THE EVOLUTION

It is not a surprise that evolution of insects is an unsolved puzzle! The reason being lack of concrete fossil records to prove its most primitive ancestors identity. The oldest known definite insect fossil is from the Devonian Era Rhyniognatha hirsti, estimated at 396-407 million years. It is interesting to know that this species had mandibles (a mouth part, jaw) which are similar to that of winged insects; these are similar to some insects that are present today. And so we can conclude that there were insects prior to the Devonian period. According to the accepted theories of evolution, insects, like other animals, have descended from more primitive forms of life that existed in earlier geological periods. But the preservation of insect fossils are relatively poor and limited to only a few sites unlike mammals, reptiles and trilobites (extinct marine arthropods). The reason for this scarcity of insect fossil is the poor preservation potential of an insect’s exoskeleton. Like other Arthropods, the insect exoskeleton is made up of chitin (a type of carbohydrate), and a tough protein. This material is not hard (calcified) like the exoskeleton of a trilobite. Hence, in spite of abundant insect fossil there is a complete absence of evidence to trace the primitive stages of insect evolution.

The gap in fossil evidence leaves insect evolution open to several speculations. There are three aspects of insect evolution that several evolutionists are still working on, i.e., their wings, compound eyes and metamorphosis. Let us try and understand these critical aspects of insect evolution…

Insect wings are made up of chitin, which is extremely light, but amazingly strong. The wings are strengthened by a complex set of veins that not only provide support but also resist bending and twisting of wing during flight. Insect possess some 30-odd wing muscles in their thorax which are known to be the most powerful muscles in any animals, in per square millimeter area of cross-section area. Although, on an average, insects can beat their wings 200 times per second, they can go as fast as 1,000 times per second! Wings are also opened to absorb heat. In case of birds, it has been concluded that their wings are modified limbs. However, in case of insects, it is only assumed that their wings are developed from a body part; a process better known as cooption, but it is difficult to determine which organ could have been co-opted.

Now coming to the insect eye, it is a complex structure consisting of a large number of closely packed ‘lenses’ also known as ommatidia. Insects like honey bees and some flies may have 4,000 lenses in each eye. Apart from a pair of compound eyes many insects have two or three spot like eyes called ocelli. Compound eyes can detect the sky’s plane of polarization which helps them to navigate. These eyes are also very sensitive to movement.

Eye - Insect

Eye - Trilobite

One of the most characteristic and magical features of insects is metamorphosis! Insects almost always hatch in a condition morphologically different from that of the adult. In order to reach to adulthood they consequently have to pass through changes of form (Larva > Pupa > Adult) which are collectively termed as metamorphosis. Hence, a caterpillar is different from a butterfly not only externally but it also differs in internal anatomy. Many insects show complete metamorphosis involving larva, pupa and adult stages. The evidence of complete metamorphosis is found early in the fossil record, but no record of its evolution has ever been found.