A butterfly’s life cycle provides an excellent way to teach students about the stages of development found in all living things and introduce scientific vocabulary.
Female butterflies lay their eggs on leaves. Within days, these eggs hatch into caterpillars that consume leaves while eating more and growing larger as their skin sheds many times before becoming adults.
Egg
Eggs are female reproductive cells produced by animals, plants or humans and serve as their gametes – this term also refers to their protective container which holds embryos until they can support themselves independently.
Butterflies begin their lives as eggs that can be found anywhere, such as on leaves. Their protection comes in the form of two tough membranes; one adheres to their shell while the other secures them to their egg white, called albumen. Both these super strong, transparent protein membranes are comprised of fibrous material called keratin for maximum durability.
These membranes allow water to pass in and out without letting bacteria in, yet are very difficult to puncture – taking more force than the weight of one million elephants to break a butterfly egg!
Similar to reptile eggs, most insect eggs are relatively soft so sperm can more easily access and fertilize the egg within. Once inside, fertilization occurs and growth commences within that fertilized egg known as pre-pupa.
Insect eggs may appear small, but that doesn’t mean there’s nothing going on inside! An embryo’s development into an adult butterfly starts in various areas within its egg. Chemical gradients within its cells allow certain regions to specialize further and form parts such as heads and tails for specific parts of its development – for instance the head develops from one end while tail formation starts on another!
Most modern mammals give birth to live babies, yet some species still lay eggs, including platypus and echidna. Their eggs closely resemble reptile eggs; those not fertilized may even be sold as “naked eggs.”
Naked eggs provide students with an effective visual for how a semipermeable membrane allows molecules such as water to pass in and out through its semipermeability, creating diffusion and osmosis processes. By pairing this activity with others such as Cellular Soap Opera Snack, students will develop a more comprehensive conceptual model of diffusion and osmosis.
Caterpillar
As it grows into its larger form, caterpillars feed on plants. Their voracious appetites cause them to consume up to 1,000 times their birthweight during this stage, providing enough calories for their miraculous metamorphosis into an adult butterfly.
Caterpillars use silk as a protective covering, sometimes building a cocoon around themselves; other times simply rolling up in leaves or branches for shelter. Many hairy caterpillars protect their pupa with body hair from within their own bodies to further cushion its protection.
As a caterpillar develops, it must regularly shed its outer skin in order to gain space for expansion and feeding. This process is known as molting; some caterpillars may molt dozens of times before reaching maximum growth; once this point has been reached, feeding stops and it prepares to move into its next life stage, known as pupal phase.
As soon as a caterpillar reaches maturity, it sheds its final molt and enters the pupal stage. Unlike most moths, butterflies do not construct cocoons for protection in this stage – instead shedding hard coverings known as chrysalises that often feature camouflaging colors or warning colors to alert predators that the caterpillar might contain distasteful materials or poisonous spines that will make for easy prey.
Scientists are amazed by the speed of this transformation. In one study, researchers observed a painted lady butterfly as it entered its chrysalis, and discovered that its guts quickly rearranged while its breathing tubes only changed slightly shape during this process.
Inside a chrysalis, fluid dissolves old caterpillar tissues into undifferentiated cells that will become new butterfly body parts – these imaginal cells also give the chrysalis its color.
Once complete, chrysalises usually emerge within two weeks – although some species require months – ready for their new homes: whether on grassy lawns or tree bark.
Butterflies have evolved to thrive in various habitats. They don’t rely on one specific host plant for laying eggs and they feed on different substances such as nectar from flowers, honeydew from ants, rotting fruit, sweat, urine, dung and dead animals – not all at the same time!
Pupa
As soon as the caterpillar reaches its last instar, it ceases eating and burrows or forms a chrysalis – an outer covering with hard shell covering it – in which protein enzymes begin breaking down muscles and other tissues and special groups of cells start creating wings, new mouthparts, and reproductive organs for its emerging insect self. After several days, its dark color indicates its readiness for emerge.
Scientists have long attempted to capture the dramatic transformation from caterpillar to butterfly on film, but disturbing a living caterpillar in its cocoon or chrysalis is risky and may disrupt its transformation into an adult moth or butterfly. A notable exception occurred recently when researchers got an unusual look at an uncocooned Tussah silkmoth that provided researchers with a unique glimpse at its wings, antennae and legs (more about that in future columns).
Finding an accurate view of this transformation process remains challenging, however. Care should also be taken not to disturb a pupa that has yet to emerge or it could die (some caterpillars and flies use “cocoonase” to loosen their cocoon). For an idea of what chrysalis looks like prior to transformation, check out these amazing photographs of an unsuccessful caterpillar cocoon attempt.
Richard Rolff used an electron microscope to observe the initial stages of this transformation, publishing his findings in PNAS journal and showing that proteins dissolved from within a caterpillar’s body reorganized and rebuilt its muscles and other tissues. Proteins are large molecules found throughout living things including muscles and tissue as well as being key building blocks of drugs like antibodies and hemoglobin production.
Researchers involved with Rolff’s study demonstrated that E93, one of the proteins activated by steroid pulses in larva, can activated through hormone pulses in its larva. When E93 becomes active, caterpillars begin forming wings and other body parts while their chrysalis becomes darker and opaquer over time – although scientists still do not fully comprehend why this happens – although they have discovered the remodeling process is controlled by hormones.
Adult
Once a caterpillar stops feeding and begins to shed its skin, it is ready for its transformation into an adult butterfly or moth. This process occurs within a chrysalis; depending on the species of butterfly or moth this could mean suspended from a branch, hidden under leaves or even underground burial. Though appearing dead from the outside, special cells present during larval development begin growing rapidly inside this seemingly dead shell – eventually giving rise to wings, legs and eyes of an insect – while digestive juices provide energy needed for this growth process – digesting caterpillar digestive juices turn into “tissue cell soup” providing fuel to these growing cells within.
Developing the chrysalis may take several weeks to a month. When complete, it can be distinguished from previous stages by its distinctive pattern of black (or dark brown) and yellow bands; or its proboscis which extends from its head like a straw to sip nectar; its body no longer looks transparent or shiny, and its setae on the thorax appear shorter and less stubble-like than they did during second instar larval development.
When an adult butterfly or moth is ready to emerge from its chrysalis, it opens it, crawls out and hangs upside down until its wings have hardened enough for flight. At that point, its veins pump fluid into its wings in order to elongate and inflate them – until then, waiting patiently until its wings have dried enough.
Contrary to caterpillars, which must find a host plant to lay its eggs on, butterflies have more freedom in terms of traveling and feeding wherever they wish. But this freedom comes with risks; many butterflies and moths can be preyed upon by lizards, birds or dragonflies; to combat this, some adult butterflies employ camouflage techniques as well as produce toxic or foul-tasting chemicals to deter potential predators.
Certain species of butterfly, like the Mourning Cloak, spend winter months in an inactive state known as diapause before returning to flight in spring to breed at their respective breeding grounds. Watching a butterfly life cycle unfold is truly incredible to observe and serves as an astounding testament to nature’s remarkable power to sustain life.