The question of how periodical cicadas perceive time has been a long-standing one， unfortunately， there are no definitive studies yet that precisely explain this phenomenon.
The prevailing theory suggests that periodical cicadas might determine their lifespan spent underground by observing the seasonal changes in the trees they feed on.
It’s a common misconception that cicadas in their subterranean phase are dormant or “hibernating.” In reality， this is their nymphal stage， during which they are highly active.
Cicada nymphs sustain themselves by siphoning plant sap. They burrow underground， seeking out tree roots to access this vital nourishment. Both adult cicadas and nymphs consume nutrients that are actively being transported within the plant， such as sugars produced through photosynthesis and various amino acids generated by other metabolic processes.
The availability of these nutrients in plants exhibits clear seasonality. For instance， the concentration of amino acids in a tree’s xylem experiences a brief but significant surge during its annual budding and flowering phases.
It is highly plausible that cicada nymphs utilize these seasonal fluctuations as an indicator of the years that have passed.
This hypothesis is not merely conjecture; it is supported by observations and has been tested through experiments， though designing such studies for 17-year cicadas presents considerable challenges.
In 2007， an unusual warm spell occurred in Cincinnati， Ohio. January of that year experienced temperatures sufficiently mild for trees to bud prematurely. However， a severe cold snap in February followed， killing many of these new buds. Subsequently， spring temperatures returned to normal， and the trees resumed their growth cycle as expected.
This anomaly resulted in trees budding twice within the same year. This event seemingly triggered the premature emergence of some 17-year cicadas， which were expected to emerge in 2008. The number of these early emergers was relatively small， numbering only in the hundreds [1].
The climate anomaly effectively caused trees to experience 17 budding events over 16 years. This could have misled some 17-year cicadas into believing they had completed 17 years underground， leading to their early emergence.
Furthermore， researchers have conducted experiments where a group of 15-year-old cicada nymphs were manipulated to consume tree sap from trees that budded twice within a single year. These cicadas also emerged prematurely [2].
These observations and experiments strongly indicate a connection between the “sense of time” in periodical cicadas and the seasonal cycles of trees. However， the precise mechanisms by which cicadas “remember” their 17 or 13 years underground remain unknown.
Frankly， even for humans， it would be challenging to accurately recall living in a particular place for 17 years without any dated records. Some individuals might resort to constantly marking seasonal changes as reminders. It is conceivable that cicadas employ a similar method， perhaps by tracking the number of seasonal cycles through their tree-feeding activities.
Finally， here’s another interesting fact about cicadas.
Previously， I believed cicadas were solely categorized as “annual cicadas” and “periodical cicadas.” However， I recently discovered the existence of “primitive periodical cicadas.”
“Annual” cicadas do not necessarily have a one-year life cycle. Instead， large broods of their nymphs emerge above ground annually to complete their final molt into adults. Their actual life cycles can range from 2 to 9 years or even longer. It’s important to note that it remains unclear whether all annual cicadas strictly adhere to a specific lifespan.
This contrasts sharply with periodical cicadas， which complete their life cycles in synchronization and emerge en masse.
There are over 3，000 known species of cicadas (Cicadoidea). However， only about nine are classified as periodical cicadas， with seven of these found in North America — the 17-year and 13-year cicadas. Two other species’ periodical nature is less certain: one is the “World Cup cicada” (4-year cicada) found in Northern India， and the other is an 8-year cicada from Fiji.
Primitive periodical cicadas are characterized by distinct “big years” of exceptionally large emergences， with significantly fewer appearing in other years. This is why they are termed primitive periodical cicadas.
However， it is currently unknown if primitive periodical cicadas represent an evolutionary intermediate stage between annual and periodical cicadas.
Notably， regardless of their classification or life cycle duration， most cicada species undergo five molts throughout their lives. Four of these molts occur underground， with the final molt taking place above ground as they emerge as adults.
