EXPERIMENTS ON THE BREEDING OF THE MANGOLD FLY. 91 



In this experiment no more specimens were introduced after oviposition took 

 place. 



Everything considered, the season was not very favourable for the successful 

 prosecution of the experiments. The period of drought which persisted from the 

 time of sowing, 26th May until 25th June, was followed by wet weather, which 

 more or less continued up to the time that the work was concluded in the middle of 

 September. Duriuc; these dull, wet days the flies confined in the cage were inactive 

 and sluggish. Bricht, warm days, which appear to offer the best conditions for 

 copulation, were conspicuous by their absence. Still, this does not offer a satisfactory 

 explanation of the results ; but the weather conditions served to impress us with 

 the necessity of repeating the experiments for two or three years if the work is to 

 have a determinative value. 



Oviposition was first observed on 9th August in compartment V., where the adult 

 females of P. hyoscyami, reared from maggots feeding on the leaves of belladonna, 

 were actively engaged in laying their eggs on the mangold plants. A few days later 

 the leaves of almost every individual plant had their quota of eggs, and in some cases 

 the tiny maggots had hatched out and were busily engaged making incipient mines. 

 On 3rd September the leaves were badly blistered. In order to determine any 

 differences between the adults which were reared from these mangolds and those 

 reared on belladonna, a number of puparia of both were separately isolated and the 

 emerging adults retained. It was found that the differences were insignificant and 

 amounted to a slight variation of colour. Those reared from mangold leaves were 

 generally slightly darker, the abdomen being rather more cinereous than in the 

 specimens reared on belladonna. 



The fact that the result was positive in the case just considered would appear to 

 necessitate an abandonment of the theory of " biologic " species which the author 

 has postulated {loc. cit. p. 70), or, at the most, it can only be maintained in a modified 

 form. It might be argued that in a confined space with only one kind of plant 

 present, the fly had no choice but to oviposit on the mangold leaves. But since it is 

 recognised that the presence of the food-plant is a stimulus to oviposition, we must 

 believe that under certain circumstances mangolds will supply this stimulus equally 

 w^ith belladonna and induce the fertilised females of P. hyoscyanii reared on the latter 

 to lay their eggs on mangold leaves, if belladonna be not available. 



A curious circumstance is that the P. hyoscyanii of belladonna refused to oviposit 

 on sugar-beet, whereas they did so freely on mangold in compartment V., although 

 sugar-beet is merely a selected variety of mangold specially grown for its sugar content. 

 Similarly in the case of compartment III., specimens of P. hyoscyanii reared from 

 mangold leaves failed to oviposit on the leaves of sugar-beet. In these two instances 

 the theory of " biologic " species would appear to be established, but the facts ought 

 to be confirmed by repeating the experiments. 



In the case of compartments I. and IV. where P. bicolor reared on dock was confined 

 with sugar-beet and mangold, the results turned out as the author expected. In 

 both experiments the flies failed to oviposit. This rather strengthens our belief that 

 the precaution of clearing out docks from the vicinity of mangold and sugar-beet 

 crops is not likely to produce the results generally anticipated. Not that the farmer 



