Ladybugs and metamorphosis

Many insects undergo what is known as complete metamorphosis, the technical term being holometabolism. The larvae, or young, of insects that undergo complete metamorphosis are quite different than the adults. A familiar example are caterpillars and butterflies. A butterfly egg hatches into a caterpillar (the larval stage) which spends some time eating and growing. The caterpillar then creates a cocoon and becomes a pupae. It is during this pupal stage that the caterpillar transforms into a butterfly (the adult stage). Ants, bees, wasps, dragonflies, butterflies, flies, and beetles are some examples of insects that undergo complete metamorphosis. 

Why complete metamorphosis arose is still a mystery, but entomologists have been able to identify a few of the advantages to undergoing complete metamorphosis.

1. A decrease in competition. In biology, competition refers to an interaction between organisms that results in harm to each organism. For example, lets say there are 100 spiders living in a field, but there are only 50 crickets. There is going to be competition between the spiders for food because there are not enough crickets for each spider to have one (and spiders aren't particularly fond of sharing). Complete metamorphosis allows some insects to avoid this type of competition. Dragonfly larvae live in ponds and streams and eat aquatic insects. Adult dragonflies spend their time in the air and eat small flying insects. There can still be competition among the dragonfly larvae or among the dragonfly adults, but since the larvae live in a different environment than the adults, there won't be any competition between the larvae and the adults.
2. Different roles. Complete metamorphosis can also allow insects to fulfill different roles for different life stages. The cecropia moth divides its life into two stages. The larval stage (the caterpillars) is in charge of eating and growing. The adults (the moths) are in charge of reproduction; the adults don't have functioning digestive systems. Each developmental stage of the cecropia moth is best adapted to what it needs to do.
3. Protection. Complete metamorphosis is also a good way for the adults to care for the young. This is commonly seen in the ants, bees, and wasps (order Hymenoptera) which build nests to shelter their larvae. The foraging ants/bees/wasps collect food not only for themselves, but also for their larvae.

While not as cute as their parents, ladybug larvae did inherit their parents' predatory instincts!

Now that we have a basic understanding of complete metamorphosis and of its advantages, let's turn our attention to the ladybug. The ladybug is a welcome addition to any garden because ladybugs eat aphids. Aphids can be quite harmful to plants, so having ladybugs around helps make sure the plants stay healthy. However, ladybug larvae also eat aphids. While good for the garden, how does this fit in with the advantages we just listed above? After all, ladybugs are beetles and beetles undergo complete metamorphosis.

Let's examine ladybug larvae according to our three advantages listed above. Since ladybug larvae eat aphids, advantage number one about avoiding competition does not apply to ladybugs. We can also discount advantage number two because the only difference between ladybug larvae and the adults (besides their appearance) is that the adults can reproduce while the larvae cannot. Advantage number three is eliminated because the larvae exist without the adults watching over them.

What I find most interesting about ladybug larvae and adults sharing the same habitat and food source and not utilizing our three advantages is what it means in terms of how complete metamorphosis arose. Did complete metamorphosis arise because it provided the advantages listed above, but then the ladybugs reverted back to a model where the larvae and adults share the same lifestyle? Or did complete metamorphosis arise for a completely different reason and then the advantages listed above came later? Either way, the next time you see a ladybug (or any insect for that matter), I hope you take a moment to think about the role it fills in its ecological niche.