LIFE-HISTORY 



45 



course of its growth it casts its skin twice, and therefore passes 

 through three distinct stages of growth. In the first stage the 

 anal spiracles, or breathing holes, on the last segment, are con- 

 tained 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" (Howard, 1911, p. 20). 



The first stage usually occupies 24 to 36 hours, but may last 

 as long as three or four days. The second stage lasts between 

 24 hours and several days. 



" The full-grown larva is a creamy white legless maggot 

 measuring 12 mm. {h inch) in length. It is slender and tapering 



Fig. 18. Full-grown larva of house-fly; greatly enlarged; upper figure, side view; 

 lower, view of under surface ; middle figure, anal spiracle still more enlarged. 

 (From Howard, 191 1, Fig. 7, p. 22.) 



in front, large and terminating bluntl}- behind. Twelve distinct 

 segments can be recognized ; in reality there are thirteen, the 

 second segment being of a double nature" (Hewitt, 1912, p. 22). 

 The body gradually tapers oft" from the middle to the anterior 

 end where it terminates in a pair of oral lobes, each of which 

 bears two small sensory tubercles. The mouth opens on the under 

 side of and between the oral lobes, which can be withdrawn into 

 the succeeding segment. Above the orifice of the mouth there 

 projects from between the oral lobes a black hook-shaped process. 

 *' This is part of the skeleton of the larval head, and is used in 



