308 THE CAUSES WHICH PROPAGATE 



the office of alar expansion in the new-born insect on quitting the 

 swathing pupa mask ; or afterwards, where the hind-wings plait 

 and fold in repose, they inflate and spread them balloon-like 

 previous to taking flight. It is evident, therefore, this similar 

 lateral and corresponding disposition of the respiratory organs in 

 each segment, typical of insects and sj^iders, excludes simple 

 laryngeal voice, where the lungs are the bellows, and the vocal 

 cords, palate, and teeth the facile notes of the flute ; so insects 

 can neither cry, sing, nor speak, and the term vocal can here only 

 be technically retained in treating of any spiracular sounds. 



If we may suppose the instrumental music of Articulata had 

 its origin in muscular contractions when under the stimulus of 

 emotion, spiracular music may have taken origin in a wing 

 movement performed under similar conditions. And while in 

 some insects these murmurs are in degree alar in character, in 

 others their production becomes more evidently dependent on 

 the internal muscular system, and more evidently produced by 

 the action of the tracheal : a specialisation which confers new 

 powers in the expression or communication of the emotions. 

 But before entering on the elucidation of this higher feature of 

 vocal music, it will be well to entertain clear views of the earlier 

 wing movements and^their import. 



Certain moths produce a sound by simple wing-percussion, 

 that appears to promote the intercourse of the sexes. In proof 

 of this we may turn to the broad-wing silk-spinners, or to their 

 representatives among those with looping caterpillars, and ob- 

 serve the sonorous beating of wangs that accompanies the brisk 

 gyrations of the male subsequent to its scenting a female. This 

 is well exhibited by the common White Mulberry Silkworm 

 Moth. The female also, I believe, thus assembles its suitors ; 

 for it is not unfrequently the well-attuned ear of the country fly- 

 catcher catches this soft fluttering sound, when out with his gauze 

 net on the dewy meadows, at the hush and solitude of nightfall, 

 when some bulky cynosure announces the term of her siesta by 

 climbing the flowery herbage, winnowi'ng her echoing wings. 

 Then, if a male of one of these beating moths be confined in an 

 apartment, its period of flight will be intimated by its trilling at 

 intervals, with a sound not unlike that of ^ a policeman^s rattle, 

 which begins low and increases in loudness. The grey-suited 



