330 
LUMINOSITY OF INSECTS. 
beneath the hinder part of the thorax, which is only seen 
during flight. The light proceeding from these points is 
sufficiently intense to allow small print to be read in the pro- 
foundest darkness, if the insect be held in the fingers and 
be moved along the lines; and the natives of the coun¬ 
tries where they are found (particularly in St. Domingo, 
where they are abundant) use them instead of candles in 
their houses, and tie them to their feet and heads, when 
travelling at night, to give light to their path through the forest. 
In all the luminous species of this family, the two sexes are 
equally phosphorescent. 
398. The family Lampyridoe contains about 200 species 
known to be luminous ; the greater part of these are natives 
of America, whilst others are 
widely diffused through the 
Old World. In most of these, 
the light is most strongly dis¬ 
played by the female, which 
is usually destitute of wings, 
so that it might be mistaken 
for a larva. The species of 
Fig. 174.— Male and Female Glow- OUr Own Country is known as 
WORM * the Glow-worm (fig. 174). 
399. The light of the Glow-worm issues from the under 
surface of the last three abdominal rings. The luminous 
matter, which consists of little granules, is contained in 
minute sacs covered with a transparent horny lid; and this 
exhibits a number of flattened surfaces, so contrived as to 
diffuse the light in the most advantageous manner. The sacs 
are mostly composed of a close network of finely-divided air- 
tubes (§ 321), which ramify through every part of the granular 
substance; and it appears that the access of air through these 
is a necessary condition of the phosphorescence. For if the 
aperture of the large trachea which supplies the luminous sac 
be closed, the light ceases; but if the sac be lifted from its 
place, without injuring the trachea, the light is not inter¬ 
rupted. All the luminous insects appear to have the power 
of extinguishing their light; and this they probably do when 
alarmed by approaching danger. The sudden extinction of 
the light is probably due to the animals power of closing the 
aperture of the trachea. 
