PREOVIPOSITION PERIOD OF THE HOUSE FLY. 11 



Bishopp, Dove, and Parman are doubtless correct in saying that in 

 general "increased humidity seems to hasten egg laying." 



The present results agree also with the Dallas observations in 

 showing that the food supply of the adult flies has an important 

 influence on the length of the prcoviposition period. Banana, water, 

 and manure was found to be a favorable combination in these experi- 

 ments. Lack of moisture always proved fatal to flies, and without 

 food they die within two or three days. 



Yet temperature, humidity, and food supply are not the only 

 factors which influence egg laying. If Table I be examined, such 

 apparently contradictory results as an 8-day record at an average 

 mean temperature of 72° F. (see No. 20) and a 12-day record at an 

 average mean temperature of 76° F. (see No. 39) will be found. 

 Mr. Pomeroy has suggested, and the writer is strongly of the opinion, 

 that the kind and quality of the larval food is partly responsible for such 

 apparent discrepancies. It is a well-known fact that the amount and 

 kind of larval food have a marked influence on the size of the resulting 

 adult flies, and there is no reason to doubt that the physiological 

 activities of the adult are likewise affected. It appears in Table I 

 that most of the shorter preoviposition records were obtained with 

 flies which had been bred from larvae fed on moist bran or a bran- 

 manure mixture. From media which fermented less actively than these, 

 or which were allowed to dry out, or were too small in quantity, the re- 

 sulting flies were not only small but often failed to oviposit at all 

 and usually lived a shorter time than large flies under the same con- 

 ditions. Some larvae bred exclusively on rotten potato produced a 

 small race of flies which did not oviposit, due perhaps to the pre- 

 ponderance of starch in their diet. Griffith (1908) observes that 

 small flies are incapable of reproduction. 



In the experiments with isolated pairs of flies only 7 out of 30, or 

 about 24 per cent, gave any positive results as regards the deposition 

 of eggs. In the other experiments where a number of flies (8 to 200) 

 were used in each lot, 70 out of 110, or about 63 per cent, gave positive 

 results. It is possible that this may be accounted for by what may 

 be called a psychological factor. The habit of the adult females of 

 associating in large numbers in one favored spot in depositing eggs has 

 been previously pointed out (Bishopp, Dove, and Parman, 1915), and 

 it is probable that the isolation of a pair of flies is an abnormal con- 

 dition which has its inhibiting effect. 



As regards the number of depositions, the Arlington observations 

 again agree with those made at Dallas in that it was impossible to 

 obtain more than two lots of eggs from a single female, but this does 

 not prove, of course, that under normal conditions the adult fly can 

 not oviposit oftener. Griffith (1908) gives no details of experiments 



