434 PHYSIOLOGY OF THE INVERTEBRATA. 



express emotion postulates adaptive acquisition, consequent 

 on assumed integumental tendency under attrition to 

 determine a smooth undulatory surface, and propagation by 

 hereditary transmission ; a rudimentary structure of this 

 description exists in the Stag Beetle at the inferior and pos- 

 terior extremity of the head ; and whenever a number or 

 group of insects is capable of music, we may establish a 

 degradation of the organs almost invariably in mute indi- 

 viduals of the opposite sex, or in other members of the genus 

 or family. Practically, the microscope establishes the essential 

 constituent, the file (lima), to be a dermal or skin excrescence, 

 with a systematic exaggeration or coalescence of external 

 callosities, wrinkles, tubercles, or a protrusion of the' spiral 

 thread of the wing-veins or other tracheal .organ. Theo- 

 retically, this active or passive source of sonorous vibration 

 is a variously-placed more or less /-shaped tumour, provided 

 with denticulations more or less regular, which are vibrated 

 and sounded diagonally over a narrow raised callosity or 

 ridge, on the chitinous integument or modified alar vein. 

 These latter, constituting the passive or active clasping organ, 

 assume the function of a violin bow or plectrum." Many of 

 the musical sounds emitted by insects are said to express 

 fear, anger, and " the more complex emotions of love and 

 rivalry, causing, at certain seasons, the music to assume the 

 character of a stimulus to reproduction and migration." 



" The action of stridulation with the majority of beetles 

 and one of the bee group is a more or less rapid protrusion 

 and contraction of the abdominal segments, a respiratory 

 movement which we shall show results from tracheal dis- 

 position in the Insecta. In some moths and grasshoppers, 

 music is implicated with a bladdery inflation of the skin ; 

 but in other insects it is not directly dependent on respira- 

 tion. With some the action is a sharp nid-nodding, 

 performed by the elevator and depressor muscles of the 

 prothorax or head. Many butterflies and the crickets pro- 

 du(!e their music by wing friction, resulting from a rapid 



