Darkling beetles (Tenebrionidae) in the Dunes of Western Arizona

 At the end of February, Charlie O'Brien and I went on an early collecting trip to the dunes of western Arizona, from Yuma and San Luis in the south to Parker and Bouze in the north. After the cold spell in early February,  desert annuals were only sprouting and most bushes and perennials were just coming out of  dormancy.

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

Surprisingly, many of these desert species are dark and easy to spot on the light sand, especially since most of them become active by early dusk.

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.  

Asbolus verrucosus and Asbolus laevis

Some teneb species have thick wax coatings, but their smooth black relatives seem to be just as heat and aridity tolerant.

In dune habitats, many insect species burrow into the sand during the day to escape from heat and predators. Sifting dry sand through fine sieves is a successful collection method. The best place to start seems to be  the area around Burr Sage and Creosote bushes, especially under hanging branches that touch the soil.

Edrotes arens and Trichiasida hirsuta are covered in soft hair

Many of the insects found by sifting show special adaptations to burrowing in shifting dune sand.  We collected several species of Burrowing Bugs, Weevils and Darkling Beetles that are covered by coats of soft hair. This feature seems to help them to 'swimm' through the loose fine sand while keeping the grains from clogging the tracheal openings (spiracles) needed for respiration.

Cnemodinus testaceus and Notibius puberulus

  The tibiae of these little tenebs found by sifting under Creosote bushes show adaptations to their burrowing lifestyle. The tibiae of Cnemodinus remind of the dentate tibiae of many Scarabs that also burrow underground during the day. Notibius also has widened, shovel-like tibiae.

From left to right: Helops sp., Embaphion depressum, Helops sp., Parasidina hispicula, Metoponium ? sp.

A single Parasidina hispicula under dead wood and several Blaps and Helops-like beetles caught in a pit trap,  plus the  elegant little Ambaphion depressum that was found dead,  brought the number of teneb species for this early spring dune trip to thirteen. Not bad for an area with on-going night-time freezes, still nearly devoid of fresh vegetation.

I like to thank Prof. emerit. Charlie O'Brien who knows all the locations and who did all the driving and sifting, and Kojun Kanda Ph.D. and Aaron Smith Ph. D. for their help with identifications.