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. 



