arrest crack propagation between each layer. Compositional analysis has found that the microstructure of the beetle's exoskeleton is protein-rich and contains no inorganic structures (common in crustacean exoskeletons), while also containing a thicker endocuticle than other insects.
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 hardened elytra: they are locked in place with a zipper-like connection, which increases the exoskeleton's strength and ability to resist pressure but prevents the beetle from opening its wings to fly. 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.
The structure of its shell has inspired efforts to design similar materials and joints for use in submillimeter engineering.