Stridulation

“Even insects express anger, terror, jealousy, and love by their stridulation.”

-Charles Darwin, The Expression of the Emotions in Man and Animals, 1872

Stridulation is the production of sound by an insect, caused by the rubbing together of body parts. The bioacoustic (or zoosemiotic) ambience of summertime in Minnesota is replete with cricket, grasshopper, and katydid calls, all of which insects stridulate. Crickets and katydids produce their calls by rubbing together their wings. They have a pair of structures, file and scraper, on each forewing – the sawing of the rigid scraper across the comb-like file produces vibrations which become sound.

Crickets. In Minnesota, the most cosmopolitan cricket is the field cricket, the common name for several cricket species belonging to the genus Gryllus. A multitude of other cricket species, including the house, ground, camel, tree, and prairie crickets, reside in Minnesota as well.

The relationship between air temperature and the rate at which crickets chirp is expressed by Dolbear’s Law as follows, where T is the temperature (in Farenheit) and N is the number of chirps per minute:

Amos Dolbear published this formula in his 1897 article entitled, “The Cricket as a Thermometer.” Dolbear is more well known for his less whimsical 1865 invention of a telephone receiver 11 years before the similar machine for which Alexander Graham Bell, not Dolbear, was recipient of a patent.

Alternately, it is said that by counting the number of chirps produced by a field cricket during a 15-second period and adding 40 to that number, a rough approximation of the temperature can be derived.

Katydid is a name onomatopoeic for that insect’s call, at least for “true katydids” (Pterophylla camellifolia), whose call it is said, sounds like “katy did, katy didn’t” (audioclip). True katydids have only recently pushed the north-most boundary of their territory into Minnesota, where they have been found in the Twin Cities metropolitan area. Formerly, true katydids were known only in the southern and eastern United States. This is probably accounted for by the overall rise in temperature in Minnesota – since 1981 Minnesota has seen 8 of its 20 warmest years on record. The other katydids, the ones seemingly long-resident in Minnesota, include bush, meadow, cone-headed, and shield-bearing type katydids.

Grasshoppers, unlike crickets and katydids, stridulate with their legs and wings or abdomen; a small row of toothy structures on the inside of their hind legs is drawn across a ridge on their wings or abdomen to produce calls similar to those of crickets and katydids. Grasshoppers also crepitate, by snapping together their wings during flight. In Minnesota there are about 60 different kinds of grasshoppers. In the mid-1870s, floods of grasshoppers called Rocky Mountain Locusts (locusts are grasshoppers in “swarming” phase) scoured the Great Plains, devouring crops, stripping trees, and caking railways to such an extent that they became inoperable. The visitations of the “Rocky Mountain Locust” are recalled in Laura Ingalls Wilder’s On the Banks of Plum Creek:

The Cloud was hailing grasshoppers. The cloud was grasshoppers. [ ] Their thin, large wings gleamed and glittered. The rasping whirring of their wings filled the whole air and they hit the ground and the house with the noise of a hailstorm.

It is said that the sound of their chewing was also deafening. It is also said that where crops and greenery were already obliterated, the clothes were eaten from clotheslines, the wood of axe handles and leather of saddles were gnawed on, and supposedly the wool was consumed from the very backs of sheep. The United States government in 1876 declared the grasshoppers “the single greatest impediment to the settlement of the country.”

Estimates suggest that the locusts from a swarm that descended on Nebraska in July of 1874 covered 198,000 square miles, and numbered approximately 12.5 trillion insects, amounting in sheer biomass to as much as 27.5 million tons. The locusts came five consecutive years – from 1873 to 1878. At that time, 69% of agricultural lands in Minnesota were consigned to the production of wheat. In an effort to arrest the devastation being wrought on the local wheat crop, the state put a bounty on locusts and their brood, which varied in award from 50 cents a bushel for locusts to $5 dollars per bushel for locust eggs (a bushel is just over 35 liters).

Minnesota Governor John Pillsbury (Pillsbury of milling fame) declared a state-wide “day of prayer” on April 26th, 1877, to ask mercy from the depredations of the locusts. Grasshopper Assumption Chapel in Cold Spring, MN is a memorial to the fabled insecticidal storm of freezing rain that supposedly came and “smote” the locusts following the so-called “day of prayer.”

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Shortly after the 2003 North American Blackout an article appeared in The New Yorker (A Rare and Different Tune) in response to the theretofore collectively overlooked insect singing with which New Yorkers were greeted during the Blackout.

Some people go through life never knowing what a katydid is, or what kind of bug makes which sound, when, and for what reason. But the particular bug sounds chronicled below took place under a big linden, just outside the city. This was during the blackout, a few weeks ago, when anyone who was near trees might suddenly have noticed, in the absence of some of the other usual night noises—air-conditioners, floor waxers, Ernie Anastos—that the bugs, whatever they were, were making a hell of a racket.

A discussion, not quite an argument, arose over what was making which sounds. The more you listened to the trees, the stupider you felt.

Thank God the electricity came back on, because it gave everyone a chance to log on to the Internet for some insta-entomology.

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What distinguishes stridulation from the production of the more bracing strains for which cicadas are known are the actual sound-producing organs cicadas possess. Cicadas do not stridulate, instead male cicadas have abdominal structures called tymbals, which are fine exoskeletal membranes that vibrate rapidly. Cavities within the cicada’s body augment this sound like resonance chambers. I read somewhere that philologists once thought the Spanish word for cigar (cigarra) derived from the word cicada, claiming that the resemblance between the two words owed to the resemblance of the cigars first seen by Spanish conquistadores to cidadas – these cigars were the kind tapered at both ends, now called perfecto.

The cicadas commonly known as “seventeen-year locusts” are periodical cicadas well-known in the eastern United States for exilic, 17-year periods of collective dormancy followed by the sudden, exuberant emergence of incalculable multitudes. The cyclical emergence of these cicadas are to entomology what the return of certain comets are to astronomy. 2004 was the year of the emergence of “Brood X” in anticipation of which an article on the subject appeared in The New York Times, May 20th of 2004, entitled, The Orgy in Your Backyard:

They overwhelm the cornerstone of rationality: our ability to quantify nature. In fact, if we do want to try to quantify cicadas, we have to deal with some incomprehensibly big numbers. When the periodical cicadas are in their full glory, there will be an average of about 100,000 insects per acre spread across an area four times the size of Pennsylvania. That works out to about 10 trillion cicadas, 1,500 for each human on earth.

The cicadas will outweigh the population of the
United States (even with our obesity problems) by a factor of nearly two.

A few weeks after their arrival, the cicadas will die, leaving piles of depleted corpses and more than 500 trillion eggs. In a single square mile of forest with the densest populations, there will be as many eggs as there are stars in the Milky Way.

A cicada shell;
it sang itself
utterly away.

-Matsuo Bashō

For diagnostic reference of insect sounds, I recommend the website Singing Insects of North America.