20 THE HOUSE FLY—DISEASE CARRIER 
creature tapering from the blunt, round, hinder end 
to the pointed head end. It is glistening white in 
color and only about two mm. in length. It is ex¬ 
tremely active and burrows at once into the substance 
upon which the egg from which it hatched had been 
laid, rapidly disappearing from sight. In the course 
of its growth it casts its skin twice, and therefore 
passes through three distinct stages of growth. In the 
first one the anal spiracles, or breathing holes, on the 
last segment, are contained in a heart-shaped aperture. 
After the first molt these spiracles issue in two slits, and 
after the second molt there are three winding slits. 
In the third and last stage the larva is still white, 
sometimes appearing yellowish. It is slender and taper¬ 
ing in front, large and truncate behind. The head has 
a tiny papilla on each side. There is one great hook 
above the mouth orifice. On each side of the pro¬ 
thorax there are spiracles which show six or seven 
lobes. On the ventral base of the sixth and following 
segments there is a transverse fusiform, swollen area 
provided with minute teeth. The anal area is only 
slightly prominent, and shows two processes close to¬ 
gether. The anal spiracles are prominent, less than 
their diameter apart, and each with three sinuous slits 
and a button at the base. In some cases two of the 
winding slits are apparently connected together. With¬ 
in the head, or rather the anterior part of the body, 
is a chitinous framework consisting of several articu¬ 
lated parts called the cephalopharyngeal skeleton, which 
is indicated in figure 7 . 
