resonant cavities, which are classified accord- 
ing to their physical construction. They are 
the coaxial resonator, the waveguide cavity, 
and the rhumbatron or doughnut-shaped cavity. 
They take the shape of cylinders, rectangular 
prisms, or spheres. Rectangular waveguides 
are commonly used as resonant cavity devices 
in microwave radar. The cylindrical hollow 
waveguide and cylindrical coaxial resonator 
are the two configurations most nearly re- 
sembling the structural form of the antennal 
sensilla. 
The present state of antennal morphology 
indicates that although some sensory sensilla 
may be hollow, others have processes originat- 
ing at the sensory nerve cell that are channeled 
through a cuticular sheath and end in pores at 
the spine cuticle (Schneider 1964), If we con- 
sider these two types electrically, the former 
may resonate as hollow waveguides and the 
latter as coaxial resonators. Either type might 
also be filled with a liquid or other substance 
and thus be similar to a solid or liquid-filled 
dielectric resonator. 
Okress (1965) stated, "In the case of solid 
or liquid dielectric waveguides, the electro- 
magnetic energy exists in both solid or liquid 
dielectric and the surrounding air. For high 
real dielectric constants and for frequencies 
well above the cut-off valve, the energy is 
essentially confined in the solid or liquid di- 
electric. Since the cut-off wavelength depends 
upon the dielectric constant, no simple ex- 
pression as previously given for the metallic 
case is possible, 
"For single lobed directional radiators the 
asymmetric hybrid HE;,; mode in dielectric 
waveguide has similar field configuration to 
that of the dominant (H,,) mode in hollow cir- 
cular metal waveguide, except that the former 
(HE,,) has no cut-off (i.e., propagates at all 
frequencies), However, there is cut-off be- 
havior with respect to all, including the H,, and 
E,,, other modes, 
"In the case of the hollow dielectric cylinder 
waveguide, modes arise corresponding to those 
of the solid dielectric rod, but characterized 
by greater cut-off frequencies. However, in 
the case of the HE;; mode, no cut-off frequency 
exists again. Furthermore, hollow dielectric 
cylindrical waveguide can support a single 
mode (i.e., the fundamental HE,, mode), This 
is especially important for use as an antenna. 
159 
This is done by reducing the ratio of wall thick- 
ness to real dielectric constant to a sufficiently 
small value. Furthermore, for thin wall dielec- 
tric waveguide essentially single-lobed radia- 
tion pattern may be realized, This follows for 
the HE 1. mode when the wall thickness is small 
enough to make the hollow dielectric waveguide 
beyond cut-off for all other modes. 
"It is for these reasons important to deter- 
mine whether the insect's antennae are filled 
with high or low or no real dielectric constant 
liquid (e.g., water), It is also important to 
determine the real and complex dielectric 
constants of the dielectric antennae tubes and 
liquids, if any, contained therein." 
Generally speaking, if we allow the inner 
conductor in a coaxial line to decrease in size, 
we can eventually remove it, and the magnetic 
field will be self-supporting, This then be- 
comes the H,, mode ina circular waveguide 
(fig. 3), The resonant modes, which correspond 
to the lowest possible generated electromag- 
netic frequency, are labeled Ey and Hy modes, 
They are the lowest of an infinite number of 
frequencies at which a cavity will resonate. 
The cutoff wavelength of all modes in a cir- 
TAS: etic att 


cular waveguide isAc = 

wavelength for the H,, mode in acircular wave- 
guide isA c = 1.6.40 a. 
E- waves (fig. 4) are produced by calculating 
modes in rectangular waveguides and allowing 
the configuration to change from rectangular to 
circular. Where the configuration is changed, 
it may be necessary to have circular wave- 
guides of different sizes for different modes 
if we are considering the same wavelength 
energy. The different sizes would be neces- 
sary, because the cutoff wavelength changes 
and the circular waveguide must be able to 
propagate the energy of the wavelength in the 
rectangular waveguide (Mariner 1961), Such 
is, of course, exactly what we have in the 
moth antennae. There are many different an- 
tennal sensilla and certain of them are of 
variable lengths in graduated steps (Callahan 
1965b), 
E- modes have an electric field parallel to 
the axis of the cavity (Young 1960), The Eo; 
mode has a field that is constant with an angle 
a half wavelength in width along the diameter 
and a full wavelength along the axis. The Ep; 
mode of the cylindrical cavity has a resonant 
