Cicadas, a type of winged insect known for their distinctive droning sound, are often associated with hot summer days. However, one peculiar thing that many people wonder about is why some cicada species only emerge every 17 years.
Cicadas spend 99.5% of their lives underground. Most of them live in large colonies underground for years at a time. However, there are some species that live underground and only emerge every 13 or 17 years – notably the Magicicada, which includes three main species: Magicicada septendecim, Magicicada cassini, and Magicicada septendecula. These cicadas primarily inhabit the eastern United States.
Magicicada cicadas undergo a lengthy larval stage underground, which can last up to 17 years (or 13 years) before emerging to molt, mate, and reproduce. Compared to regular cicadas, Magicicada are larger in size.
In fact, not all cicada species live and molt in cycles. Some species emerge annually, appearing every summer.
The most distinctive feature of Magicicada cicadas is their long life cycle of 13 or 17 years. Their larval stage occurs entirely underground, feeding on tree sap to grow. Once they complete this stage, they burrow up to the surface, molt, and undergo a brief adult stage lasting only a few weeks to mate and reproduce. The loud calls of the 17-year cicadas can be quite bothersome to humans, especially at night.
In 1907, entomologist Charles Lester Marlatt studied different broods of cicadas in the United States, recording their life cycle durations. These cicada broods can consist of millions of individuals – this is also part of the cicada’s evolutionary strategy.
Some broods have more than 1.5 million individuals per acre. When they first emerge from the ground, they become easy prey for reptiles, birds, cats, and even squirrels.
Thus, cicadas do something quite simple: they overwhelm these predators with sheer numbers. This is a survival trait known as predator satiation, which essentially means that there are too many cicadas for predators to consume them all.
Magicicada cicadas are generally larger than regular cicadas. They are black or brown with orange or red stripes, while regular cicadas can come in various colors, including green, brown, and black. The calls of Magicicada cicadas are typically louder and higher-pitched than those of regular cicadas. Overall, Magicicada cicadas and regular cicadas are two distinct types of insects with unique characteristics and behaviors. The unique 17-year life cycle and specific habitat make Magicicada cicadas a remarkable species that captures the attention of scientists and the public alike.
Contrary to popular belief, cicadas do not hibernate underground; they are quite active. In their wingless nymph form, they dig tunnels and move around, feeding on the xylem from tree roots. They primarily live within 2 feet (61 cm) of the surface, moving deeper and feeding on thicker roots as they mature.
However, cicadas cannot obtain all the necessary nutrients from tree roots alone. They also rely on symbiotic bacteria to provide essential vitamins and nutrients in exchange for a portion of the cicada’s energy.
As cicada nymphs burrow underground, they contribute to aerating the soil and promoting leaf litter decomposition, helping trees absorb nutrients more effectively. Magicicada cicadas play a crucial role in controlling other insect populations, maintaining ecological balance in deciduous forest ecosystems.
The exact mechanisms that cause cicadas to emerge simultaneously after 17 years are not fully understood, but researchers believe it is related to a combination of hormonal changes triggered by the tree sap they consume and environmental signals.
Additionally, scientists have proposed several theories to explain this unusual life cycle of cicadas. Some of the most common theories include:
- Exploiting food sources: Trees shed leaves cyclically, and tree roots are the primary food source for cicada nymphs. The 17-year cycle coincides with the leaf drop cycle of many tree species, ensuring cicada nymphs have a plentiful food supply throughout their lengthy underground development.
- Energy conservation: The long life cycle allows cicada nymphs to grow slowly but steadily, conserving energy and avoiding resource waste. Spending many years underground helps them evade dangers above ground, such as extreme weather changes and human threats.
- Reproductive advantage: When cicadas emerge simultaneously after 17 years, they have a greater chance of successful mating, ensuring high fertilization rates and producing subsequent generations. Concentrating reproduction in a short time frame minimizes competition for food and shelter for nymphs.
The 17-year life cycle of Magicicada cicadas is not only a biological mystery but also a testament to the remarkable adaptability of this organism to its environment. It highlights the importance of survival strategies in preserving and maintaining species.
