Mimicry or Protective Colouration
Mimicry is the ability to appear to be or to imitate something other than what you really are. The use of mimicry is prevalent throughout nature and is a prime example of evolution by natural selection. Butterflies use it as a protection mechanism in their larva stage and in the final adult stage. Either to trick predators into thinking they are an inedible species or perhaps an entirely different organism all together. Foremost, the intention of mimicry is to draw attention to yourself. This is usually achieved, but not always, by advertising your presence with bright colours and is known as "aposematism". These bright colours are probably easier for predators to learn and therefore likely reduces the number of casualties necessary before the predator learns the pattern to avoid and providing the mimic with protection.
Aposematic caterpillars and butterflies are essentially warning predators of impending unpalitability or other physical dangers. This is achieved in several different ways. Some caterpillars and butterflies are poisonous and others are not? Poisonous caterpillars have ability to ingest the toxins of their host plants as in the classic example of the Monarch and the cardiac glycosides of milkweed. These poisons are absorbed and retained during the larval stage and passed on, through the transitional stage of metamorphosis, to the adult butterfly. Still others are protected by irritating hairs of caterpillars such as the Mourning Cloak or by the foul odours from caterpillars like the Eastern Swallowtail.
In the final adult stage we can find mimicry. One of the most striking examples is that of the Viceroy (fig.2) intimating the Monarch (fig.1). This type of mimicry was first described in 1862 by Henry W. Bates, while studying lepodoptera in Brazil. Subsequently the following can be considered a typical example of "Batesian" mimicry. Since Monarchs are distasteful and will cause vomiting if consumed by a predator the lesson of avoiding Monarchs is quickly learned. Viceroys finds protection through resemblance. A very important facet to this approach and the key to it's success is that the numbers of the imposter should not be too high in relationship the one being imitated. The reasoning here is that if the ratio was as high as or approaching say, 50/50, it would be possible for the predator to eventually learn the deception through trial and error and soon be able to recognize the perpetrator. In order for this method to be successful the ratio needs to remain low.
A further point pertaining to the relationship between the Viceroy and the Monarch is that recently some studies lepidopterists have concluded that the Viceroy itself is a distasteful quarry. If this is the case what would be the benefit in mimicking the Monarch? This type of advantage has been describe by Fritz Muller whereby certain protected species sometimes seek to augment their protection by mimicking other protected species as in the case of this model......if indeed the Viceroy is distasteful to predators. Known as "Mullerian" mimicry the difference between these two forms of mimicry is that the "Batesian" mimics have no protection of their own and "Mullerian" mimics already have a form of protection.
Other Canadian butterflies, of a "Batesian" example, are the Red Spotted Purple and the female Eastern Tiger Swallowtail which mimic the Pipevine Swallowtail, a distasteful butterfly due to the host plants it eats (Dutchman's Pipevine). The Eastern Tiger Swallowtail females are an intriguing example of mimicry in that they are dimorphic.....meaning that there are two forms (one yellow like the male and one black form). Only the black form of the female is a mimic. The number of females presenting themselves in this black mimic form is proportional to the numbers of Pipevine Swallowtails in the area. Again there is no advantage in high ratios between the mimic and the protected specie.
As larva, the Tiger Swallowtail caterpillar has developed an aposematic approach of presenting (fig.4) a swollen end with two large eye spots giving the appearance of much more ferocious quarry than a predator might care to tackle. If indeed they are prodded they will rear up projecting their two osmeteria (horn like appendages) from their neck. These give off a foul odour which, in combination with the somewhat ferocious appearance this presents, is usually enough to deter even the most persistent attackers. Still another and one of the more striking examples of mimicry is used by Giant Swallowtail caterpillars and is known as "transformational" mimicry. The Giant Swallowtail caterpillar (fig.3) resembles something more like a bird dropping. It will rest motionless during the day only moving at night to feed. In fact all Swallowtail larvae seem to have chosen to resemble something other than caterpillars.
Another approach to survival is protective colouration or crypsis. The formula for success with this survival technique is quite the opposite to mimicry. Here, instead of drawing attention to yourself, the implementers goal is to camouflage their presence.
The larvae of species such as Hairstreaks and Skippers choose this more subtle approach of protective colouration by resembling their host plant colour and patterning. With their uncanny colours and markings they are virtually undetectable and are able to feed in relative obscurity.
While in the pupa stage butterflies are potentially most vulnerable. In order to increase their odds of survival they have developed the ability to camouflage themselves (fig.5&6) by resembling their surroundings. This is an example of protective colouration and not mimicry because the intent is not to draw attention. Once a caterpillar enters this stage it will not have flight or fight at it's disposal......so by looking like a live or dead leaf or even a small branch, this can make it virtually undetectable. No matter how brightly coloured the larva might be, when transformation into a chrysalis occurs, the colour will most likely be green or brownish when complete like the photo below. Trying to locate this in the wild is all but impossible.
Colouring is a powerful method used by butterflies to blend in with their surroundings. For instance when a Mourning Cloak or Comma or Northern Pearly Eye alights on a tree the cryptic patterning on the underside of their wings very much looks like the bark or a dried leaf....therefore concealing their presence. I have often lost track of these butterflies when I know that they have just landed. Sometimes a diligent search will turn up their resting spot......otherwise I have to wait for them to take to flight again in order to relocate them. When you first witness this you will soon realize how effective protective colouration can be.
Hairstreaks present a slightly different form of mimicry. The best way to understand how this works is to first examine the pattern (fig.7) on the underside of the wings. You will notice a very distinct spot along the outer margin of the hind wing called the "thecla spot". Also the hind wing has little hair like tails located behind the spot. When the butterfly is at rest it rubs it's hind wings together in a circular motion. By doing so attention is drawn to that area of movement and upon closer observation one can be fooled into thinking that the thecla spot looks like an eye and the tails look like antennae. Effectively making the butterfly appear to be something other than a butterfly and certainly a creature that is facing the opposite direction. A confused predator, when striking at the mimic, most likely comes up with nothing more than a piece of wing and the butterfly gets a second chance. I have spent some time observing this function and it is a very convincing form of deception. It would appear as if the Eastern Tailed Blue uses this technique as well. Watch for it yourself - it is very cool.