Invertebrate Ears Only a few of the millions of insect species are known to possess organs capable of detecting sound. These include grasshoppers and crickets, cicadas, the waterboatman, moths, and mosquitoes. The ears of invertebrates are located on various parts of their bodies: Crickets and katydids have ears on the first walking legs; grasshoppers' ears are on the first segment of the abdomen; while the waterboatman hears through its first thorax segment. Moths have uncomplicated ears located either on the first segment of the abdomen or the rear part of the thorax; mosquitoes hear through sensors associated with their antennae. Insects can produce and perceive a variety of different highly species-specific sounds for communication and mating. Although not sensitive to pitch, information is conveyed by changes in intensity, duration, and sound patterns. One of several different structures may serve as the hearing organ of insects: tympanal membranes, cercal organs, or antennae. The auditory system of grasshoppers and crickets is anatomically connected to the respiratory system, which conducts air from openings in the thorax to the muscles of the legs. Tympana, very thin membranes located on the forelegs or on the abdomen, are found on the body surface of a respiratory tube. Impinging sound waves cause the tympana to flex, which in turn induces tension changes in the attached scolophores, highly specialized sensory structures that transmit nerve impulses to the central nervous system. Moths have simple tympanal organs containing only two or four scolophores, while the highly developed auditory mechanism of cicadas may contain over a thousand sclorophores. The grasshopper ear, with about one hundred sclorophores, has a tympanic membrane hidden beneath the base of the wing cover. Roaches and certain crickets respond to a wide range of sound frequencies by means of cercal organs located at the tip of the abdomen. The mosquito ear, located at the base of the antenna where an expanded sac contains many scolophores, is stimulated when the antenna shaft vibrates. The stimulation is greatest when the antenna is pointed toward the source of sound, which enables mosquitoes to determine the direction of sounds. For male mosquitoes, the frequency region which responds with the greatest intensity is the same as the hum of the female's vibrating wings, which enables him to find her when mating. The bodies of spiders contain many slitlike openings, called lyriform organs, one of which (located on the next to last segment of each of the eight legs) is close to the joint between this segment and the last leg segment (tarsus). The tarsus is the sensing element which transmits vibrations to the lyriform organ. The small leg segments respond to the changing velocity of oscillating air particles, enabling some species to hear over a frequency range from 20 hertz to 45,000 hertz.
Studying Regeneration Developmental geneticists have studied regeneration in a variety of ways.Amongthe principal experimental techniques have been fate map determination of imaginal disks and limb regeneration, already discussed above, transdetermination of imaginal disks, and studies of simple organismal development. Geneticists have found that under special circumstances, an imaginal disk or a portion of an imaginal disk can change its pattern of determination, that is, it transdetermines. A wing occasionally grows from an eye, for example, or a leg from a wing. Cells that are programmed to follow one developmental route follow another route instead. The cause of transdetermination is unknown, but the process does follow specific patterns. For example, a genital imaginal disk can transdetermine to form an antenna or leg, but not vice versa. An antenna disk can transdetermine to produce an eye, wing, or leg, but the wing and leg disks cannot transdetermine to an antenna. Further regenerative studies involve the model developmental systems, including the cellular slime mold Dictyostelium discoideum. In the presence of adequate food, this exists as single, amoeba-like cells. If the cells are starved, they release a chemical attractant (chemotaxic) substance, cyclicAMP, that attracts the cells to one another. The resulting cellular mass moves as a single unit until the organism finds a suitable food source, upon which the cells differentiate and specialize to produce and release spores, each of which subsequently gives rise to a new amoebalike stage. Such studies are necessary because regeneration ultimately involves changes in the determination and differentiation of cells.
