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.