788 Marvels of the Universe 
pair of strong and flexible knives toothed along both their edges. These blades vary somewhat 
in form in the fleas of different animals, and between them is a suctorial tongue, also coarsely toothed, 
by means of which the flea increases and absorbs the flow of blood from the wound made by the 
saw-like lancets. 
The flea, like other insects, deposits eggs, which are of a comparatively large size, and in the 
case of the human flea twelve is a maximum batch. 
The larvee which hatch from the eggs are bristly worm- 
like animals, and they will feed on almost any organic 
dust and dirt that happens to be near, such as that 
rubbed from clothes, fur, feathers, etc. ; they will also 
thrive on powdered bread crumbs and _ similar food 
materials. As a consequence, they are often reared 
between the boards of houses where floors are left 
unswept. After ten or twelve days’ feeding they 
change into the pupa stage, and a week later emerge 
as perfect fleas. In cold weather their development 
takes longer. It is a curious fact that the human 
flea proper, although common in Europe, yet does 
not thrive in America, and is there very rare. A 
common species which infests both the cat and dog 
there, also becomes a pest in houses. There are a 
hundred or more kinds of fleas known to infest 
mammals and birds. 
THE MIRAGE 
BY F. W. HENKEL, B.A., F.R.A.S. 
THE name of Mirage is commonly given to certain 
illusory optical phenomena whereby objects are 
apparently seen in positions far removed from their 
true places, sometimes clear and distinct, at others 
distorted, the images being erect or inverted (2.e., 
seen upside-down). Special local varieties of Mirage, 
such as the Fata Morgana of Italy, the “‘serab”’ of 
the desert, are well known, and the phenomena 
known to the sailor by the name of looming are of 
kindred nature. 
In ordinary cases an object is seen by light trans- 
mitted directly from itself to the eye, and when 
Ee a Mane TS the intervening medium (usually air) is of approxi- 
THE HEAD OF A FLEA. mately uniform density and the object is not very 
Showing the complex drill which probes into the far above or below the level of the eye, it is seen in 
flesh of its victim. An outer sheath of two strong . qs: : : : : : 
faves ats coated! cee Gallbecs a onicrell mom, mS Me kineeviom, Since ligat Unen travels in straight 
also coarsely toothed. lines. If, however, the medium is ot of uniform 
density, or if a change of medium takes place (e.g., if, for instance, the light passing from 
an object to the eye travels first through air and then through water), the path of the rays is no 
longer straight but broken or refracted. In a similar manner a straight stick held obliquely 
partly in water and partly outside will appeay bent at the surface of the water, the immersed 
part being apparently bent upwards. If a coin be placed*at the bottom of a vessel with opaque 
