Sunday, February 8, 2015

Batesian and Muellerian Mimicry

Some defenseless arthropods that would make great prey have evolved to resemble other species that are toxic, bad tasting or heavily armed. One precondition must be that the models are not just foul tasting or aggressive, but memorably so: So the mimicked models are announcing their in-edibility with warning colors and patterns (aposematic) and the imitators share in the protection. This is the Batesian mimicry that is often explained using toxic Monarch Butterflies and their harmless look-alikes, the Viceroys as an example. Also similar looking is the Queen Butterfly. Queen and Monarch caterpillars both grow up incorporating toxins from their mutual host, the milkweed plant. In the case of Monarchs and Queens the warning function of colors and patterns is enhanced because they are so similar. A predator who has learned to avoid one will also leave the other one alone. Strength that grows with numbers. This kind of mimicry is called Muellerian.

                     Vespula pensylvanica, Western Yellow Jacket (social), Ancistrocerus tuberculocephalus (solitary), Mischocyttarus navajo (social), Polistes comanchus (social)
A striking and well known Muellerian mimicry group is that of the yellow/black banded, stinging wasps, who share a pattern of warning colors that stayed evolutionary stable even as wasps spread over nearly all continents and formed many different species, genera and tribes. (I am assuming here that this phenotype relies on an ancient gene (group of ). Did the pattern evolve before social hives appeared? The threat of the social group that attacks  together certainly gave the warning pattern its power. Solitary wasps that share it are profiting from the fierceness of their social sisters.

Climaciella brunnea (Wasp Mantidfly), and Polistes comanchus
Batesian mimicry:
Social wasps are probably among the most aggressive defenders of their hive area, so they have many very close mimics. Previously I showed an examples for Muellerian mimicry. Here are some of many examples of Batesian imitators: The Wasp Mantisfly (Neuroptera) is shown with one of its models Polistes comanchus. But other individual in the same mantispid species strikingly imitates solid brown and more strongly banded Polistes species.
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A moth in the family Sesiidae (Clearwing Moths) and a Syrphid Fly. The moth was so convincing that I watched an experienced entomologist hesitate to take it out of his net by hand. The fly goes the extra mile to camouflage a feature that partly gives away the wolf to Red Riding Hood: Grandma, what big eyes you have! (Wasp eyes are much smaller than fly eyes). You may also notice another obvious difference: the wasp has much larger antennae. So several species of syrphids compensate by flying with their front-legs stretched forward like antennae
Also note the locations (on the individual images): all occur together at higher elevations..

Longhorned Beetle Strophiona tigrina
Several Acmaeodera species in the family of the Metallic Wood-boring Beetles 
 Even many beetles are using the 'yellow jacket' color pattern, especially many flower visiting species. But when beetles want to take off, most (like Strophiona tigrina, above) have to open their hard front wings (elytra) to make room for the membranous hind-wings that are used to fly. So there goes the nice imitation of a wasp. Or does it? Most Buprestides in the genus Acmaeodera have nicely banded elytra and they keep them closed during their frequent flights. The side margins of the elytra are bend upwards and the hind-wings can be stretched out from underneath. The beetles fly with closed elytra and the illusion of a wasp stays intact. There is also a group of scarabs that flies like that (Euphoria) and they very much resemble bumblebees or carpenter bees when flying, they even hit the right buzz. Silberglied, R.E. and T. Eisner. 1969. Mimicry of Hymenoptera by beetles with unconventional flight. Science 163:486-488. DOI: 10.1126/science.163.3866.486

Longhorn Beetle Tragidion decipiens, Tarantula Hawk, Hemipepsis ustulata, and Mydas Fly Mydas xanthopterus, Photo Bob Barber
One of the most painful stings according to Justin Schmidt's Pain index is delivered by one of our big, solitary wasps, the Tarantula Hawk (female only) Of course, there are several Muellerian mimicry 'gang members' like painfully biting Robber Flies and other wasps like a big Sphecid species. But there are also harmless mimics like several Cerambycids (Longhorn Beetles) in the genus Tragidion. These beetles also visit some of the places where the big wasps lick up sweet tree juices. After landing, the beetles often keep their elytra open for a while - very unlike other beetles but in striking similarity with the wasp.

Phidippus apacheanus and Dasymutilla sp.
I am not sure whether the connection between Phidippus apacheanus (probably including several other Phidippus species) and the very painfully stinging mutillids, the orange and red species of Dasymutilla should be called Muellerian or Batesian. Are jumping spiders dangerous prey to a bird or lizard? Probably not, so Batesian. Anyway, the spiders and wingless wasp females don't only look alike, they also frequently crawl around together on the branches of Desert Broom. The spiders are hunting for insects that get attracted by the sweet juices and the wasp is a sweets-lover herself.

Dasymutilla male
Dasymutilla males are also often sleeping close by. They sport the same colors as the females and their wings are folded and hardly visible. Like ALL males of stinging Hymenopterans, they are also just sharing the protection that the aposematic coloration of their females provides. In that sense, most brightly colored male wasps and bees are Batesian mimics of their female counterparts, because NO male hymenopteran has a stinger.

Here is a little Coreid (Leaf-footed Bug) nymph in the genus Narnia and her mom. The little ones were all over the juicy fruit of a Barrel Cactus and its extra-floral nectaries while the adult was hiding among the thorns, very much out of reach. Where the little ones just naively taking a risk or were they protected? The barrel cactus fruit and nectaries are often visited (and owned) by the very defensive local fireants Solenopsis xyloni. The bug nymphs resemble the ants in size, color and preferred location. Not so much in shape. But the resemblance really does not have to confuse the human eye as long as it repels a predator

To dispel the idea that only the very painful stingers among hymenoptera can be mimicry models: Here is a peaceful, flightless, dusk-active darkling beetle, Eleodes armatus on the right. A stink beetle, like all of his genus and many in his family). There are 2 huge glands in his abdomen that douse him and the prospective predator's nose with very obnoxious chemicals. To get good coverage, he lift his hind end where the glands open, high into the air and lets gravity do some of the work. The same-sized Cactus Longhorn Beetles here Moneilema appressum is also dark, flightless, and has to move between cacti across the light desert sand at dusk. So it does a stop-and-go walk, just like Eleodes and even lifts its behind when disturbed. The big dark Calosoma species (Ground Beetle, Carabidae) also joins in the behavioral mimicry, but those guys can release a nice stink by themselves ....

Pipvine Swallowtail, Battus philenor 

Red-spotted Purple, Limentis arthemis
In AZ (and elsewhere in the southern US) where riparian areas interface with desert habitats, we find two butterflies that can be easily mixed up: One is the Pipvine Swallowtail whose caterpillar picks up enough toxins from its foodplant, the pipevine, to make not only the caterpillar but also the adult inedible for many predators, the other one is the Red-spotted Purple. Interestingly, that same species, Limentis arthemis, has northern color forms that look quite different. Check them out on bugguide 
So where the model is not around in sufficient numbers, the similarity provided no advantage and did not evolve.

Lycus simulans and  Elytroleptus ignitus
A final twist: Netwing Beetles in the genus Lycus are highly toxic and announce that fact with bright red warning colors. They also often congregate in great numbers to mate and are hard to overlook. Not surprisingly, several species of Lycus are similarly colored and patterned, forming a tight Muellerian mimicry group. There are several moths and  beetles of unrelated families that mimic them as Batesian groupies.
Were there enough mimics to endanger the whole system? Did naive predators get lucky too often so they did not learn to avoid the insects?
Anyway, there is something very interesting going on in SE Arizona: Several species of the Longhorn Beetle genus Elytroleptus usually associate with the toxic Lycids around oak trees (extra floral nectaries) and on flowers. In those mixed groups, many Lycids looked like they had been chewed on. Holes in the elytra, leaking heamolymph ... Normally we think of all Longhorn Beetles as strictly vegetarian. But it turned out that Elytroleptus were chewing on Lycus, and chemical analyses revealed that the Longhorns were actually incorporating (sequestering) the toxins of the Lycids.  A Batesian mimic becoming Muellerian at the cost of its model!

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