Stink Beetles

One of the most conspicuous insects in our area is a relatively large black beetle. It can be encountered out in the desert or along the dirt paths and roads of the local canyons as well as the Nature Center. This beetle walks with its abdomen elevated and at times appears to attempt a head stand. If it is particularly irritated it may spray or ooze a foul smelling liquid from the tip of its abdomen. (This has not happened on any bug walks, yet.) This beetle is one of the stink beetles - members of the family Tenebrionidae, also called the darkling ground beetles. Of the 400 plus Tenebrionid species known in California about 100 are stink beetles.

In general, adult beetles have two sets of wings, the front set, called the elytra, hard and often colorful, folds down protecting the second set which are the membranous flight wings. (In flight, small beetles can be distinguished from other flying insects because they must hold the elytra up to free the wings.) Stink beetles cannot fly, in fact, the elytra are fused and the flight wings are absent. This is an adaptation to the environment that these beetles live in, one that is often very hot and very dry.

Insects breath through spiracles, pore-like holes, located mainly along the sides of their abdomens and controlled by the nervous system. The spiracles of stink beetles open into the empty sub-elytral cavity. The humidity level of this air can be controlled because there is only one small opening to the outside world, right at the tip of the abdomen. This air also serves as an insulation layer. Serious damage to the elytra will affect the insect's ability to control its temperature and water balance. In order to avoid overheating or dehydrating, the beetles are normally active only during time periods when conditions are favorable.

Chemical analyses of the liquids involved in the sprays has generally revealed more than 25 compounds. The spray is an important defense mechanism against predators. In one species, it is also used by the females to discourage persistent but unwanted male suitors. (Apparently it is quite effective causing the male to enter a coma-like state for several hours.) Some predators have learned to avoid the problem. Grasshopper mice reportedly push the beetles, abdomen first, into the ground so that the ground absorbs the spray. If the spray works, the insect enters a period of increased vulnerability due to the loss of water and the need to replenish its liquid irritant.

Other important sources of water loss are transpiration through the cuticle, the layer that covers the entire body, and loss due to defecation.

Because they are so small, insects are highly susceptible to small changes in external temperature and humidity. As a result they become adept at using their physical structure in combination with subtle differences in their habitat to satisfy their current needs. Looking around, it is evident that every species has slightly different coping strategies and that these vary somewhat depending on the individual and the conditions. You might want to consider the strategies you have adopted, or been forced to adopt, to exist in your own environment.

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Ron Lyons (volunteer 1990-1999)
Chula Vista Nature Center, 1000 Gunpowder Point Drive, Chula Vista, CA 91910-1201