Morphological mimicry

Many accounts of mimicry in insects have concentrated on the morphological similarities, particularly the evolution of warning colors by palatable mimetic organisms to resemble their unpalatable or protected models with aposematic coloration. There are many well-studied examples, particularly in butterflies, which display their warning coloration on their large, conspicuous wings. The well-documented geographical correlations in color pattern between model and mimic species described by Bates and well illustrated by Moulton [15] have since been explored from a genetic perspective [16-20], and the potential for birds to act as selective agents of prey coloration and pattern, as suggested by Carpenter [21], has been verified experimentally for captive birds [4,22] and wild birds [23]. Many moths that have black and yellow banding on their body appear to be Batesian mimics of wasps [24].

Similarly, some beetles display black and yellow banding on the elytra [3]. Morphological mimicry in insects occurs widely throughout the tropics, but there are good examples occurring in temperate areas as well. Dipterans, including some asilids, conopids, tachinids, bombyliids, and most notably syrphids (hoverflies), mimic solitary and social wasps, honeybees, and bumblebees. The morphological similarities, particularly in color and markings, have been well-documented [25-28], and the abundance, distribution, and phenology of model and mimic species have also been intensively studied [29-31].

10.3 behavioral mimicry

It has been widely acknowledged that behavior plays a major role in mimicry. Members of mimicry complexes dominated by unpalatable neotropical butterflies have been found to roost at similar heights in the canopy to their comimics [32] and to utilize host plants at similar heights [33]. It is also generally accepted that prey that are unprofitable because they are poisonous or unpalatable often exhibit slow and predictable movement; there is no selection pressure on them to adopt rapid movement to escape predators. Rather, conversely, there is selection pressure on such prey to advertise their defenses [34]. Bates [1] was probably the first person to observe that unpalatable or noxious butterflies flew slowly and deliberately, so that their warning coloration was easily visible, whereas palatable ones flew faster and more erratically. Aposematic beetles also adopt slow, sluggish behavior whereas palatable ones run quickly to avoid predatory birds [35].

On the other hand, prey may be unprofitable because they are simply hard to catch. Humphries and Driver [36] suggested that certain erratic behaviors shown by some prey animals when attacked by a predator were not accidental but specifically evolved as antipredator devices, confusing or disorientating the predator and thus increasing the prey's reaction time. Such behaviors, which seem to have no obvious aerodynamic or physiological function, appear highly erratic and include zigzagging, looping, and spinning. Driver and Humphries suggested this occurs in a wide range of animals, calling it protean behavior [37]. Examples include noctuid and geometrid moths, which show a bewildering range of seemingly unorientated maneuvers when exposed to the ultrasonics of hunting bats, a behavior that confers a 40% selective advantage for the moths [38]. Driver and Humphries [37] suggested that the behavior is advertising that the prey is difficult to catch and therefore unprofitable. This seems to suggest that a predator might not bother to attack, or another explanation is that such behavior could result in confusion, delaying an attack by a predator and allowing the prey to escape. Certainly erratic behavior is commonly observed in many insects including moths, orthopterans, dipterans, hemipterans, and homopter-ans [37], and Marden and Chai [39] described uncharacteristic upward movements shown by butterflies escaping predation.

Animals may even evolve morphological signals to reinforce or replace their behavioral ones that indicate they are hard to catch, which would help dissuade predators from attacking them. Of course, possession by one species of these signals can then lead to the evolution of such signals in other species, to produce what Srygley [40] has termed escape mimicry. There do not appear to be any clear examples of escape mimicry involving two or more species that are all hard to catch, though several candidates among tropical butterflies have been put forward [40]. However, an example in which an easy-to-catch mimic resembles a hard-to-catch model was given by Hespenheide [41]. He described an unusual and novel case of mimicry in which a group of Central American beetles, mostly weevils from the subfamily Zygopinae, mimic agile flies, notably robust-bodied species, such as tachinids, muscids, and tabanids. The weevils share common color patterns with the flies, which are unlike those of other beetles, none of which is considered distasteful. The weevils and flies share a behavioral characteristic that puts them in close association spatially; most perch on the same relatively isolated and exposed tree boles at midelevation in the canopy. Hespenheide [41] estimated that flies accounted for between 65 to 70% of flying insects in the area, and yet work carried out on the diet of neotropical birds found that flies, particularly robust-bodied species, formed a very small proportion of their diet. Hespenheide hypothesized, therefore, that the mimicry was based not on distastefulness but on the speed and maneuverability of the flies, which advertise they are difficult to catch. Gibson [42,43], in a series of experiments on captive birds, showed that escape mimicry is potentially plausible since over a period of several days, two species of birds both learned to avoid models of evasive prey and were also confused by escape mimics. However, Brower [10] wrote that erratic flight as an aversion tactic employed by insects and their Batesian mimics is unlikely to result in long-term learning by a predator, and so he was skeptical that such escape mimicry could evolve.

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