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 Lampyridce 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- OUT own country is known as 



the Glow-worm (fig. 174). 



399. The light of the Glow-worm issues from the under 

 surface of the last three abdominal rings. The lumiaous 

 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 animal's power of closing the 

 aperture of the trachea. 



