THE GLOWWORM* S LIGHT-EMITTING APPARATUS. 319 
surfaces as there exist organs, is thus diffused by aid of the 
interstitial substance, and when the light function is active, the 
brilliancy of the whole is nearly equal. But when the 
intensity of action lessens , the interspaces are less luminous, 
and the spots of light corresponding to the position of the 
light-organs continuing to shine (though with less splendour), 
appear distinct and isolated. Having no illumination of its 
own, the interstitial substance is the first to grow dark when 
the light diminishes, and thus lead to the erroneous idea of 
Marcartney*s, that this substance is more under the influence 
of the insect's volition. The truth rather appears to be that 
the fatty interstitial substance is well adapted to multiply, by 
refraction and reflection, the light emitted by the proper light- 
organs, just as the catoptric apparatus of the lighthouse 
spreads the light of the lamp placed in its focus far and wide. 
This physical effect of the interstitial substance is further 
evidenced in other luminous insects. In the glowworm, when 
shining brightly, the light appears at the sides and the back 
of the insect (particularly that coming from the deeper-seated 
organs), through the transparent membrane which connects 
the segments of the body. In the fire-fly the principal light is 
given out from two transparent patches situated upon the 
thorax, and from two similar patches concealed under the 
wing-cases, which are not visible except when the insect is 
flying. But the whole body is full of light , which shines out 
between the segments when stretched. In the lantern-fly the 
material which diffuses the light is contained in a snout-like 
projection from the head. The light of Pausus sphoerocerus 
shines dimly from the antennae. In some moths, and even 
gnats, phosphoric light has been seen. In all insects an 
interstitial fatty matter is present, but in luminous insects a 
more or less perfect apparatus or light-organ probably exists. 
The histology of this fatty matter of the glowworm has not 
been specially described by any writer, for which reason the 
following account may be worthy of record. In the adult 
insect a small portion of this substance, when unravelled and 
placed under the microscope, exhibits a series of globular- 
shaped bodies composed of granular and highly-refractive 
fatty particles inclosed in a membranous envelope, and con- 
nected together like beads on a string by one or more fila- 
ments, which are a direct continuation of the structureless 
membranous sheath. The ends of these filaments are attached 
to the inner surface of the skin, and the fatty bodies — hung, as 
it were, on the filaments — are suspended freely in the thoracic 
and abdominal cavities of the insect, chiefly along the sides 
of the abdomen, and form a kind of epiploon. As there is no 
other connective or areolar tissue, the whole structure differs 
