In bright daylight the pigment of the om- 
matidia is dispersed and protects the "instru- 
ment'' from the excessive IR radiation of sun- 
light. As evening falls and incandescence is 
reduced, just as with a rheostat and incandes- 
cent lamp, the visible (hot) incandescent dis- 
appears but longwave (IR warm) incandescent 
light remains. Toward the night horizon, long- 
wave radiation is emitted from a deep layer 
of air and water vapor, where the sharpest 
intensity increase occurs at low elevations. 
The horizon, therefore, behaves like a true 
blackbody at its ambient temperature, All other 
objects in nature would be either hotter or 
cooler than this blackbody background and thus 
would be either lighter or darker to an IR 
detector. 
Mazokhin-Porshnyakov's high value for his 
ERG, obtained when he oriented the moth's 
eye to the midnight horizon, might have been 
due to the IR radiation of the dark midnight 
horizon and not to the low-intensity reflected 
visible light at all. The spectral energy of 
such wavelengths in the 0-mu region is far 
greater than the small radiation remaining 
from the scattering of sunlight by air molecules 
and other particles. 
It would seem to me that an instrument 
designed to detect the nighttime IR incandes- 
cence or IR reflected light of other moths and 
plants would be much more useful than one 
designed to detect objects illuminated by ex- 
tremely low-intensity reflected visible light. 
The latter instrument would seem to be apar- 
ticularly ineffective nighttime visual apparatus 
for detecting radiations if the insect has poor 
visual acuity, as many of the workers who have 
studied insect vision maintain (Dethier 1963). 
The work of Smith and Kimeldorf (1964) 
showing a high bioelectrical response of the 
noctuid eye to beta-radiation, as well as my 
own work with ultraviolet light (Callahan 
1965c), serves only to reinforce my belief that 
the moth eye is an IR detector at night. The 
shorter the wavelength, the faster the eye 
"light adapts" (i.e., shifts toward the short 
wavelengths and away from the longer IR 
wavelengths), The strong d.c. bias demon- 
strated by Burtt and Catton (1964) in noctuid 
eyes would certainly be expected of an IR 
image-conversioninstrument, especially in a 
sensitivity instrument with the ability to shift 
162 
its wavelength sensitivity back and forth along 
the spectrum. 
In my opinion, the dilemma of the moth 
when confronted with incandescent light is 
caused by emissions of visible light from the 
lamp as well as tremendous amounts of IR 
energy (fig. 1). The moth is in the same 
dilemma when confronted with blacklight and 
mercury lamps (table 2), From a distance a 
moth might start for such a point source, 
attracted by the intense longwave IR output. 
As the moth moves toward the light, the in- 
tensity of the source is quadrupled as the 
distance is halved, causing its eyes to grad- 
ually become light-conditioned and to move 
along the spectrum toward a visible or ultra- 
violet sensitivity. Thus, the moth's eyes would 
never be in a static condition. The spectral 
sensitivity at any one moment would depend 
on the distance from the point source and the 
flight speed of the insect approaching the 
source. If the moth flew close enough, it would, 
in a sense, be sitting in the light of a small 
sun, and after a time its eyes would become 
completely daylight-conditioned. Its eyes would 
thus have shifted from longwave IR detection 
to shortwave detection of visible light. The 
eye pigment would have dispersed from the 
Table 2.--IR lines in low-pressure mercury 
are-argon daylight and blacklight tubes 
Microns Atmospheric transmission 


[FS O13 9 Berets 
AA 286 Geen vere tae 
SOLO Sicrereyersrs Peak of tungsten filament at 
2200° K. 
df S628 eer 
ILS eR OU Nercstoidis.c Fer 
a BPA Pe oo.o1o.e Good atmospheric transmis- 
sion. 
W69202 405 408 Do. 
LAG O4N 9 Ae Do. 
USHA SG ad5 os Do. 
LO 99s even Do. 
A 8TSONe seers Poor atmospheric transmis- 
sion. 
PE OY PNG C ec Excellent atmospheric 
transmission. 
DASZOADeeieiens Do. 
Se9400h ee Do. 
AO2Z00 aero Do. 
Pee) A548 Ged Secondary emission. 

