Nature’s Thermometers – Cicadas

by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station

Insects are an important part of summer and of our collective impression of the passing seasons. When I reflect upon a quintessential summer, I think of June bugs, grasshoppers, butterflies, perhaps on more cynical days, deer flies, mosquitoes, wasps…back to good days…fireflies, moths, and as the dog days of summer come, the cicada. For the past two to three weeks we have been able to hear the rasping,  buzzing sound of cicadas emanating from trees from downtown to ‘Sconset. Often heard but rarely seen, these harbingers of late summer warm weather days remind us that fall is around the corner. According to folk legend, when you hear the first song of the dog-day cicadas, it means there’s just six weeks until frost. While this may not be a precise predictor, there is some merit to the claim. Dog-day cicadas, as their name implies, appear during the long, hot summer days of late July and August.

Cicadas are insects of the order Hemiptera (“hemi” or “half” + pteron “wing”), suborder  Auchenorrhyncha (formerly erroneously known as Homoptera; it’s a long story, to piece out the  phylogenetic details go to http://en.wikipedia.org/wiki/Homoptera), in the superfamily Cicadoidea, with large eyes wide apart on the head and usually transparent, stronglyveined wings. Their name is a direct  derivation of the Latin “cicada,” meaning “tree cricket.” In classical Greek, it was called a “tettix,” and in  modern Greek “tzitzikas”—both names being onomatopoeic. The suborder name Auchenorrhyncha is from the Latin words for “neck-beaks” and refers to the location of the mouthparts. Hemipteras are “true bugs” whose forewings have a thickened base and membranous tip in many families.

In the Ancient Greek myth, Tithonus (often shown carrying a lyre) eventually turns into a cicada after being granted immortality, but not eternal youth, by Zeus after being bugged incessantly (pun intended) by the goddess Eos to be her immortal lover (the original “be careful of what you ask for”).

Cicadas are depicted in pottery and art in almost every culture going back thousands of years. To learn more about cicadas in classical literature, explore “Cicadas in Ancient Greece. Ventures in Classical Tettigology” by Rory B. Egan, which may be found at http://www.insects.org/ced3/cicada_ancgrcult.html . This excellent site: “DRMetcalf” http://www.lib.ncsu.edu/specialcollections/digital/metcalf/cicadas.html) is a compendium of all things cicada. The website is named for Zeno Payne Metcalf, “whose life’s work was inseparably linked to the study of the Auchenorrhyncha. Fascinated by both the scientific and economic aspects of these plant-feeding insects, Z.P. Metcalf amassed the world’s most comprehensive collection of literature on the Auchenorrhyncha, including nearly all  publications on the group through 1955.” From here we can find out some specifics about cicada morphology (body structure). Members of the suborder Auchenorrhyncha share the following combination of features that distinguish them from other Hemiptera:

  1. the mouthparts arise from the back of the head (in contrast to arising from the front of the head in Heteroptera or from between the forelegs in Sternorrhyncha—which sounds kind of painful);
  2. the antennae are relatively short and bristlelike; and
  3. the adult tarsi (“feet”) have 3 subsegments. Cicadas’ relatives in Auchenorrhyncha include the leafhoppers, treehoppers, planthoppers, and spittlebugs (froghoppers).

Cicadas range in length from a colossal 110 mm (4.33 inches! including the wings) down to 14 mm. The hind legs of adult cicadas are not modified for jumping. Cicadas have three small eyes, or ocelli, located on the top of the head between the two large eyes that match the color of the large eyes. Males of most species (and also females of the Tettigarctidae) have abdominal organs called tymbals, which are used to produce acoustic signals. In addition, unlike most Auchenorrhyncha, members of the family Cicadidae (both males and females) have distinctive sound receptor organs (tympana) on the underside of the abdomen. Nymphs of many cicadas resemble adults in morphology and habits, but lack ocelli (eyes), may have fewer tarsal segments than the adults, and develop wing buds only in their later instars. Cicada nymphs differ from the immatures of other Auchenorrhyncha in having fossorial forelegs (modified for digging) which makes sense since they need to burrow into the soil and hang out there for two to seventeen years.

There are about 2,500 species of cicada around the world, and many of them remain unclassified. Cicadas live in temperate to tropical climates where they are among the most widely recognized of all insects, mainly due to their large size and unique sound. Cicadas are a popular protein source and are eaten in China, Malaysia, Burma, Latin America, and the Congo. Although abundant, cicadas are far more often heard than seen. Males make a variety of sounds to attract females. Most commonly heard are loud, often shrill, buzzing, sometimes with several individual insects synchronizing their songs. Other cicadas make clicking noises.

Cicadas are plant eaters or phytophagous fauna and they are dependent on trees or woody shrubs which explain why for quite some time they were rarely heard on our relatively treeless island. Cicada nymphs suck sap from the xylem of various species of tree, including oak, cypress, willow, ash, and maple. Cicadas are sometimes mistakenly called locusts, a term properly used to describe certain migratory grasshoppers. This error originated when early European settlers encountered huge masses of periodical cicadas in the Northeast. As they had not previously seen cicada outbreaks, they likened them to the locusts described in the Bible. While it is common folklore that adults do not eat, in reality they do have their own sucking mouthparts, and also drink plant sap. Cicadas cannot bite or sting; although they may try to taste you to see if you are a tree if you leave one on your skin long enough!

We often think about cicadas in the “dog days of summer” because many of them start their singing now. The “Dog Days” are named after the period when Sirius (the Dog Star) and the constellation Canis Major begin to appear in the early morning sky. There are about four species of cicadas in Massachusetts. Our local cicadas are typically Tibicen lyricen. Did you know you can approximately tell  the temperature if you hear a cricket sing?? I remember this bit of folklore going back (way back) to my childhood and I was happy to ready find out that it is true. Crickets are pretty accurate thermometers; you can count the number of chirps in 15 seconds and then add 37 to arrive at an approximate  temperature.

Cicadas also can gauge the temperature to figure out when to emerge from the soil and for synchronicity and variance in the loudness and syncopation of their songs. Cicadas like heat and do their most spirited singing during the hotter hours of a summer day, in a roughly 24-hour cycle. Scientists have found that their song will vary in pitch and intensity based on temperature cues. I found at least a dozen papers on this subject. More random temperature facts: desert cicadas are one of the few insects who can “sweat” and several species of cicadas can voluntarily increase their body temperature.

You may recall the recent outburst of the periodical cicadas that inundated the mid-Atlantic States a few years back. These are members of the genus Magicicada (love that name!). Often when people think of cicadas they are thinking of these periodical cicadas whose long life span and spectacular emergence every 13 or 17 years tends to be a show-stopper. Twelve broods of 17-year cicadas appear in different areas of the northeastern U.S. in different years, emerging from late May through June. Their bright red eyes and reddish markings distinguish the periodical cicadas from the annual or “dog day” cicadas which emerge later in the summer (July through August) and have green markings. That biblical outpouring of periodical adults is a “satiation” strategy which is designed to produce so much “meat on the hoof” that predators like birds or squirrels can’t eat them all which allows cicadas to sing their song and mate in peace. The massive Magicicada emergence last seen in 2004 on the East Coast is known as Brood X, or the Big Brood. Information on Massachusetts (including the somewhat elusive Nantucket sightings) and East Coast cicadas can be found at http://www.masscic.org.

So who is doing all that singing? As usual, it is the male cicada who is singing his heart out in order to attract a, now slightly deaf, female. Most cicadas have a pair of tymbals or domed, drum-like organs on the sides of the abdomen. They alternately contract and release muscles to make the tymbals resonate. A large air sac in the abdomen with a thin exterior eardrum acts as an echo chamber that greatly amplifies the sound. These muscles do get tired as you may notice when you hear a cicada’s racket taper off. Some cicadas produce sounds up to 120 dB, among the loudest of all insect-produced sounds. In Costa Rica, cicadas are so loud you often have to stop talking to let them do their thing. I think my husband may start importing those tropical species just for that reason. Some scientists have determined that a cicada is technically loud enough to cause permanent hearing loss in humans, should the cicada sing just outside the listener’s ear. So your deafness might not be that Metallica concert  years ago.

A cicada’s high-pitched song not only attracts females (and other males who join in the cacophony) but it also repels birds. In some cases the din is enough to throw off avian communication and disrupt their group huntingbehavior. Even cicadas must protect themselves from the volume of their own singing. Both male and female cicadas have a pair of large, mirror-like membranes called the tympana, which function as ears. The tympana are connected to an auditory organ by a short tendon. When a male sings, the tendon retracts, creasing the tympana so that it won’t be damaged by the sound.

Some tiny cicada species have songs so high in pitch that the noise is inaudible to humans. Every species of cicada has a unique call to ensure they attract the appropriate mate. It can be difficult to determine from which direction(s) cicada song is coming, because the low pitch carries well and because it may, in fact, be coming from many directions at once, as cicadas in various trees all raise one another to make noise in unison. Females can make a clicking noise with their wings, but it’s nothing like the noise the males make (ain’t that the truth). In addition to the mating song, many species also have a distinct distress call, usually a somewhat broken and erratic sound emitted when an individual is seized. A number of species also have a courtship song, which is often a quieter, Barry White type melody produced after a female has been drawn by the calling song.

After mating, the female woodland cicadas use their specially modified ovipositors to cut short slits in the twigs of the trees they are in. They lay their eggs in the slits and the young nymphs drop to the ground when they hatch. The nymphs burrow underground and spend the next two to five years feeding on plant roots. When they are ready to become adults, and if the soil temperature is just right, the nymphs tunnel to the surface and crawl up on the side of the nearest object. Then the skin splits down the middle of their back and they emerge to inflate and dry their wings, gradually pumping blood throughout their bodies to harden their exoskeleton. This is usually done at night. Adults are present for about four to six weeks following emergence. After mating, females lay eggs in bark or twigs and the cycle starts again. As a quick note, if you are trying to protect young ornamental trees from cicadas  (which usually do minimal damage to trees), simply wrap a 1.5-2.00 inch layer of blue painter’s tape  around the bark; the cicadas cannot grip it with their limbs and will stop below the tape.

Cicadas are important in the food chain and are commonly eaten by birds, and sometimes by squirrels, but Massospora cicadina (a fungal disease) is the biggest enemy of cicadas. Another known predator is the cicada killer wasp (in Massachusetts typically the Sphecius speciousus, or Eastern cicada killer), which as the name suggests, have evolved simply to feed on one creature. When they’re underground they’re often eaten by moles and other furry insectivores, but enough of them escape the moles to survive. Cicadas can survive freezing temperatures underground. But they only emerge when the soil is warm, typically a relatively toasty 64 degrees Fahrenheit. So they are nature’s thermometers in the sense that they are broadcasting the soil temperature. It has to be relatively warm to reach 63- 66 degrees Fahrenheit at a depth of 20 cm in the soil. When it gets that warm, the cicada nymphs began to emerge. A great summer research project for kids is to monitor this soil temperature in an area where cicadas have been found. In some of our cities and suburbs, fewer cicadas are seen due to a combination of factors like tree removal, pesticide use, and construction. The Cicada Mania web site is a treasure trove of cicada songs, pictures and general nuttiness which will be featured on the BBC’s “Nature’s Weirdest” soon: http://www.cicadamania.com/cicadas. Missing the sound of cicadas this winter? Get a copy of the “Songs of Insects” at http://www.musicofnature.org/songsofinsects/index.html.