Cicadas are big bugs1. Members of the biggest, tropical varieties can be as long as 10cm (4 inches). They have fat, chunky looking bodies, short, broad heads, beaks, boggly looking bug eyes, stumpy little antenna and two pairs of membraneous wings. They cannot bite or sting but have claw-like feet that they use to get a firm grip on tree bark, so if they were feeling spiteful, they could give you a little pinch.
Cicadas feed on sap (xylem fluids) which they get mainly from trees so they have syringe-like mouth parts to stab into the tree and suck up the fluid. The Homoptera sub-order of insects includes more than 45,000 species that feed on sap by piercing the plant with similar stabbing mouth arrangements. Aphids, whiteflies leafhoppers, spittlebugs and scale insects are also members of this sub-order.
It is not known how many species of cicada there are in existence but there are believed to be thousands. They live all over the world in tropical and temperate climates. America, for example, has hundreds of species, New Zealand has around 40, Europe has several and there is just one species in Britain.
The life cycle of cicadas begins with the emergence from the ground of the nymphs2, usually in huge numbers (often many millions), in spring or summer. They tunnel to the surface and immediately try to find a tree to climb. When they have found a tree (or some vertical structure they have mistaken for a tree) and located a suitable position on it, they take a firm grip of the bark with their claw-like front legs and shed their exoskeleton3. The moult takes about an hour to complete and the new adult cicada then has to pump fluid into its wings and wait for its body to harden before continuing up to the tree-top.
After a few days (the time it takes for the males' drum-kits or tymbals and the females' egg laying tubes or ovipositors4 to harden) they fly from the tree-tops in search of a 'lek' - a location where males and females gather to find mates: females choose their mates and males mate more than once.
Male cicadas sing in order to attract mates. All the males in an area, depending on the species, produce a loud monotonous or throbbing buzz in harmony with one another. The sound is made using their inbuilt drum kit, which consists of a couple of membranes on either side of the abdomen (the drums) and a couple of internal cord-like strips of muscle (the drum sticks) which they rapidly tense and relax in order to beat their drums.
After mating, the females fly off to find a suitable site to deposit their eggs. They normally choose deciduous tree twigs, occasionally shrub twigs, but they avoid trees that produce a lot of resin, like fir trees. They lay up to 600 eggs in batches of ten or so, each batch occupying a little slit that they have cut in the twig with their ovipositor. Once the eggs have been laid, the female's work is finished and she may die almost straight away, exhausted from her efforts, or she may live for a few more days. Males also die soon after the end of mating.
The eggs normally hatch after six to ten weeks. The larvae are lively and run about the branches. They have been described as looking rather like tiny lobsters because of their large forelegs which will enable them to tunnel into the ground. Soon after hatching they make their way to the trunk end of their branch and then they simply let go and fall to the ground. As soon as they reach the ground, they start burrowing, find a root and feed. Their life as nymphs starts here and they may spend anywhere between one and 17 years underground. They dig down to deeper levels during winter then migrate up to shallower depths for the summer. Then, when they are mature, they emerge en masse and the cycle starts again.
The life cycle of some cicadas is just fascinating. Different species of cicada have different life cycle periods, some of which span many years. The longest lived are the magicicadas, also known as 'periodic cicadas' of North America. It had been believed that there were six distinct species: three that live 13 years and three that live 17 years5. However, it has recently been found that there are actually three different species each of which has a 13 and a 17 year population. Furthermore, a proportion of any 17 year brood6 can, seemingly, spontaneously accelerate their development and emerge after 13 years7. If the new 13 year brood survives, they will be separated from the 'sibling' brood by a four year gap and can, over time, gradually move away geographically. They may maintain the accelerated cycle or they may return to a 17 year period. Either way, there will clearly be no further opportunity for the two broods to interbreed for at least a couple of hundred years, by which time the geographical locations of the broods may no longer overlap. This is exactly the sort of isolation that is required for speciation8 to occur. That is what seems to be happening to the three species mentioned. The 13 and 17 year 'siblings' are gradually drifting away from one another genetically.
One of the most remarkable things about the periods of different cicada species, is the specific lengths of their periods. There are species with three, five, 13 and 17 year periods. These are all prime numbers. It is a wonderful survival strategy because it means that their predators will find it impossible to get into the usual predator/prey 'arms race' with them unless they can synchronise their life cycles to match the life cycle of a cicada species. Take any species of bird, for example. They time their breeding season to coincide with the time of most plentiful food supply. The cicadas, depending on their species and location, emerge in spring or summer, but not every spring or summer or even every other spring or summer. So although there may be a glut of cicadas this year, there is no point in a bird planning to have its next brood this time next year, to coincide with the emergence of a bug that only shows up every three, five, 13 or 17 years.
If the cicadas had a two, four or even a six year cycle, predatory animals could take advantage, especially if there were, say, two or three cicada broods emerging in adjacent areas in different years. The magicicadas have another clever ploy though; not only do they have long life cycle periods of inconvenient length, but they have somehow managed to synchronise their emergence so that all the cicadas in a brood - that is, all the cicadas of any species in contiguous areas emerge together, within about a week of each other.
Another interesting part of the story is the distribution of broods. Broods very rarely overlap geographically unless there is at least a four year gap between the years they are due to emerge. For predators it again means that they cannot adapt their breeding cycle to coincide with different broods emerging each year in the same location. For the cicadas it means there will not be large populations of expectant predators awaiting their arrival. It also means there will be less competition for resources below ground.
No species, other than a fungus, has managed to synchronise with the periodic cicadas so, although there is an enormous loss to predation, it is proportionally a small loss and it is like a 'windfall' to the predators, who are not adapted to the emergence of the periodic glut.
Roman Numeral Designation and Distribution
In the 19th Century, 30 broods were mapped and assigned Roman numeral designations. The 17 year broods were assigned numbers I to XVII and the 13 year broods were assigned numbers XVIII to XXX. Some of these broods have become extinct in the intervening years. One example is Brood XI, one of the 17 year broods, which last emerged in Connecticut in 1954, but no further emergence has been recorded since then. The New Jersey brood has also disappeared. These extinctions are largely driven by disturbance of habitat such as deforestation and urbanisation. Currently, there are 12 broods of 17 year and three broods of 13 year cicadas in existence.
The Magicicadas can survive a certain amount of urbanisation as has been demonstrated in recent years by the Chicago brood (XIV).
A Typical Brood
In the summer of 1956 we had a plague of Periodical Cicadas, or 17-year locusts9 in the Chicago region. There were countless millions of them during June and early July, clattering through the air or crawling on trees and bushes. Insect-eating birds and mammals got fat. The song of just one male cicada sounds like a buzz saw going through a log, and the metallic screeching of millions - sometimes a continuous clamour, sometimes rising and falling in waves - made a nerve-wracking din. People were frightened when those big insects, actually harmless but fearsome in appearance, lit and crawled upon them. It was like a bad dream. Then they disappeared but their progeny will return in 1973.
- Forest Preserve District of Cook County, Nature Bulletin Number 273-A, 1967
Then, in 1973, return they did - in their millions! Interestingly, hundreds of thousands also emerged four years early, potentially forming a new sibling brood. However, the new brood was not big enough to withstand predation in sufficient numbers to survive.
Geographically, the 17 year broods tend to be located to the north, west and east of the 13 year broods which are found mainly in the southern and mid-western states. The territory covered by a single brood can be vast. The largest covers the greater part of 15 states.
Fruit growers go to considerable lengths to control cicadas with chemical pesticides. However, the weight of evidence appears to indicate that these fascinating insects actually cause very little damage to trees and the little damage that does occur as a result of their breeding activities is not permanent. They are completely harmless to humans. Cicadas can neither bite nor sting. Pesticides are expensive and many have been shown to damage the environment, kill beneficial species and pollute the food chain. It seems unreasonable to go to these extraordinary lengths to eliminate a bug that really is not causing very much of a problem.
It is generally agreed by the folks who love bugs that cicadas are wonderful and interesting insects that we should appreciate and preserve, not abhor and persecute.