What is the main topic of this lecture?

The butterfly’s life cycle has captured the hearts of people for ages. It’s a rather romantic story of transformation. Everyone here knows where butterflies come from? They are born as caterpillars, and then later transform. Today, we’re going to look a little more closely at this life cycle.
    So let’s start at the beginning--laying eggs! Butterflies usually lay their eggs on leaves, often on the same kind of plant that caterpillars eat. The female can instinctively sense whether the area is safe from predators or not. When the egg is laid, it has sticky glue around it, which hardens onto the leaf’s surface and fixes the egg in place. And the eggs will usually stay for a few weeks after being fertilized before the caterpillar is born.
    Then, we come to the caterpillar phase, or larvae, as they are also called. Eating as much as possible is the caterpillar’s main purpose. These multi-footed little creatures are hardwired to chew through as many leaves as possible. Have you ever noticed those small chunks missing from green tree leaves? Those are caterpillar bites. Caterpillars are, for the most part, herbivores, meaning that they only eat plants. The caterpillar’s body grows very rapidly with all this eating, and it will shed its skin three or four times in its lifetime in order to make room for its growing body. Now, in the last couple of skin sheddings, the caterpillar’s body begins to develop butterfly’s qualities. Small wings begin to develop under the surface of the skin, for example. The caterpillar is now reaching its apex; at a certain point, the body registers that it has reached its maximum size. It is ready now for the next phase of its life. It stops searching constantly for food, and begins the wandering stage, in which it searches for the perfect place to make its cocoon. Once it does, it begins the pupation phase.
    The pupa, also called a chrysalis, is the little silk bundle the caterpillar makes around itself. It makes the pupa by fixing itself to the surface of a leaf or branch, and then by shedding its skin one last time. This final shedding produces the nutrient-rich chrysalis substance. The chrysalis is hard and firmly fixed to its surface, so much so that ... very few predators can remove it. The transforming caterpillar stays in the pupa for weeks, often months --they can survive the winter in a chrysalis. During that time, the caterpillar transforms into a butterfly. This process is called metamorphosis. During metamorphosis, the wings develop fully from the caterpillar’s body. To do this, the body must absorb an incredible amount of nutrients from the pupa.
    Then when the pupation period is over, the butterfly emerges from the pupa fully formed. The butterfly is the mature, adult version of the organism, and represents the reproductive phase of its life cycle. Although butterflies have a relatively short life span -- between several days and several weeks--they have a few jobs. The first one is eating; the butterfly feeds primarily on liquids, mostly nectar from flowers. It also needs to find a mate, and then the female must lay her eggs. Also, this is the migration phase for the species, which means that the butterfly may migrate to new habitats and lay eggs there.
    The butterfly species are, in a sense, born twice. They have different incarnations and different new beginnings. This is probably why humans are so enraptured with the butterfly’s metamorphosis. The idea that the caterpillar dies-only for the beautiful butterfly to emerge for a few short weeks-is quite touching.
P Small wings begin to develop under the surface of the skin, for example. The caterpillar is now reaching its apex; at a certain point, the body registers that it has reached its maximum size.
What does the professor imply when he says this:
P The caterpillar is now reaching its apex.