The "Oil Fly" of Southern California 



By 

 Calvin 0. Esterly, Ph.D., Occidental College. 



These insects are remarkable and unique in that the larvae 

 or maggots (often spoken of as "oil-worms") live in crude 

 petroleum. The fly is named Psilopa petrolei, and the larvae 

 may be found wherever there is exposed oil, though they seem 

 to be more abundant in puddles or where the oil covers boards 

 with a fairly thick coating. Psilopa lielongs to the family 

 Ephydridae, many of w^hose members are notable for breeding 

 in unusual places. Some of the genus Ephydra pass their 

 larval existence in such highly alkaline water as that of the 

 Great Salt Lake or Mono Lake. 



Very little is known, apparently, about the habits of the 

 oil-flies outside the laboratory. It seems probable that the 

 eggs are not laid in the oil. I have been alile to discover only 

 tlu-ee eggs and these were in my laboratory on some oil-soaked, 

 but dry, leaves. It is hard to see how tlie oxygen necessary 

 for the processes of development could be obtained if the eggs 

 are laid directly in the petroleum. The larvae, certainly, can- 

 not live very long outside of the oil. They often crawd out of 

 the petroleum but soon l)ecome clean and then begin to curl 

 up, and grow motionless and somewhat dried out; they die in 

 twelve or flfteen hours if not returned to the oil. 



The larvae may become completely submerged, but most 

 of them move about just under the surface of the oil, with 

 the tips of their air-tubes showing as minute points above 

 the surface film. Mr. Crawford has shown (Pomona Journal 

 of Entomology, Vol. 4, No. 2) that the i)rocesses which bear 

 the tracheal openings can be telescoped so that the spiracles 

 do not come in contact with the oil. It seems not to be known 

 how long the larvae can live wuthout access to air, but it is 

 likely that this protection of the air openings makes it possible 

 for them to exist for some time submerged in oil. 



How animals living in such a medium get their food is 

 not definitely known, as yet. It is possible that the larvae 

 get nutrition from the oil by osmosis through the body wall. 

 It is clainu^d, on arguments that merit consideration, that many 

 small -marine organisms subsist on nutrient substances dis- 

 solved in the sea-water instead of on ingested food. Similar 

 conditions may obtain for the oil-maggots, though it must be 

 said that it does not seem very likely. Another suggestion, 

 made by Dr. Howard, is that the larvae feed on organic parti- 

 cles contained in the crude oil. I have seen the bodies of uu>ths 

 and caterpillars, caught in the oil, covered with maggots wliich 

 were doubtless actually eating. But so far as my observa- 



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