One of the interesting features of insect life cycles is that the juvenile life stages will often live in a very different habitat than the adult, and may also make use of very different food sources. For example, caterpillars generally eat leaves, while butterflies drink nectar from flowers.
Cicadas are no exception: they spend a lengthy period (sometimes as long as 17 years) underground in their juvenile forms, before emerging as adults. This feature of the lifespan can lead to interesting consequences, as we shall see. In the UK, we only have one cicada species, and it is very restricted in distribution, being found only in the New Forest.
With a seventeen year life cycle, the periodical cicada is very long-lived for an insect. Yet even this is dwarfed by the wood-boring beetle Buprestis aurulenta, which has been known to live for 51 years! [Source: University of Florida]
What is a Cicada?
Cicadas are a kind of bug, related to greenfly and to shield bugs. They are not harmful to humans, and feed on plant sap. You may have been aware of cicadas when on holiday in a warm climate - the adult males make a loud call, using specialised structures in the abdomen, to attract females. Each cicada species has its own unique "song", presumably to avoid attempted mating with the wrong species.
The life cycle of most cicadas generally takes from two to eight years, though as we shall see, some have considerably longer cycles. The bulk of this period is spent underground. The juvenile stages, or nymphs, are well adapted to underground life, having robust spade-like front legs to enable them to dig their way to plant roots.
For the majority of cicada species, the developmental stages are not synchronised. That is, adults emerge each year, even though they may have spent several years underground as nymphs. However, some species, the periodical cicadas, develop synchronously - the adults emerge in one year, lay eggs and die. The eggs, which are laid in stems of trees, hatch into nymphs, which drop to the ground and burrow underground to feed for many years, finally emerging as adult cicadas in the same season as the others which hatched at the same time. Thus, these cicadas are not seen as adults in most years. In fact, the periodical cicadas have very long developmental periods, spending thirteen or seventeen years as nymphs, with adults only seen every thirteen or seventeen years.
It is likely that one factor that explains why these cicadas spend so long feeding in their juvenile stages is that their food source (the rising sap of trees) is not a rich source of nutrients.
There are seven species of periodical cicada, found in North America: four have 13 year cycles, and three have 17 year cycles. The 17 year cicadas have more northern distributions than do the 13 year cicadas.
The 17 year cicadas don't only emerge in one year out of 17: in fact out of every seventeen years, there are twelve years in which these species emerge. Each of these years' emergence is referred to as a "brood". Similarly, for the 13 year cicadas, there are three broods.
Because the broods differ in population size, geographical distribution and in which species are included, some broods produce more cicadas than others. This can lead to very dramatic "cicada years" when a large 17 year broods emerge simultaneously with a large 13 year brood.
Cicadas emerging in a particular brood have a distinct geographical range - these brood maps illustrate this nicely
Some of the years in which large broods coincide can yield astonishing numbers of insects. It is thought that the Pilgrim Fathers were so taken aback that they regarded it as a "plague of locusts" as described in the Bible, and hence in New England the periodical cicadas are sometimes still colloquially known as locusts (they are in fact not closely related to locusts).
It is likely that the synchronisation of cicada emergence is a way for the cicadas to survive predators - by overwhelming the predators with huge numbers, some will be certain to survive and lay eggs. Despite this, the huge abundance of cicadas represents an enormous bounty of food for cicada predators.
Humans also enjoy a cicada meal, as these recipes using cicadas demonstrate
How Does It Work?
How periodical cicadas regulate their development is unknown. Most insects regulate their life cycles by measuring how long days and nights are, and can thereby predict cold winter weather before it arrives. However, cicada nymphs live about 60 cm underground, and cannot use day length. It has been suggested that they could use the annual growth rhythms of the trees on whose's sap they feed, but no direct evidence has been obtained.