in the oil. Tt is noticeable that in phiees where shallow pools 

 are literally masses of wriggling larvae, the number of adult 

 flies is surprisingly small in comparison. This may indicate 

 that but few larvae pupate in the held. In the laboratory, 

 too, the number of pupae obtained is but a small proportion 

 of the number of larvae. It may be that the dead larvae in 

 the oil furnish a source of food for the living, either through 

 extracted juices or actual consumption of the bodies. 



Pupation takes place readily enough in the laboratory, 

 and the cases are generally found on the vertical side of the 

 dish (sometimes on the cover), and nearly always on the side 

 nearest the window. The larvae thus show a very marked 

 positive reaction to light, for a short time, though there does 

 not seem to be any response to light in the pre- and post-pupal 

 stages. I cannot state where pupation occurs in the open, 

 for all the larvae I have seen were reared in the laboratory. 



An interesting question for speculation is : How did this 

 habit of breeding in oil originate'? It is evident that as far 

 as competition of the larvae with other organisms is concerned, 

 the struggle for existence must result favorably to Psilopa, for 

 the immature stages are perfectly protected from other ani- 

 mals that might prey on them. If the larvae are able to get 

 food in the oil, how simple their problem of existence seems 

 to be! It is not probable that tlie remarkable hal)it of Psilopa 

 petrolei could have been develo])ed gradually. On the con- 

 trary, must not this habit and the adaptive structures and 

 constitution of the larvae have arisen suddenly, as "muta- 

 tions?" At any rate, no matter what views one may hold re- 

 garding the method of evolution, the oil-fly shows a most ex- 

 traordinary adaptation to environment. 



Another Mustard Pest 



A. liavidson, M.D. 



About 1909 the w^riter first observed an unfamiliar wild 

 mustard on W. Washington St., Los Angeles. Attention was 

 attracted by observing a mustard plant still in flower late in 

 the autumn, in a plot of unirrigated ground. It reappeared 

 or more properly speaking it continues in that locality (for it 

 is apparently perennial) yet and has been observed in many 

 other parts of the city. At the present writing it may be said 

 to be fairly common in many places and extends outside the 

 city along the railway to Chatsworth and East as far as Po- 

 mona and Ontario, Avith a few plants near Riverside. The com- 

 mon mustard, Brassica nigra, prefers the heavy clay soils of 

 the region and is purely a spring plant. This mustard, Brassica 

 adpressa Moench, like "the other, is an old-w^orld emigrant, but 



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