Cicadas form a small part of the order Hemiptera, a diverse group of insects whose mouthparts comprise a jointed rostrum for piercing and sucking up liquid food. They make up the superfamily Cicadoidea, distinguished by having three ocelli, an antennal flagellum usually of five segments, and a complete ten-torium (internal development of the head for attachment of muscles); nymphs burrow and develop underground. The Cicadoidea are divided into two families, the majority falling within the Cicadidae, and just two extant species plus some fossil species in the Tettigarctidae. There are almost 2000 named species, with perhaps as many again awaiting description.
Cicadas are mostly tropical or subtropical insects, but many also inhabit temperate regions. Some are minor pests of sugarcane, rice, coffee, and fruit trees, either reducing the vigor of the plants by nymphal feeding or weakening branches by oviposition which, in turn, may cause the branches to break under crop load.
STRUCTURE AND FUNCTION
Cicadas typically possess a broad head delimited by a pair of large compound eyes, a large pro- and mesothorax housing mostly wing and leg muscles, a small metathorax, an abdomen that in the male is highly modified to accommodate the organs of sound production and reception, and two pairs of membranous wings that are usually held tentlike over the body at rest.
The head is dominated by a large, noselike postclypeus that houses muscles for sucking sap through the rostrum; the three jewellike ocelli detect the direction of light sources and, if asymmetrically covered, cause erratic flight.
The foreleg femora are characteristically enlarged and swollen. On the nymph these are even more enlarged (Fig. 1), serving the nymph for subterranean tunneling.
The abdomen carries the organs of reproduction and of hearing and, in males, also sound production.
FIGURE 1 Mature nymph of Cyclochila australasiae, lateral view.
SOUND PRODUCTION AND RECEPTION
Cicadas are best known for their ability to produce loud sound. No other insect has developed such an effective and specialized means of doing so. The calls are mating songs produced only by the males. Each species has its own distinctive song and attracts only females of its own kind (Fig. 2 ).
FIGURE 2 A mating pair of northern cherrynose, Macrotristria sylvara (family Cicadidae). This large and colorful species is found in tropical northeastern Australia.
The organs of sound production are the tymbals, a pair of ribbed cuticular membranes located on either side of the first abdominal tergite (Fig. 3). In many species the tymbals are partly or entirely concealed by tymbal covers, platelike anterior projections of the second abdominal tergite. Contraction of internal tymbal muscles causes the tymbals to buckle inward, and relaxation of these muscles allows the tymbals to pop back to their original position. The sound produced is amplified by the substantially hollow abdomen, which acts as a resonator.
Many species sing during the heat of the day, but some restrict their calling to semidarkness at dusk. Often the species that sing at dusk are cryptic in coloration and gain further protection from predatory birds by confining their activity to dusk. The loud noise produced by some communal day-singing species actually repels birds, probably because the noise is painful to the birds’ ears and interferes with their normal communication. The American periodical cicadas have mass emergences, and although their song is not sufficiently loud to repel birds, the number of individuals is so large that predatory birds soon lose their appetite for them.
FIGURE 3 Transverse section of male abdomen of Tamasa tristigma at the first abdominal segment with the thorax removed. Exposed are the large tymbal muscles anchored basally to a chitinous V and attached dorsally via an apodeme to the sound-producing tymbals. Sound received by the tympana is transferred to the auditory capsules.
Both sexes have organs for hearing. Sound is received by a pair of large, mirrorlike membranes, the tympana (Fig. 3), which are often concealed below the opercula. The tympana are connected to an auditory organ by a short slender apodeme. A singing male creases the tympana to avoid being deafened by his own song.
Eggs are laid in branches of trees and shrubs or in the stems of grasses (the nymphal food plants) in small slits cut into the surface by the female’s spearlike ovipositor. The number of eggs laid in each slit varies between both species and individuals. Usually it is about 10-16, although the number laid per slit by a single female can range from 3 or fewer to more than 20.
A female makes many egg slits and often distributes her eggs at more than one site. A batch of eggs can number 300 or more. Some species, such as many Cicadetta, select only living tissue for oviposi-tion, whereas others choose only dead or dying tissue. Many days, often in excess of 100, may pass before the nymphs hatch.
On hatching, the young nymphs are encased in a thin transparent skin that encloses the appendages separately but restricts their function. These pronymphs quickly wriggle their way along the egg slit to its entrance. A spine at the apex of the abdomen probably assists this exit and also in casting off the pronymphal skin. The young nymphs fall to the ground, whereupon they immediately seek shelter in the soil and later search for a root from which to feed by sucking sap.
Cicadas spend most of their life underground, slowly growing to maturity through five instars (Fig. 1). The length of life cycle is known only for a small number of species. Some grass-feeding species mature within a year. The American periodical cicadas, Magicicada species, have a life cycle spanning 13 or 17 years, the longest known for any insect. Periodical cicadas are consistently regular in their life cycle length, but most other cicadas change by a year or two, and even individuals from a single egg batch can mature at different rates.
For most species, emergence from the final nymphal skin occurs during the first few hours after dark; the laborious process can last an hour or more. The adult life usually lasts 2-4 weeks, but some
grass-dwelling species possibly live only 3-4 days. Some of the larger tree-inhabiting species probably live 8 or more weeks.
The family Cicadidae includes all cicadas except two extant species. Three subfamilies are now recognized: the Cicadinae, the males of which have tymbal covers present, are mostly large species and are found on all continents except Antarctica; the Cicadettinae, which have tymbal covers absent and often (but not always) veins M and CuA of the fore wing fused, are mostly small species that also occur on all continents except Antarctica; and the Tettigadinae, with veins CuP and 1A of the fore wing and veins RP and M of the hind wing unfused, have comparatively few species that are mostly found in the neotropics but also across the Holarctic.
The family Tettigarctidae includes the other two extant species, Tettigarcta crinita and T. tomentosa, both found only in Australia. This family also includes 13 genera known from Cenozoic fossils.
The Tettigarctidae differ from other cicadas in several features. Most notable is the presence of tymbals in both sexes, but instead of producing airborne songs, they create low-level vibrations of the substrate below the adult. These substrate vibrations are detected by sensory empodia between the claws on all legs; the tympana used for hearing in other cicadas are lacking.