|Dune along the Gila River north of Yuma|
Besides Charlie's target group, Curculionidae, we found mostly Tenebrionidae or Darkling beetles. In American Beetles, Vol 2, Chapter 106, the authors describe the Tenebrionidae as highly evolved and diverse. In fact, they state that because of the many exceptions that exist in most diagnostic characters, these beetles are often difficult to recognize as a family. A tarsal formula of 5/5/4 and a facial ridge covering the base of the antennae is common to most, but not all of the family members. On the other hand, many beetle collectors assume jokingly that any beetle that doesn't obviously conform with any other family is is most likely a teneb (nick name for Tenebrionidae). The following links represent some examples from Bug Guide, showing tenebs looking deceivingly like June Bugs (Scarabidae), Ground Beetles (Carabidae), Flat Bark Beetles (Silvanidae), and even Lady Bugs (Coccinellidae).
Below is an assortment of 'typical' Tenebs from Arizona.
|Some Arizona Tenebs (not to scale)|
With 20,000 species, Tenebrionidae populate most parts of the world. They feed on fresh and decaying plant material, which includes grain products in human storage. Easy to raise and feed, adults and larvae (meal worm) have become popular as laboratory animal models and as food sources for insectivores in captivity.
Tenebs occupy moist forest floors and dark corners under rocks and bark of decaying wood and are freeze resistant or freeze tolerant even during arctic winters. But undoubtedly, this family found most of its adaptive niches in hot, arid and sandy habitats. Very striking adaptations can be found from the Mediterranean to south African and Asian deserts and of course also in the Sonoran and Mohave Deserts of Arizona.
|Eleodes longicollis, Madera Canyon|
Tenebs use several strategies to deter predators. Besides being very good at playing dead, many of them can emit a pungent smell. They announce with a clear behavioral signal (head stand) when they are going to use this chemical defense. The avoidance reaction of predators must be strong, because there are several non-teneb dark desert beetles that join in on this defense with Muellerian (Ground Beetle Calosoma peregrinator) and Batesian (Cactus Longhorn Moneilema) mimicry. To humans of the Southwest, the teneb species are generally known by the popular names Stink Beetle and Pinacate Beetle.
|Meal leftovers of a Grasshopper Mouse|
Still, some predators do specialize in hunting the big beetles as a substantial prey. Coyote scat and Sonoran Desert Toad droppings are often full of their exosceletons. Grashopper mice simply stuff the offending hind end of a Stink Beetle into the sand and then chew off its front portion.
|Eleodes armatus stored by a Shrike|
Most birds do not have a well developed sense of smell. A shrike took this Eleodes armatus and stored it on a pointed branch for a later meal.
Interestingly, the beetle's head-stand that serves as a warning to predators here in Arizona is used by Namibian Tenebrionids to collect fog-moisture. Our local beetles, also suffering from water deprivation, may or may not employ the same mechanism, but dew and fog are less common here than in the coastal South African desert. Instead, all teneb bodies are superbly built for water conservation. Collectors soon learn how well the carapax of a teneb prevents evaporation even after death. While all other pinned beetles of the same size are long since dry and ready to be stored in insect drawers, the legs of pinned tenebs seem to remain endlessly flexible and the retained moisture sometimes causes the insect pins to rust and break.
|Edrotes ventricosus, Mohawk Dunes, Yuma Co.|
Shapes approaching the ideal of a round disk or better yet, a sphere, optimize the surface to mass relation in Eusattus and Edrotes species and thus minimize water-loss.
|Eusattus reticulatus, Molino Basin, Catalinas, Pima Co.|
|Eusattus dubius arizonensis, Kofa, La Paz Co.|
Interestingly, the Eusattus species from a moister location at a creek bed in Kofa is not quite as disc-shaped as the Eusattus reticulatus from dry, sandy areas of Molino Basin in Tucson.