1975] 
Willey — Sound Production in Arphia 
335 
phase shift of right and left hind femora. In these grasshoppers 
which produce several clearly defined amplitude-modulated (AM) 
pulses per stroke by each femur, phase shift serves to blur or erase 
the amplitude-modulation which each femur alone is producing. His 
careful analysis shows, however, that this AM-erasure is due not to 
a sloppy coordination of the two femora, but rather that the pro- 
prioceptors of the two legs produce a feedback which precisely times 
the movements to the msec. He postulates that the main signal is a 
frequency modulated (FM) pulse series superimposed on the com- 
bined AM system, ranging between 250 Hz to 8 KHz. Eisner sug- 
gests that there may be discrete frequencies in this range which 
would activate specific groups of sensory cells or neuronal circuits 
in a manner already known in locusts. If we reexamine Figs. 8-43 of 
the present paper, even the single AM-pulsed chirp of Arphia sul- 
phurea has a rather regular, though weak, FM signal which peaks 
several times at about 10 KHz in addition to the carrier frequency 
of about 5 KHz (average). This FM signal would appear in a 
sonagram as a faint octave overtone merging with the instrumental 
artifact (see Willey & Willey, 1969), perhaps indistinguishably. 
Nevertheless, one of the earliest determinations of an insect’s acous- 
tic sensitivity was performed on A. sulphurea by Wever (1935) 
who showed that the maximum sensitivity of the tympanal organ 
was reached at 10 KHz — a result which makes the oscillographi- 
cally observed FM signal more biologically significant. 
I present the following hypothesis: that the necessity to place the 
femora out of phase, because the femoro-tibial joints overlap the 
midline, also allowed a metachronous mechanism to develop which 
could control the frequency modulation of the signal. Arphia sul- 
phurea and other oedipodines are at the least modified end of the 
spectrum and show perhaps how the more elaborate gomphocerine 
signal systems began. 
Technique Recommendations 
The technique of superimposing synchronizable, visually trans- 
duced signals on a high-speed film of a moving structure is elegant, 
but expensive. However, good preparations are permanent and can 
be analyzed over and over for new permutations. I advise one to 
copy the best sections by means of an internegative, before they are 
viewed in a standard projector or motion analyzer. A careful check 
through a clean 16 mm movie editor is the maximum handling the 
original should receive. Then the original should be filed away 
