I have decided to begin a new, recurring feature here called "ID Tips" that will be designed to help naturalists, amateur and student entomologists, and others solve common and basic problems in insect and spider identification. Our first installment will explain differences between ground beetles (family Carabidae) and darkling beetles (family Tenebrionidae). The two are easily confused, but generally straightforward once you know what to look for.
One glaring difference between carabids and tenebrionids is the structure of the antennae. In ground beetles, the segments of the antennae are usually longer than wide, giving the antenna the appearance of a near single filament. Darkling beetles have mostly bead-like segments, such that the antenna resembles one of those keychains that closes by pushing the filament between the beads into a clasp. This is not a foolproof character, but works enough of the time that it will help you make a quick determination.
Look also at the front of the head. Are the mandibles (jaws) exposed and directed forward? If so, then you have a ground beetle or another kind of predatory beetle. Conversely, if the mandibles are concealed beneath the clypeus ("upper lip" if you will), then it is likely your specimen is a darkling beetle or another beetle that feeds on vegetation or detritus.
Does your beetle move very rapidly by running? This is another characteristic of ground beetles. Many species are so fast that it is a real challenge to catch them for closer examination. Does your beetle lumber along slowly or only run fast when prodded? Chances are you have a darkling beetle then. Many darkling beetles rely on a very dense exoskeleton to repel attacks by predators, and consequently don't feel the need to flee in haste. Desert and prairie species may even have the wing covers fused to help limit water loss.
Does your beetle stand on its head when disturbed? This head-standing behavior is characteristic of the darkling beetle genus Eleodes, and some other closely-related genera. Both darkling beetles and ground beetles can have extremely effective chemical defenses that include pungent, permeating aromatic compounds released from abdominal glands, so smelling your beetle to make an identification is not only a potential exercise in futility, it can be highly unpleasant if not injury-inducing.
Habitat is not a good way to distinguish ground beetles from darkling beetles. Both can be found in a variety of habitats, and are especially abundant and diverse in aridland ecosystems.
Because both carabids and tenebrionids are well-defended insects, they are mimicked by a variety of unrelated insects. One prime example is the longhorned beetle genus Moneilema, which mimics Eleodes darkling beetles not only in appearance, but behavior. The longhorned beetles, which feed mostly at night, on cactus pads, move slowly and even stand on their heads like Eleodes. They have no chemical defense to back-up their warning posture, but the bluff works fine on all but the most desperate of insectivores.
Another complication is the relatively recent assimilation of two former beetle families into the Tenebrionidae, many of which do not resemble "classic" darkling beetles. These are the comb-clawed beetles (subfamily Alleculinae) and long-jointed beetles (Lagriinae).
In contrast, the "ironclad beetles" in the family Zopheridae were once members of the Tenebrionidae. They include beetles so well-armored that entomologists have extreme difficulty in pinning specimens that they collect.
No single character is probably reliable by itself, and it is recommended that one use a dichotomous key, in a technical book or online, to reach a firm conclusion on the identity of your beetle. It is also a great idea to invest in a good dissecting (binocular) microscope. Good luck!