Passive defense mechanisms used by animals cover a wide range of tactics, including camouflage, disruptve colouring, mimicry, and alarming or distracting startle displays. Many animals use a combination of defenses, relying on camouflage as a first defense and flash colouring for the cases where camouflage does not work. Most of these protective mechanisms are not merely a matter of morphology, but of behaviour as well. Animals that survive by mimicry must act like the species they mimic; camouflage is ineffectual unless the camouflaged creature moves slowly or sits still.
Camouflage, or crypsis, is the most widespread form of protective morphology. Visual camouflage involves shape, colour, and patterning, as well as behavioral modifications, to make the animal blend into its background or resemble common features of the environment.
Colour Matching - the simplest sort of background matching is colouring that matches the surroundings. This can be as simple as the monochromatic coats of lions and polar bears, or as complex as the barklike wing striping found in many types of moth. Colour matching is central to almost all forms of camouflage, although a few animals seem to employ disruptive colouring alone.
Contour Elimination - the contours and shadows of the animal's body must also be hidden in order for the camouflage to be complete. This is accomplished through countershading and disruptive colouration. Countershading, wherein an animal's skin fades from a dark colour on top to a light one on the ventral side, helps to eliminate the telltale shadows of an animal's body and make it appear depthless. Caterpillars and other insects use countershading to make them appear flat, like the leaves they live amongst. (Note that this is not the only way countershading can be used - sharks, orcas, and many birds use countershading for improved colour matching, as they are typically viewed against light surroundings from underneath and dark surrounding from above. This is also the reason military ships and airplanes are countershaded.) Disruptive camouflage, which is most famously used by zebras and modern infantry forces, is another method of contour elimination, which makes different parts of the animal's body fade into different features of the background, thereby eliminating the outline of the animal's body. The stripes of a zebra are one of the most outstanding examples of disruptive camouflage, not background matching - their function is not to make the zebra blend into the grass as many people think , but to make it blend into other zebras. A herd of zebras is a conspicuous target for lions and other predators, but the disruptive striping makes it difficult to target individual animals in a startled herd of quick-moving stripes. Body flanges and dorsoventral flattening are used mainly by insects and some fish to further eliminate shadows and clear outlines.
Aposematic colouration and behaviour is that which conspicuously advertises an animal's unpalatability. Bright, highly contrasting colours and conspicuous behaviour are often warning signs that an animal is poisonous (as in many varieties of tree frogs), venomous (snakes and spiders) or noxious (beetles, skunks and caterpillars). Many insects use a combination of aposematic colouring and loud, crackling or buzzing noises, as well as smells, to dissuade would-be predators.
Mimicry is the technique of imitating other creatures, and it falls into three basic types:
Mullerian Mimicry - convergence of a group of aposematic species coming to resemble each other in their basic forms. This reduces the number of warning cues that predators must learn before deciding that an entire gestalt is unpalatable. Examples are the bees and wasps, almost all bearing yellow and black bands, and the tiger stripe butterfly complex, a group of butterflies and moths of varying unpalatability, all of which bear yellow, brown, black and orange stripes. Many of the species in the latter group are related, but not all.
Batesian Mimicry - imitation of aposematic species by palatable ones. The classic example of this is the non-toxic viceroy butterfly, which closely mimics the unpalatable monarch butterfly. Batesian mimicry is only effective when the mimic species is less common than the real unpalatable species, which results in strong pressure towards mutation into new forms resembling other unpalatable species.
Wasmannian Mimicry - imitating a host species. This is typically seen in beetles and other insects that live inside ant colonies.
When an animal's camouflage is detected, it may find a need for a second method of defense. In many species this need is met by an alarming startle response, which can be an aggressive posturing (as in grasshoppers and spiders) or, more commonly, the sudden exposure of small patches of aposematic colouring or body parts that mimic predator species. Eye spots on wings, usually mimicking owls' or other raptors' eyes, are favoured by many insect species, while lizards and birds tend to favour aposematic flash colouring. Some cephalopods manufacture flash colouring as needed, and will display strong red and white striping when trapped or annoyed, if their other camouflage mechanisms do not avail.
While this is not actually a separate form of camouflage, it is worth noting that some animals are capable of changing their own colouration and texture to match a variety of surroundings. Locusts change their colour slowly to match whatever environment they are born into , and even adult locusts can change their colour over a period of several days. Chameleons, of course, can change their hues much more quickly, and as I have discussed previously, most cephalopods are capable of instantaneous changes in colour and texture. This is perhaps the only kind of camouflage not yet employed by human military forces, although I'm sure it is being researched.