Sensory Systems

base of flagellum sensory nerve

FIGURE 12.6. Johnston's organ. [After H. Autrum, 1963, Anatomy and physiology of sound receptors in invertebrates, in: Acoustic Behaviour of Animals (R. G. Busnel, ed.). By permission of Elsevier/North-Holland Biomedical Press, Amsterdam.]

and Pyralidae, metathorax in Noctuidae and Notodontidae, fore or hind wing base in Nymphalidae and Hedylidae, mouthparts in Sphingidae), Hemiptera (abdomen in Cica-didae, thorax in Corixidae), Coleoptera (abdomen in Cicindelinae, cervical membrane in Scarabaeidae), Dictyoptera (metathorax in Mantodea, metathoracic leg in Blattodea), Neu-roptera (wing base), and Diptera (ventral prosternum). Though their detailed structure varies, almost all tympanal organs have three common features: a cuticular membrane (the tympanum); a large tracheal air sac appressed to the membrane, the two structures forming a "drum"; and a group of chordotonal sensilla (Figure 12.7) (Yager, 1999). The tympanum is much thinner (1 in cicadas, 40-100 |¿m in some ensiferans) than the surrounding cuticle, providing the sensitivity required for sound reception.

Sound waves that strike the drum cause it to vibrate and, therefore, the sensilla to be stimulated. The range of frequency of the waves that stimulate tympanal organs is high. For example, in Acrididae, it extends from less than 1 kHz to about 50 kHz. Over this range the sensitivity of the organ varies greatly, with a maximum in the 2- to 15-kHz range (Figure 12.8). In contrast, the human ear is most sensitive to a frequency of 1-3 kHz. As pressure difference receivers, insect tympanal organs have directional sensitivity. Thus, insects with these organs can locate the source of a sound.

The functional significance of tympanal organs varies (Spangler, 1988; Hoy and Robert, 1996). In Orthoptera and Hemiptera, the ability to hear is complemented by the ability

base of flagellum sensory nerve

opening into tympanal cavity tibia opening into tympanal cavity tibia

FIGURE 12.7. (A) Surface view of tibial tympanal organ of Decticus (Tettigoniidae); and (B) transverse section through tympanal organ of Decticus. [After V. G. Dethier, 1963, The Physiology of Insect Senses, John Wiley and Sons, Inc. By permission of the author.]

crista acoustica with chordotonal organ auditory nerve sense cell rigid wall between _ enlarged tracheae anterior he mo lymph space opening into tympanal cavity basal lamina tympanal cavity anterior he mo lymph space opening into tympanal cavity basal lamina tympanal cavity sense cell rigid wall between _ enlarged tracheae

posterior hemolymph space muscle posterior hemolymph space muscle to produce sounds (Chapter 3, Section 4.3.1), and in these orders the organs are important in species aggregation and/or mate location. Experimentally, it has been shown that the tympanal organs of nocturnal Lepidoptera, Dictyoptera, Neuroptera, Orthoptera and Coleoptera are sensitive to high-frequency sounds, and it is generally assumed that this enables these insects to detect the approach of predators, principally insectivorous bats. It must be noted, however, that for most groups this has not been observed under field conditions. In Lepidoptera, for which good evidence is available, the tympanal organs are most sensitive

Beekeeping for Beginners

Beekeeping for Beginners

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