PHASES IN THE LIFE HISTORY OF NON-BITING FLIES 129 



may take place, according to Hutchison (1916), as early as the day 

 following emergence. Oviposition may begin on the third day. 



He cites a large series of observations on the preoviposition period 

 showing that eggs may be laid from 21/2 to 23 days after emergence, and 

 that the period corresponds to temperature and humidity changes. At 

 Washington the shortest period was obtained at 82° to 84° F., and in 

 general the length of period increased with the decrease of temperature. 

 Increase in humidity seems to hasten egg laying. 



The eggs are white, cylindrically oval, slightly broader at the pos- 

 terior end with two distinct curved rib-like thickenings on the dorsal 

 surface, along one of which the egg splits on hatching. These eggs 

 are laid in masses averaging about 120, and a female may lay as many 

 as four such batches, and probably under favorable conditions more. The 

 eggs usually hatch in less than 24 hours, the time of course depending 

 upon the climatic conditions. At 10° C. (40° F.) the egg period is two 

 or three days; at 15 to 20° C. (59-68° F.) it is 24 hours; at 25-35° C. 

 (77-95° F.) only 8 to 12 hours, according to Hewitt (1917). 



The larvae are white, smooth, cylindrical maggots, tapering at the head 

 end and considerably enlarged at the tail end. When viewed by trans- 

 mitted light a dark chitinous structure can be seen in the anterior 

 regions. This is called the cephalopharyngeal skeleton and is partially 

 extrusible. Each species of fly larva is distinguished by the form of this 

 skeleton and hence if a slide mount is made of a skin boiled in potash, the 

 species can be identified by this and one or more other characters. The 

 three larval stages differ somewhat in the form of this skeleton so that it 

 becomes possible to determine exactly the stage of development. The 

 body is composed of fourteen segments of which the second is the pro- 

 thorax. This segment at its posterior margin bears the anterior spiracles 

 which are fan-shaped and have six or seven lobes. This segment is fol- 

 lowed by the mesothorax, metathorax, and eight abdominal segments. 

 The ninth and tenth (anal) segments are small and ventral. The 

 anterior portion of the venter of each of the first eight abdominal 

 segments bears spiniferous pads which assist in locomotion. The eighth 

 or last apparent segment bears the spiracular plates. These spiracular 

 plates afford the best means of identification of fly larvae. In the first 

 two stages each plate consists merely of two oblique slits on a slight 

 prominence. In the third stage they are well defined plates, D-shaped, 

 closer together than their width, with flat faces opposed, each with three 

 sinuous slits. 



In connection with this larval description, we may call attention to 

 errors existing in many larval descriptions. The thoracic spiracles belong 

 to mesothorax but often appear to have migrated to the prothorax. The 

 large terminal spiracles of Dipterous larvae are always on the eighth 



