THE CHEESE SKIPPER AS A PEST IN CURED MEATS 13 



PREOVIPOSITION PERIOD 



At summer temperatures and in the presence of a moist food 

 medium, the laying of fertile eggs usually begins in about 24 hours 

 after newly-emerged adults have been mated. With less favorable 

 conditions the preoviposition period is lengthened, the determining 

 factors being temperature and food supply. Table 4 shows typical 

 preoviposition periods. One female 24 hours old laid eggs 3 hours 

 after mating, but these did not hatch. Fertile eggs have been secured 

 lO 1 /^ hours after the mating of newly emerged flies. 



OVIPOSITION 



The process of egg laying is accomplished in a rapid manner, eggs 

 being deposited every three or four seconds or at longer intervals. 

 The female runs about over the meat in nervous haste with her ovi- 

 positor extended and its tip, held downward at an angle with the 

 ovipositor, touching the surface. As she goes actively about she feels 

 the surface with the tactile extremity of the ovipositor moving from 

 side to side and exploring crevices in the meat. No sooner has an egg 

 been deposited than another appears in the translucent basal segment 

 of the ovipositor, whence it is rapidly ejected at the will of the fly. 

 As the egg passes through the opening in the ovipositor the latter 

 often bends sharply for an instant, whereupon the egg is wiped off 

 by contact with the meat. The eggs are laid singly or in groups on 

 the surface of the meat or, where many flies are present, are packed 

 by thousands into crevices where the membranous connective tissue 

 seems to fill the requirements of the females for an ideal location. 

 The laying of eggs in masses favors the development of the progeny, 

 as indicated later. In vials where the meat was placed on cardboard, 

 eggs were often laid between the meat and paper, arranged in groups 

 like the pleats of an open fan. 



In the time of Redi (1688) the process of oviposition had evidently 

 not been observed. Most people believed in the spontaneous genera- 

 tion of low forms of life, particularly those found in filth. An 

 interesting theory, mentioned also by Redi (58, p. 73), was that of 

 Gassendi, who believed that the skipper flies deposited their eggs on 

 grass, which was eaten by cows, sheep, and goats and thereby intro- 

 duced into milk and cheese. 



A female fly in the act of thrusting her ovipositor through the 

 meshes of a linen bag covering a ham was observed by Dufour (19). 



The observations of Kellogg (37, pp. HJ^-115) in infested packing 

 houses at Kansas City showed that the adults were literally swarming 

 in the smoky sacking rooms where hams were being wrapped and in 

 the smoke-filled shaft where the meats were smoked. The eggs were 

 laid upon the hams even while the meat was in the wrappers' hands, 

 with the result that the wrapped hams were infested before being 

 shipped. ' 



Murtfeldt (53, p. 173), however, found that the eggs were de- 

 posited on the wrapper, preferably among the folds or in spots where 

 the fat had penetrated and loosened the yellow wash. Sakharov (67) 

 asserted that eggs are not deposited on a dry medium, but the writer 

 has observed eggs upon- the dry muslin cover of a jar of strong- 

 smelling cheese. It is evident that actual contact with the proposed 



