THE CHEESE SKIPPER AS A PEST IN CURED MEATS 27 



of food ; seldom because of its exhaustion. The usual clustering habit 

 is exhibited on dry food, but in the case of dry meat the food resists 

 the agency which enables the maggots to soften cheese, and the 

 larvae become quiescent and develop alate margins on account of im- 

 poverishment. Under such adverse conditions, however, their hold 

 on life is extremely tenacious, and even in midsummer they are able 

 to resist starvation for weeks, and when moistened with a few drops 

 of water become active at once. 



It is worth while to note that masses of well-grown larvae, which 

 often congregate in the lower part of culture dishes, give rise to heat. 

 In one case the bottom of a glass dish containing several thousand 

 maggots clustered in a moist mass was found to be 13° warmer than 

 the room temperature of 69° F. 



The entire larval stage requires, under optimum conditions, but 

 five days for completion. Unfavorable food and temperature condi- 

 tions lengthen this period ; in one case a starved larva lived for over 

 six months, from October 2 to April 7, after which it pupated and 

 became adult. Pavloski (56) reported that the larvae have lived 

 confined in a corked test tube without food for eight months, and 

 later pupated. 



MIGRATION OF THE MATURE LARVA 



It is during migration that the insinuating ability of the skipper 

 and its leaping powers are particularly useful. Like many other 

 dipterous larvae, the full-fed larva of Piophila casei is impelled by a 

 strong instinct to leave the food and gain a dry, dark, close location. 

 Of these three specifications, closeness is least important and darkness 

 comes next, whereas dryness and freedom from grease are nearly 

 always matters of necessity. When darkness and dryness are pro- 

 vided, closeness in the form of a tight crevice is sought for in addi- 

 tion. According to the writer's observations, puparia are formed 

 somewhat more readily in darkness than in the light, but in hot 

 weather, even when light was provided without interruption, the 

 rapidity of pupation of both illuminated larvae and those in darkness 

 was such that the difference between them was negligible. 



The behavior of full-grown larvae which are prevented from leav- 

 ing the greasy medium in which they developed is of much interest. 

 This situation retards pupation or, if the grease is abundant, prevents 

 it and the larvae become quiescent. There follows a prolonged period 

 of rest during which the insect literally has nothing to do with the 

 exception, perhaps, of restoring slight metabolic losses by occasional 

 feeding. Cold likewise retards or prevents pupation. At 45° to 50° 

 F. pupation does occur, but slowly and irregularly, and mam^ larvae 

 do not pupate for weeks. When hundreds of migrant larvae are placed 

 together in a dry container at room temperatures, the mass of mag- 

 gots keeps in constant motion from the futile efforts of each indi- 

 vidual to skip or crawl away, and this condition likewise retards 

 pupation. When larvae which are held in greasy or cold locations are 

 removed to favorable surroundings, pupation takes place promptly. 



In the case of a certain culture reared by the writer, a number of 

 mature larvae were confined with their greasy food beyond the normal 

 time for migration. Upon their removal the pupation process was 

 so precipitate that in 35 cases the usual prepupal contraction was 



