Sunday, April 7, 2013

Bug-on-Bug Violence: Wing Diversity and the evolutionary Arm's race

Hey ya'll!

Weaver ants brutalizing a caterpillar
It's a bug eat bug world, and in the insect kingdom, everything is almost certainly food for something else, as I was explicitly reminded on my way home from dinner! Pictured here are an army of red weaver ants (Oecophylla sp.) brutalizing an unidentified caterpillar. Now caterpillars are essentially the sitting ducks of the insect world. They're soft, and vulnerable, and they can't even move very quickly! Consequently, many of them have developed a variety of strategies and means through which they can defend themselves from predators (to read more about the amazing ways they do this, click here). 

Ants targeting the weak spots of a spined caterpillar
Nature, however, always finds a way and many predators have eventually learnt how to bypass the natural defenses of these animals. Take this case, for example: The caterpillar in mention is what I believe to be the larvae of the Tawny coster (acraea terpsicore/violae), a brightly colored butterfly with leathery wings and a lazy flight. Under normal circumstances, the caterpillars of these butterflies are rather bold insects. They possess a unique chemical defense system, sequestered from the mildly poisonous leaves of the passiflora vines on which they feed on, that make them severely distasteful to birds and most vertebrate predators. Invertebrate predators, on the other hand, aren't so choosy about taste and so to protect themselves from such animals, the caterpillars have evolved a series of sharp and rigid spines that adorn the length of their entire bodies. These spines are not poisonous and do not break easily and it is speculated that what they do, is create (quite literally!) and arm's length of space between any potential predator and the caterpillar's vulnerable body.

When attacking armored prey, like beetles, many ants have
learnt to target the joints, the weak spots in the larger
insect's armor. Here they can inject their poison to
eventually overcome their prey.
These ants in question, however, seemed to have figured out a way to bypass all of that. By literally clipping off the spines bit by bit, the same way a gardener might trim the hedge, the ants whittle away at the caterpillar's defenses until they are able to expose the soft, succulent flesh beneath. And this is not something new. Similar behavior have been observed in the infamous Army ants (Eciton sp.) of Southern America. In fact, many of ants have even taken things one step further and are similarly known to exploit the weak points in the armor of armored invertebrates such as beetles and scorpions. Though the process can take up quite a significant bit of time and energy, the ants will often persevere until the defenses of the prey are completely whittled away. Protein can be very rare and is subsequently a very valuable natural resource in the insect world. Any amount of it (however small) is clearly worth every bit of effort!

The brightly colored, and patterned wings of many
butterflies are thought to function as biological
billboards that provide various signals to other butterflies
as well as animals of other species.
And so it has always been, the evolutionary arms race that began when the first invertebrate crawled out of the ocean, and so it continues to this very day. The need to feed and breed, coupled with the drive to escaped predation has resulted in the diversity and magnificence of insects we have come to know of today. Every insect's uniqueness, every part of it that stands out (aside from the basic anatomy) evolved, often for specific purposes that would aid its survival as an individual and as a species. Take the structure of insect wings for example. The wings of insects are probably one of the most diverse flight structures in the natural world as we know it and are believed to have their origin in gill-like structures possessed by some primitive invertebrates. When mosses and plants began to creep out of the ocean to colonize the shores, invertebrates were soon to follow. Not all of them, however, lost these gill like structures and many continued to retain these flap-like projections as a form of vestigial growth.

The frontal pair of a beetle's wings have evolved into
a virtually impenetrable,waterproof armor known as an
elytra and protects the more fragile set of flight wings
as well as the beetle's body from attack.
But as more invertebrates continued to move to land and evolved into insects, their predators were also quick to follow. It eventually became imperative once again, that insects evolved new strategies to ensure their survival and that of their species. Climbing, the unique ability of insects to scale most vertical surfaces, was believed to have evolved as such a strategy. But the most amazing of all, is perhaps the evolution of the insect wing. The flap-like projections that some insects retained as vestigial growths now developed a new purpose: it gave them the ability to glide! Gliding became an invaluable means of escape as, aside from the insects who retained this ability, there were NO flying predators at the time. This, more than anything else, led to the streamlining of the gliding process and the correspondent anatomical part to give rise to the very first insect wing making the insect quite literally, the first animal capable of sustained flight! The continued benefit of flight propelled the continued evolution of the wing resulting in the sophistication and diversity of wing design witnessed in insects today.
Some pictures of other insect wings, to give you a better idea.
Dragonfly wings have evolved to be strong and resilient structures. They are
waterproof and can function with great dexterity and independence of each other
allowing the dragonfly to be one of the most accomplished of aerial predators.
Cicada wings
Grasshopper wings
Leafhopper wings
But of course, the evolutionary arms race in insects is not limited to its wings. In fact, every part of an insect's anatomy was originally geared towards helping the species survive each other. The barbed stings of bees for instance, was thought to have evolved as a defense mechanism against invertebrate predators. Interestingly enough, bees do not die when they sting invertebrate such as hornets and moths, which frequently invade their hives. The eventual introduction of vertebrate predators into the mix only branched this evolution out further. The following illustrated chart might give you an idea as to how this might have taken place.  

This, combined with the fact that insects have been around far longer than any other living creature on the planet, and because of their fast reproduction rates, has lead insects to diversify to the point that they are quite literally one of the most successful animals on the planet! Try to remember that the next time you witness an ant carving up a caterpillar, or stop to appreciate the remarkable patterns on the wings of a butterfly. You might very well be witnessing the result and progress of the evolutionary arms race!


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