Box 14.2. Backpack bugs — dressed to kill?
Certain West African predatory assassin bugs (Hemiptera: Reduviidae) decorate themselves with a coat of dust which they adhere to their bodies with sticky secretions from abdominal setae. To this undercoat, the nymphal instars (of several species) add vegetation and cast skins of prey items, mainly ants and termites. The resultant “backpack” of trash can be much larger than the animal itself (as in this illustration derived from a photograph by M. Brandt). It had been assumed that the bugs are mistaken by their predators or prey for an innocuous pile of debris; but rather few examples of such deceptive camouflage have been tested critically.
In the first behavioral experiment, investigators Brandt and Mahsberg (2002) exposed bugs to predators typical of their surroundings, namely spiders, geckos, and centipedes. Three groups of bugs were tested experimentally: naturally occurring ones with dustcoat and backpack, individuals only with a dustcoat, and naked ones lacking both dustcoat and back-pack. Bug behavior was unaffected, but the predators’ reactions varied: spiders were slower to capture the individuals with backpacks than individuals of the other two groups; geckos also were slower to attack back-pack wearers; and centipedes never attacked back-packers although they ate most of the nymphs without backpacks. The implied anti-predatory protection certainly includes some visual disguise, but only the gecko is a visual predator: spiders are tactile predators, and centipedes hunt using chemical and tactile cues. Backpacks are conspicuous more than cryptic, but they confuse visual, tactile, and chemical-orientating predators by looking, feeling, and smelling wrong for a prey item.
Next, differently dressed bugs and their main prey, ants, were manipulated. Studied ants responded to individual naked bugs much more aggressively than they did to dustcoated or backpack-bearing nymphs. The backpack did not diminish the risk of hostile response (taken as equating to “detection”) beyond that to the dustcoat alone, rejecting any idea that ants may be lured by the odor of dead conspecifics included in the backpack. One trialed prey item, an army ant, is highly aggressive but blind and although unable to detect the predator visually, it responded as did other prey ants — with aggression directed preferentially towards naked bugs. Evidently, any covering confers “concealment”, but not by the visual protective mechanism assumed previously.
Thus, what appeared to be simple visual camouflage proved more a case of disguise to fool chemical- and touch-sensitive predators and prey. Additional protection is provided by the bugs’ abilities to shed their backpacks — while collecting research specimens, Brandt and Mahsberg observed that bugs readily vacated their backpacks in an inexpensive autotomy strategy resembling the metabolically expensive lizard tail-shedding. Such experimental research undoubtedly will shed more light on other cases of visual camouflage/predator deception.