INSECT DIVERSITY AND ITS SIGNIFICANCE

Insects dominate terrestrial ecosystems in terms of species, biomass, number of individuals, and importance of ecological roles. It is believed that 80% of all living animals are insects. People today, generally overlook insects due to their small size and so-called inconsequential existence. But in reality, insects play a vital role in the smooth functioning of our ecosystem. Their importance can be highlighted by giving only a single example; about 95% of pollination is done by insects. Thus, insects are indispensable for the survival of every form of life present on the Earth!

Granite Ghost Bradinopyga geminata

Order	Common names within orders	Number of species	Number of species reported from India
Colombola	Springtail	l 2,000	210
Odonata	Dragonflies and Damselflies	>5,500	463
Blattaria	Cockroaches	3,700	180
Mantodea	Mantids	>1,800	162
Isoptera	Termite	2,000	253
Orthoptera	Grasshoppers, crickets and locusts	25,000	1,750
Hemiptera	True bugs	82,000	6,500
Hymenoptera	Ants, bees, wasps	1,30,000	10,000
Coleoptera	Beetles	3,50,000	15,500
Diptera	True flies, mosquitoes	2,40,000	6,093
Lepidoptera	Butterfly and Moths	1,80,000	15,000

TABLE showing diversity of selected insect groups (all figures are approximate)

We generally consider them either ‘useful’ or ‘harmful’. But,insects provide useful services like pollination, honey, silk and lac. Numerous insects also work as ‘natural cleaners’ by feeding on dead and decaying animals and plant remains; thus playing a vital role in the circulation of energy and matter within the ecosystem. Hence, it can be said that every aspect of human life have been affected by insects. In fact, within their enormous world they even somehow manage to control their numbers so no one insect species ever dominates.

Oides sp.

Though it may sound strange, but conservation  of insects is a need of the hour. Our developmental activities are responsible for environmental degradation. Such activities are causing severe threat to biological diversity and insects are an essential part of the food chain. Thus, loss of insect species will result in the collapse of the entire ecosystem, ultimately causing extinction of most of the life forms from earth including human beings. Only time has answers to the questions which we find difficult to answer today, regarding insect evolution. Maybe good fossil evidences or molecular research will unravel the mystery of insect evolution.

INSECT AND PLANT INTER-RELATIONSHIP

There is another very interesting facet to insect evolution. Understandably, fossil records show that diversity of insects is directly related to the diversity of angiosperms (flowering plants)! It is highly significant, that no highly evolved pollinators such as Lepidoptera (butterflies and moths) and bees had been found before the angiosperms appeared. It is considered, that Lepidoptera must have evolved at the same time as the early angiosperms. Evolution of land plants (especially flowering plants) is the major force driving the diversity of insects, i.e., with the increase in the diversity of land plants, the diversity of insects increases too. The interaction between plants and insects is an appropriate example of co-evolution on Earth. Co-evolution is the mutual evolutionary influence between two species (the evolution of two specie totally dependent on each other) where each of the species involved, prefer each other from several choices, and therefore evolve together. Insect and plants are engaged in an arms race of evolution, since they first appeared on Earth. Plants have developed many defence mechanisms like chemical toxins, physical barriers, to protect themselves from herbivorous insects. But sometimes they resort to a symbiotic relationship with the insect. Such co-evolution can occur between 1) a single plant and a single insect, 2) between a single plant and a group of insects, or 3) between a group of insects and a group of closely related plants. This interaction may be beneficial to both (mutualism), or is beneficial to one but harmful to another (predation). Several moth-pollinated plants have long spurs or tubes which are of the exact length of a certain moth’s “tongue” or proboscis. For example, Charles Darwin predicted the existence of a moth (Darwin’s Hawk Moth Xanthopan morgani) in Madagascar based on the size and shape of a flower (Darwin’s Orchid Angraecum sesquipedale) which he had seen there. This did turn out to be true but was discovered about 40 years after Darwin’s prediction. 

In many cases, plants try to reduce the attack of insects and insects counteract and take the battle a step forward. This type of co-evolution can be observed between plants and their insect predators. The plant kingdom shows the presence of a large number of chemicals used by the plants to reduce the impact of herbivores on their survival and reproduction. These chemical defences can act as repellents or toxins to herbivorous insects to reduce plant digestibility. But in some cases, these herbivores too, have evolved different ways to use plant defences for their own benefit. By accumulating toxic compounds from their food, the insects protect themselves from predators (sequestering). For example, the larva of Plain Tiger Butterfly Danaus chrysippus feeds on the Milkweed plant (Calotropis sp.). The Calotropis has defensive chemicals which are ingested and stored in the body of the caterpillar. These chemicals remain in the adult butterfly making it unpalatable. Thus, the toxic chemical present in the plant is used by the butterfly for its own defence.

There are also some plants that feed on insects (Carnivorous/Predatory Plants). These plants derive essential nutrients from the trapped insects. Pitcher plant, venus flytrap, sundews and bladderworts are the best known examples of insectivorous plants. These plants either trap insects by trickery or when the trigger hair is touched. On one hand, it is seen that sometimes only one or few insects can feed on a specific plant (Milkweed plant and Plain Tiger Butterfly). On the other hand, insects like grasshoppers adapt to eat almost any plant. There are also insects that learn to eat plant parts avoiding the harmful toxins.

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.

The unknown world of insects!

About one million insect species are known to inhabit every nook and corner of our planet, except seas and oceans. Scientists believe that there maybe about nine million more insect species inhabiting our planet! Insects represent an interesting branch in the tree of life, whose development equals, if not excels, that of any other branch. In terms of diversity, insects are the dominant life form on land; there are about 61,500 known species of insects in India alone.

Man Faced Bug

Man’s achievements and knowledge
about the universe are highly
commendable — the exact distance
between the Sun and the Earth is 14,94,76,000
km, the total weight of Earth is 5.9736 × 1024
kg. But, if asked, ‘how many insect species are
present on Earth?’ The answer would not be so
precise, because our information is limited when
it comes to these astonishing creatures. 

Tree Hopper

About one million insect species are known to inhabit
every nook and corner of our planet, except seas
and oceans. Scientists believe that there maybe
about nine million more insect species inhabiting
our planet!

Blister Beetle

Insects represent an interesting branch in the
tree of life, whose development equals, if not
excels, that of any other branch. In terms of
diversity, insects are the dominant life form on
land; there are about 61,500 known species of
insects in India alone. They are of different
shapes, size and colour with remarkable
adaptation capabilities. Naturally a question arises
in our mind as to why this diversity? At this point,
it becomes imperative for us to understand the
process of evolution of insects …