INSECTA. A large number of insects are mute ; some produce their sounds merely by friction, and others by the passage of the air through the thoracic spiracles. The sounds produced by friction are denominated stridu lation ; those by the air from the trachem, buzzing, or humming (bourdonnement), &c. Organs adapted to produce stridulation are found in the Orthoptera, Omoptera, and some of the Coleoptera.
Grylli.—In the Gryllus eampestris (fig. 921) the elytrum is composed of dry, thin, trans lucent membranes, forming two planes, united and strengthened at their junction by four longitudinal and parallel nervures ; one of these planes lies on the back, and the other on the side of the insect. The former of these planes is divided by a series of regularly curved nervures, into a number of arcoles. The musical apparatus may be divided into two systems, the first com posed of four oblique nervures, which ter minate in a strong nervure, serrated like a file (a); this may be considered as the bow. The second is formed of three nervures, which take their origin in a remarkable point in the internal border of the elytrum, furnished with a tuft of short stiff hairs, or brush (b); above this point is found a firm, transparent, and nearly triangular disc or sounding plate*, surrounded by a nervure. When the insect cries, the wings are crossed, and the bow rubbed rapidly across the whole length of the - - The muscles acting on the larynx in Batra chia are the dilator aditus laryngis and the constrictor aditus. Besides these, there is found in the tailless Batrachia a third muscle, the compressor glottidis, which in Brito arises from the columella*, and is inserted into the posterior point of the arytenoid cartilage. Its use is to compress the larynx, to bend the posterior angle of the arytenoid outwards, and to expand the vocal ligament. It is the most important muscle connected with the voice in the tailless Batrachia. Its course varies in different species, and is absent altogether in Pipa.
disc or sounding plate, whereby the whole of both elytra are put in a state of vibration, and the stridulous sounds peculiar to these gr3 Ili result. The pitch of the sound of the house cricket is very acute, being equivalent to about 4096 vibrations in a second.
Cicada?, Cigales. -- The Cicadm have their musical instruments inclosed in the interior of the abdomen. Reaumur gives a detailed description, illustrated by drawings, by which they may readily be recognised. These in struments, he says, are contained in the abdo minal cavity, divided into two cells by a scaly partition of a triangular form, covered by two cartilaginous plates, acting as shutters or opercula. When viewed from the abdominal surface, each cell presents exteriorly a white folded membrane with radiated reflections, which he terms the mirror. On opening from the upper surface the part of the abdomen corresponding to the cavity, we perceive on each side a plaited membrane, dry and sono rous, which is moved by a powerful muscle, composed of straight parallel fibres springing from the scaly partition ; this membrane is the tymbal. In order to bring into play an instru ment so complicated, Reaumur states that the insect alternately contracts and relaxes the muscle attached to the tymbal, and by this means produces the sound. He believes that " this sound is augmented in the drum, and that this portion of the vocal organ has no other use than to give it brilliancy." He also imagines " that the trochanter of the haunch performs the office of a curb, and prevents the operculum from being too much elevated during the song. Some doubts have arisen with regard to this simple explanation of the song of the Cicadm, and entomologists have concluded that the air performs an important part in the formation of the voice, and that it is due, at least in a great measure, to a rapid current issuing from the stigmata of the meta thorax, which resounds within the organs above described."