1504- 



VOICE. 



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 Cicadas, 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." 



THE HUMMING, OR BUZZING OF INSECTS. 



It has been supposed by some entomologists 

 that the hum of insects is produced by the os- 

 cillations of their wings during flight, and this 

 supposition is strengthened by the fact that 

 the tones are altered during the suspension 

 of the insects in the air, and that the sound 

 becomes more acute when the tips of the wings 

 are removed. This hypothesis will not, how- 

 ever, bear the test of rigid investigation. 



It was observed by John Hunter, " that in- 

 sects emitted sounds after their wings were 

 cut off.* De Geerf, finding that after he had 

 cut off the wings, winglets, and poisers, the 

 buzzing continued, placed the insect under a 

 microscope, and observing that the stumps of 

 the wings were in rapid motion, he pulled 

 them off by the roots, on which the buzzing 

 ceased , and hence he inferred that the sound 

 was produced bv the vibrations of the wings. 

 But it is not surprising that insects, after such 

 mutilation cease to emit any sound. Bur- 

 meister J is of opinion that the sounds of some 

 Diptera, such as Tabanus bovinus, are produced 

 by a stream of air rapidly transmitted through 

 the thoracic air-holes during flight. He has 

 described and figured the mechanism of the 

 thorax and the air-holes of the Eristalis tenax, 

 which is as follows: The aperture of the 

 hinder air-hole is provided with a sphincter 

 muscle, perpendicular to the inner surface of 

 which are sixteen or eighteen horny lamellae, of 

 the same breadth as the muscle, and connected in 



* Phil. Trans. 1792. t Vol. ii. p. 13. 



J Art xvi p 377. Taylor's Scientific Memoirs. . 



the middle by another longitudinal horny band. 

 The sphincter muscle is lined with a membrane 

 clothed with feathery hairs, which cover the 

 air-hole like a sieve, and exclude foreign bodies. 

 He leaves it to naturalists to decide whether 

 or not this mechanism contributes to the form- 

 ation of the sound , adding that either way 

 it is of little consequence, as many insects 

 have no such lamellae. He observes that when 

 the insect sits or crawls it breathes through 

 the air-holes of the abdomen, but during flight 

 through those of the thorax. He considers 

 the hum of insects to be in reality a whistle. 

 The pitch of this hum is hitherto unexplained, 

 although it is quite certain that it does not 

 depend upon the number of the vibrations of 

 the wings, for in a favourable light the motion 

 of the wings of many insects, whose hum is of 

 a high pitch, can be clearly detected ; but, if 

 that pitch were owing to the vibrations of the 

 wings, their number would necessarily be so 

 great as to render the motion imperceptible.* 

 The author has recently examined, with 

 Professor Queekett, the spiracles of other 

 insects, such as the blue-bottle fly, and the 

 humble bee, and has discovered in them a 

 beautifully organised valvular opening, capable 

 of producing the sounds which these insects 

 emit during flight, f In fig. 922. is shown 

 one of the large thoracic spiracles of a blow 

 fly (Musca vomitoria)', it consists of two 

 valves, one much larger than the other, each 



Fig. 922. 



Thoracic spiracle of the blow-fly, Musca vomitoria. 



being provided with numerous branching 

 horny filaments or hairs which serve as a 

 support to the thin membrane forming the 

 valve; a somewhat similar form of spiracle 

 occurs in the humble bee, (fig. 923). The 

 valves are nearly of equal size, and the 

 branching hairs are much stronger and more 

 numerous than those in the blow fly. 



* Those who wish to pursue the subject further 

 may consult the second note to p. 425. art. MOTION., 



f The pitch of the blue-bottle fly ranges, in 

 different species, from 288 to 341 vibrations in a 

 second. 





