Sound reception

Sound is a pressure fluctuation transmitted in a wave form via movement of the air or the substrate, including water. Sound and hearing are terms often applied to the quite limited range of frequencies of airborne vibration that humans perceive with their ears, usually in adults from 20 to 20,000 Hz (1 hertz (Hz) is a frequency of one cycle per second). Such a definition of sound is restrictive, particularly as amongst insects some receive vibrations ranging from as low as 1-2 Hz to ultrasound frequencies perhaps as high as 100 kHz. Specialized emission and reception across this range of frequencies of vibration are considered here. The reception of these frequencies involves a variety of organs, none of which resemble the ears of mammals.

An important role of insect sound is in intraspe-cific acoustic communication. For example, courtship in most orthopterans is acoustic, with males producing species-specific sounds ("songs") that the predominantly non-singing females detect and upon which they base their choice of mate. Hearing also allows detection of predators, such as insectivorous bats, which use ultrasound in hunting. Probably each species of insect detects sound within one or two relatively narrow ranges of frequencies that relate to these functions.

The insect mechanoreceptive communication system can be viewed as a continuum from substrate vibration reception, grading through the reception of only very near airborne vibration to hearing of even quite distant sound using thin cuticular membranes called tympana (singular: tympanum; adjective: tympanal). Substrate signaling probably appeared first in insect evolution; the sensory organs used to detect substrate vibrations appear to have been co-opted and modified many times in different insect groups to

Fig. 4.2 Proprioceptors: (a) sensilla of a hair plate located at a joint, showing how the hairs are stimulated by contacting adjacent cuticle; (b) campaniform sensillum on the haltere of a fly. ((a) After Chapman 1982; (b) after Snodgrass 1935; McIver 1985.)

allow reception of airborne sound at considerable distance and a range of frequencies.

Non-tympanal vibration reception

Two types of vibration or sound reception that do not involve tympana (see the next subsection) are the detection of substrate-borne signals and the ability to perceive the relatively large translational movements of the surrounding medium (air or water) that occur very close to a sound. The latter, referred to as near-field sound, is detected by either sensory hairs or specialized sensory organs.

A simple form of sound reception occurs in species that have very sensitive, elongate, trichoid sensilla that respond to vibrations produced by a near-field sound. For example, caterpillars of the noctuid moth Barathra brassicae have thoracic hairs about 0.5 mm long that respond optimally to vibrations of 150 Hz. Although in air this system is effective only for locally produced sounds, caterpillars can respond to the vibrations caused by audible approach of parasitic wasps.

The cerci of many insects, especially crickets, are clothed in long, fine trichoid sensilla (filiform setae or hairs) that are sensitive to air currents, which can convey information about the approach of predatory or parasitic insects or a potential mate. The direction of approach of another animal is indicated by which hairs are deflected; the sensory neuron of each hair is tuned to respond to movement in a particular direction. The dynamics (the time-varying pattern) of air movement gives information on the nature of the stimulus (and thus on what type of animal is approaching) and is indicated by the properties of the mechanosensory hairs. The length of each hair determines the response of its sensory neuron to the stimulus: neurons that innervate short hairs are most sensitive to high-intensity, high-frequency stimuli, whereas long hairs are more sensitive to low-intensity, low-frequency stimuli. The responses of many sensory neurons innervating different hairs on the cerci are integrated in the central nervous system to allow the insect to make a behaviorally appropriate response to detected air movement.

For low-frequency sounds in water (a medium more viscous than air), longer-distance transmission is possible. Currently, however, rather few aquatic insects have been shown to communicate through underwater sounds. Notable examples are the "drumming" sounds that some aquatic larvae produce to assert territory, and the noises produced by underwater diving hemipterans such as corixids and nepids.

Many insects can detect vibrations transmitted through a substrate at a solid-air or solid-water boundary or along a water-air surface. The perception of substrate vibrations is particularly important for ground-dwelling insects, especially nocturnal species, and social insects living in dark nests. Some insects living on plant surfaces, such as sawflies (Hymenoptera: Pergidae), communicate with each other by tapping the stem. Various plant-feeding bugs (Hemiptera), such as leafhoppers, planthoppers, and pentatomids, produce vibratory signals that are transmitted through the host plant. Water-striders (Hemiptera: Gerridae), which live on the aquatic surface film, send pulsed waves across the water surface to communicate in courtship and aggression. Moreover, they can detect the vibrations produced by the struggles of prey that fall onto the water surface. Whirligig beetles (Gyrinidae; Fig. 10.7) can navigate using a form of echolocation: waves that move on the water surface ahead of them and are reflected from obstacles are sensed by their antennae in time to take evasive action.

The specialized sensory organs that receive vibrations are subcuticular mechanoreceptors called chordotonal organs. An organ consists of one to many scolopidia, each of which consists of three linearly arranged cells: a sub-tympanal cap cell placed on top of a sheath cell (scolopale cell), which envelops the end of a nerve cell dendrite (Fig. 4.3). All adult insects and many larvae have a particular chordotonal organ, Johnston's organ, lying within the pedicel, the second antennal segment. The primary function is to sense movements of the antennal flagellum relative to the rest of the body, as in detection of flight speed by air movement. Additionally, it functions in hearing in some insects. In male mosquitoes (Culicidae) and midges (Chironomidae), many scolopidia are contained in the swollen pedicel. These scolopidia are attached at one end to the pedicel wall and at the other, sensory end to the base of the third antennal segment. This greatly modified Johnston's organ is the male receptor for the female wing tone (see section 4.1.4), as shown when males are rendered unreceptive to the sound of the female by amputation of the terminal flagellum or arista of the antenna.

Detection of substrate vibration involves the sub-genual organ, a chordotonal organ located in the

Fig. 4.3 (right) Longitudinal section of a scolopidium, the basic unit of a chordotonal organ. (After Gray 1960.)

proximal tibia of each leg. Subgenual organs are found in most insects except the Coleoptera and Diptera. The organ consists of a semi-circle of many sensory cells lying in the hemocoel, connected at one end to the inner cuticle of the tibia, and at the other to the trachea. There are subgenual organs within all legs: the organs of each pair of legs may respond specifically to substrate-borne sounds of differing frequencies. Vibration reception may involve either direct transfer of low-frequency substrate vibrations to the legs, or there may be more complex amplification and transfer. Airborne vibrations can be detected if they cause vibration of the substrate and hence of the legs.

Tympanal reception

The most elaborate sound-reception system in insects involves a specific receptor structure, the tympanum.

This membrane responds to distant sounds transmitted by airborne vibration. Tympanal membranes are linked to chordotonal organs and are associated with air-filled sacs, such as modifications of the trachea, that enhance sound reception. Tympanal organs are located on the:

• ventral thorax between the metathoracic legs of mantids;

• metathorax of many noctuid moths;

• prothoracic legs of many orthopterans;

• abdomen of other orthopterans, cicadas, and some moths and beetles;

• wing bases of certain moths and lacewings;

• cervical membranes of a few scarab beetles.

The differing location of these organs and their occurrence in distantly related insect groups indicates that tympanal hearing evolved several times in insects.

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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