Sound production

The commonest method of sound production by insects is by stridulation, in which one specialized body part, the scraper, is rubbed against another, the file. The file is a series of teeth, ridges, or pegs, which vibrate through contact with a ridged or plectrum-like scraper. The file itself makes little noise, and so has to be amplified to generate airborne sound. The horn-shaped burrow of the mole cricket is an excellent sound enhancer (Fig. 4.5). Other insects produce many modifications of the body, particularly of wings and internal air sacs of the tracheal system, to produce amplification and resonance.

Sound production by stridulation occurs in some species of many orders of insects, but the Orthoptera show most elaboration and diversity. All stridulating orthopterans enhance their sounds using the tegmina exit tunnel exit tunnel

Fig. 4.5 The singing burrow of a mole cricket, Scapteriscus acletus (Orthoptera: Gryllotalpidae), in which the singing male sits with his head in the bulb and tegmina raised across the throat of the horn. (After Bennet-Clark 1989.)

(the modified fore wings). The file of katydids and crickets is formed from a basal vein of one or both tegmina, and rasps against a scraper on the other wing. Grasshoppers and locusts (Acrididae) rasp a file on the fore femora against a similar scraper on the tegmen.

Many insects lack the body size, power, or sophistication to produce high-frequency airborne sounds, but they can produce and transmit low-frequency sound by vibration of the substrate (such as wood, soil, or a host plant), which is a denser medium. Substrate vibrations are also a by-product of airborne sound production as in acoustic signaling insects, such as some katydids, whose whole body vibrates whilst producing audible airborne stridulatory sounds. Body vibrations, which are transferred through the legs to the substrate (plant or ground), are of low frequencies of 1-5000 Hz. Substrate vibrations can be detected by the female and appear to be used in closer range localization of the calling male, in contrast to the airborne signal used at greater distance.

A second means of sound production involves alternate muscular distortion and relaxation of a specialized area of elastic cuticle, the tymbal, to give individual clicks or variably modulated pulses of sound. Tymbal sound production is most audible to the human ear from cicadas, but many other hemipterans and some moths produce sounds from a tymbal. In the cicadas, only the males have these paired tymbals, which are located dorsolaterally, one on each side, on the first abdominal segment. The tymbal membrane is supported by a variable number of ribs. A strong tymbal muscle distorts the membrane and ribs to produce a sound; on relaxation, the elastic tymbal returns to rest. To produce sounds of high frequency, the tymbal muscle contracts asynchronously, with many contractions per nerve impulse (section 3.1.1). A group of chordonotal sensilla is present and a smaller tensor muscle controls the shape of the tymbal, thereby allowing alteration of the acoustic property. The noise of one or more clicks is emitted as the tymbal distorts, and further sounds may be produced during the elastic return on relaxation. The first abdominal segment contains air sacs - modified tracheae - tuned to resonate at or close to the natural frequency of tymbal vibration.

The calls of cicadas generally are in the range of 4-7 kHz, usually of high intensity, carrying as far as 1 km, even in thick forest. Sound is received by both sexes via tympanic membranes that lie ventral to the position of the male tymbal on the first abdominal segment. Cicada calls are species-specific - studies in New Zealand and North America show specificity of duration and cadence of introductory cueing phases inducing timed responses from a prospective mate. Interestingly however, song structures are very homo-plasious, with similar songs found in distantly related taxa, but closely related taxa differing markedly in their song.

In other sound-producing hemipterans, both sexes may possess tymbals but because they lack abdominal air sacs, the sound is very damped compared with that of cicadas. The sounds produced by Nilaparvata lugens (the brown planthopper; Delphacidae), and probably other non-cicadan hemipterans, are transmitted by vibration of the substrate, and are specifically associated with mating.

Certain moths can hear the ultrasound produced by predatory bats, and moths themselves can produce sound using metepisternal tymbals. The high-frequency clicking sounds that arctiid moths produce can cause bats to veer away from attack, and may have the following (not mutually exclusive) roles:

• interspecific communication between moths;

• interference with bat sonar systems;

• aural mimicry of a bat to delude the predator about the presence of a prey item;

• warning of distastefulness (aposematism; see section 14.4).

The humming or buzzing sound characteristic of swarming mosquitoes, gnats, and midges is a flight tone produced by the frequency of wing beat. This tone, which can be virtually species-specific, differs between the sexes: the male produces a higher tone than the female. The tone also varies with age and ambient temperature for both sexes. Male insects that form nuptial (mating) swarms recognize the swarm site by species-specific environmental markers rather than audible cues (section 5.1); they are insensitive to the wing tone of males of their species. Neither can the male detect the wing tone of immature females - the Johnson's organ in his antenna responds only to the wing tone of physiologically receptive females.

4.2 THERMAL STIMULI 4.2.1 Thermoreception

Insects evidently detect variation in temperature, as seen by their behavior (section 4.2.2), yet the function and location of receptors is poorly known. Most studied insects have antennal sensing of temperature - those with amputated antennae respond differently from insects with intact antennae. Antennal temperature receptors are few in number (presumably ambient temperature is much the same at all points along the antenna), are exposed or concealed in pits, and are associated with humidity receptors in the same sen-sillum. In the cockroach Periplaneta americana, the arolium and pulvilli of the tarsi bear temperature receptors, and thermoreceptors have been found on the legs of certain other insects. Central temperature sensors must exist to detect internal temperature, but the only experimental evidence is from a large moth in which thoracic neural ganglia were found to have a role in instigating temperature-dependent flight muscle activity.

An extreme form of temperature detection is illustrated in jewel beetles (Buprestidae) belonging to the largely Holarctic genus Melanophila and also Merimna atrata (from Australia). These beetles can detect and orientate towards large-scale forest fires, where they oviposit in still-smoldering pine trunks. Adults of Melanophila eat insects killed by fire, and their larvae develop as pioneering colonists boring into fire-killed trees. Detection and orientation in Melanophila to distant fires is achieved by detection of infrared radiation (in the wavelength range 3.6-4.1 |lm) by pit organs next to the coxal cavities of the mesothoracic legs that are exposed when the beetle is in flight. Within the pits some of the 50-100 small sensillae can respond with heat-induced nanometer-scale deformation, converted to mechanoreceptor signal. The receptor organs in Merimna lie on the posterolateral abdomen. These pit organ receptors allow a flying adult buprestid to locate the source of infrared perhaps as far distant as 12 km -a feat of some interest to the US military.

4.2.2 Thermoregulation

Insects are poikilothermic, that is they lack the means to maintain homeothermy - a constant temperature independent of fluctuations in ambient (surrounding) conditions. Although the temperature of an inactive insect tends to track the ambient temperature, many insects can alter their temperature, both upwards and downwards, even if only for a short time. The temperature of an insect can be varied from ambient either behaviorally using external heat (ectothermy) or by physiological mechanisms (endothermy). Endothermy relies on internally generated heat, predominantly from metabolism associated with flight. As some 94% of flight energy is generated as heat (only 6% directed to mechanical force on the wings), flight is not only very energetically demanding but also produces much heat.

Understanding thermoregulation requires some appreciation of the relationship between heat and mass (or volume). The small size of insects in general means any heat generated is rapidly dissipated. In an environment at 10°C a 100 g bumble bee with a body temperature of 40°C experiences a temperature drop of 1°C per second, in the absence of any further heat generation. The larger the body the slower is this heat loss - which is one factor enabling larger organisms to be homeothermic, with the greater mass buffering against heat loss. However, a consequence of the mass-heat relationship is that a small insect can warm up quickly from an external heat source, even one as restricted as a light fleck. Clearly, with insects showing a 500,000fold variation in mass and 1000-fold variation in metabolic rate, there is scope for a range of variants on thermoregulatory physiologies and behaviors. We review the conventional range of thermoregulatory strategies below, but refer elsewhere to tolerance of extreme temperature (section 6.6.2).

Behavioral thermoregulation (ectothermy)

The extent to which radiant energy (either solar or substrate) influences body temperature is related to the aspect that a diurnal insect adopts. Basking, by which many insects maximize heat uptake, involves both posture and orientation relative to the source of heat. The setae of some "furry" caterpillars, such as gypsy moth larvae (Lymantriidae), serve to insulate the body against convective heat loss while not impairing radiant heat uptake. Wing position and orientation may enhance heat absorption or, alternatively, provide shading from excessive solar radiation. Cooling may include shade-seeking behavior, such as seeking cooler environmental microhabitats or altered orientation on plants. Many desert insects avoid temperature extremes by burrowing. Some insects living in exposed places may avoid excessive heating by "stilting"; that is raising themselves on extended legs to elevate most of the body out of the narrow boundary layer close to the ground. Conduction of heat from the substrate is reduced, and convection is enhanced in the cooler moving air above the boundary layer.

There is a complex (and disputed) relationship between temperature regulation and insect color and surface sculpturing. Amongst some desert beetles (Tenebrionidae), black species become active earlier in the day at lower ambient temperatures than do pale ones, which in turn can remain active longer during hotter times. The application of white paint to black tenebrionid beetles results in substantial body temperature changes: black beetles warm up more rapidly at a given ambient temperature and overheat more quickly compared with white ones, which have greater reflectivity to heat. These physiological differences correlate with certain observed differences in thermal ecology between dark and pale species. Further evidence of the role of color comes from a beclouded cicada (Hemiptera: Cacama valvata) in which basking involves directing the dark dorsal surface towards the sun, in contrast to cooling, when the pale ventral surface only is exposed.

For aquatic insects, in which body temperature must follow water temperature, there is little or no ability to regulate body temperature beyond seeking micro-climatic differences within a water body.

Physiological thermoregulation (endothermy)

Some insects can be endothermic because the thoracic flight muscles have a very high metabolic rate and produce much heat. The thorax can be maintained at a relatively constant high temperature during flight. Temperature regulation may involve clothing the thorax with insulating scales or hairs, but insulation must be balanced with the need to dissipate any excess heat generated during flight. Some butterflies and locusts alternate heat-producing flight with gliding, which allows cooling, but many insects must fly continuously and cannot glide. Bees and many moths prevent thoracic overheating in flight by increasing the heart rate and circulating hemolymph from the thorax to the poorly insulated abdomen where radiation and convection dissipate heat. At least in some bumble bees (Bombus) and carpenter bees (Xylocopa) a counter-current system that normally prevents heat loss is bypassed during flight to augment abdominal heat loss.

The insects that produce elevated temperatures during flight often require a warm thorax before they can take off. When ambient temperatures are low, these insects use the flight muscles to generate heat prior to switching them for use in flight. Mechanisms differ according to whether the flight muscles are synchronous or asynchronous (section 3.1.4). Insects with synchronous flight muscles warm up by contracting antagonistic muscle pairs synchronously and/or synergistic muscles alternately. This activity generally produces some wing vibration, as seen for example in odonates. Asynchronous flight muscles are warmed by operating the flight muscles whilst the wings are uncoupled, or the thoracic box is held rigid by accessory muscles to prevent wing movement. Usually no wing movement is seen, though ventilatory pumping movements of the abdomen may be visible. When the thorax is warm but the insect is sedentary (e.g. whilst feeding), many insects maintain temperature by shivering, which may be prolonged. In contrast, foraging honey bees may cool off during rest, and must then warm up before take-off.

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