A few days ago, I received an e-mail from a colleague and friend, Kenneth W. Nickerson. Ken usually has a focused technical question in mind when he reaches out, but this time he asked about my memories of events transpired over 30 years ago regarding the genesis of a project on oil flies, Helaeomyia petrolei (now Diasemocera petrolei). He recalled things began when I got excited after seeing oil flies on a TV documentary and I sent a message with my recollections. His question stirred memories of many events long passed.

I first heard of oil flies from a TV documentary that probably aired on WTTW in Chicago. More precisely, coming into the house after mowing the lawn and passing by the TV at exactly the right moment, I saw brief footage of a larva or larvae moving through a mass of black, viscous oil and thought I heard the words “oil flies.” Beyond that, I had no idea where the things had been filmed or anything else about them.

Notwithstanding my lack of knowledge, one simple thought from that that day stuck with me; those things eat oil. Working at the time on bioremediation in the Environmental Research (ER) Division of Argonne National Laboratory, I also knew how significant that could be if it were true. However, there was no clear through line from the TV show to a research proposal and scientific publications or inventions. A better description of how things developed toward eventual laboratory investigations of oil flies might be a long process of accretion and slow dawnings.

Re-discovery

It took a long time to find any references to oil flies in the scientific literature. Part of the reason was the seminal work had been done and almost forgotten a long time ago. I finally got a lead from a book on insect physiology by Vincent Wigglesworth in the stacks of the Oak Park (Illinois) Library. The index entry for oil fly in this book was cross-referenced as ‘see petroleum fly.’ That got me (eventually) to works published in the scientific literature by W. H. Thorpe which were, at that time, nearly 60 years old. Apart from a coming across a note reporting a more recent field report of oil flies, I found nothing more published on oil fly biology since the early 1930’s.     

The investigations of Thorpe (1, 2) revealed that my initial idea oil fly larvae were eating oil was much too simple. However, the question as to whether or not fly larvae metabolize any components of the tar remained both open and interesting (3). Nonetheless, other things still had to crystallize to drive more action.

Miller Meadow Tadpoles

The Summer of 1988 was hot and dry in Chicago and we (the Robert V. Miller lab group at Loyola University Chicago) were seeking bacterial viruses (bacteriophage) in water samples obtained from a small drainage in Miller Meadow, part of the expansive Cook County Forest Preserve system adjacent to the Des Plaines river. At least one sample contained some tiny black tadpoles which we kept in the lab. The little things were active and developed quickly even though I do not recall if we ever fed them. We hypothesized that a phenomenon commonly known as the ‘bottle effect’ may have provided the nutrition these tadpoles needed. When water samples are placed in containers, nutrients tend to concentrate on the vessel surfaces which often yields a bloom of microbial growth. Perhaps enhanced growth, a consequence of simply placing water in a glass jar, provided enough nutritious bacterial biofilm for the tadpoles to harvest and develop to adulthood. The adult toads were so tiny we fed them fruit flies obtained from the lab across the hall.

The Sheep Take the Lead

In early 1991, I was an Assistant Scientist at Argonne National Laboratory when Dr. A. M. “Morrie” Craig came calling with two sheep. No ordinary animals, these sheep had surgically-installed (by Morrie) access ports which allowed samples to be drawn directly from the rumen, a site of intense biochemical interactions between an anerobic microbial flora and host animal. This symbiosis between host and rumen microbes enables sheep to derive nutrition from ingested plant material. 

Sheep are not impacted by alkaloid toxins in plants like Tansy that will kill horses and cattle swiftly. The ability of sheep to detoxify ingested alkaloids was not due to unique enzymatic capacities of the ovine liver. Instead, Morrie made the surprising finding the microbial flora in the sheep rumen transformed deadly toxins into non-harmful products. Further, he found that the sheep rumen flora was quite versatile biochemically, able to transform some recalcitrant byproducts of munitions manufacture that had created serious environmental health hazards, perhaps the first step toward breaking them down completely. Morrie came to Argonne National Laboratory to demonstrate the rumen flora harvested from living sheep could transform a particular molecule of interest to the Department of Energy – and he succeeded The Bomb Bio-Remediation Squad Wore Wool. Could munitions wastes-polluted soil/water sites that looked like eternal problems be mitigated, if not restored, by feeding them into artificial bioreactors containing a microbe(s) obtained from the sheep rumen? It was an intriguing idea.                     

Combining Thorpe’s reports of gram-positive bacteria visible in the guts of oil fly larvae with Morrie Craig’s work on sheep rumen biochemistry, invites speculation these tiny insects harbor an analogous symbiotic gut microbial flora with novel, potentially useful, capabilities to metabolize and/or detoxify tar oil hydrocarbon compounds. Thinking back to the Miller Meadow tadpoles and some speculations of Thorpe (3), maybe the interaction between bacteria and larvae is not a complex symbiotic consortium, but something more simple, transient and fortuitous. Could bacteria with unique hydrocarbon compound degradative capacities be cultured from oil fly larvae using standard techniques and ultimately harnessed to bioremediate oil contamination? It looked like a crazy idea worth trying, but nothing got going until I met a kindred scientific free spirt named Ken Nickerson. 

To the Laboratory

First, we had to find the things. This effort began before the web was the comprehensive and accessible information resource it is today. Knowing that we were seeking petroleum flies once found in Southern California oil seeps and reaching out to Entomology faculty members in that state, it still required a good deal of time to get in touch with anyone who had actually seen these insects and knew they could still be found in the La Brea Tar Pits (4).     

With essential advice and aid from Christopher Shaw, Brad Plantz and Jeremy Walker captured some oil fly larvae at the La Brea Tar Pits by watching for them moving at the oil surface, dipping them out with toothpicks and placing them in small plastic film roll canisters containing a little tar for transport. In the lab we housed the captured oil fly larvae in Tupperware containers with a thin layer of La Brea tar. The fumes emanating from the tar were strong, suggesting reasonably high levels of volatile hydrocarbon compounds were present. If I remember correctly, the first oil fly larvae were collected not long after the 1994 Northridge earthquake and perhaps the La Brea tar seeps had been refreshed with an influx of new oil.

Our focus was characterizing the microbial flora of the larvae and I can’t recall now if or how we tried to feed them. However, we were surprised to see some larvae survived long enough to crawl out of the tar and pupate. Adult flies emerged not long later.

La Brea Tar Pit collection site, 1994. Photo by Jeremy Walker.

Oil Flies and Their Associated Microbes

Ken Nickerson and his doctoral student, Dana Kadavy, pursued the characterization of the culturable oil fly larvae bacterial flora deeply, producing publications in the peer-reviewed scientific literature (5, 6), the first reported investigations of these strange insects in over 60 years. Their experiments led them to formulate novel hypotheses regarding the evolutionary and medical significance of the interplay between solvent tolerance and microbial antibiotic resistance. Oil fly larvae were the subject of Dana’s doctoral dissertation, making her arguably the world’s leading authority on these remarkable organisms and the bacteria coexisting with them in the strange and hostile tar pit environment.

Undiscovered Territory

Over a century after oil pool maggots of Southern California were first brought to the attention of the scientific community (1, 7-9) and now nearly 30 years after Kenneth Nickerson and Dana Kadavy began publishing their oil fly microbiome studies (5, 6), the biology of these enigmatic insects and their extreme environments are ripe for further scientific exploration. Methodology, in particular the ability to detect and characterize microorganisms that are difficult to culture, has advanced considerably since the works of Kadavy et al. were published and been applied to examine the microbial community found in La Brea tar (7).  

Did the oil fly larvae we studied in the lab derive any nourishment from the hydrocarbons in the tar? Once again, a definitive yes-or-no answer is elusive. 

At some times and locations, oil flies were noted to be present in extraordinary abundance (8-10), enough to prompt one statement that larvae could be furnished for study “by the gallon” (8). Not doubting any accounts, the few facts in hand are puzzling. Although oil flies are endemic to the La Brea Tar Pits and I recall Christopher Shaw informing me that oil fly larvae predictably became active in the Spring, I do not remember him ever mentioning them becoming so numerous they became problematic and no one who collected larvae there for the lab ever remarked on their extraordinary abundance. If oil fly maturation depends on consuming entrapped insects or other creatures, how was it that some tar seeps once yielded such striking profusions of both larvae and adults?  

Several explanations for those observations are possible. A simple one is that some seeps end up with a lot more trapped carrion to serve as a food supply than others. Another is that there is are qualitative differences between oil just pumped out of the ground, fresh seeps and aged tar deposits. Oils and tars are extraordinarily complex chemically and perhaps freshly exposed samples contain more components that oil fly larvae could more readily metabolize directly or indirectly mediated through microbial interactions.

The idea that all tar seeps are not identical is an old one. S. F. Peckham noted (11) if fresh bitumen (tar) samples brought to the surface and allowed to pool in the open air, they were rapidly infested with maggots (oil fly larvae). Could fresh tar be both uniquely attractive to adult flies and offer larvae at least some sustenance while they awaited external food sources to get trapped and die? It is well understood that crude oil properties vary substantially, do tar seeps differ in both chemical composition and the microbial species they harbor? W. H. Thorpe (3), connecting the discovery of oil-degrading bacteria in deep samples from California wells to his prior work and observations of a paucity of entrapped insect food sources in some tar pools with abundant larvae, hypothesized microbial activity and oil fly nutrition could be linked in some manner. Perhaps oil fly larvae can consume some fractions of fresh tar seeps directly or become colonized by a unique and beneficial microbial consortium that metabolizes tar components. Nonetheless, Thorpe also duly noted that oil fly larvae are actively motile in their viscous environments, as if searching out food sources. If simply ingesting oil satisfied all nutritional requirements, he concluded there would be no necessity for such activity.

Over a century after the oil flies of Southern California were brought to the attention of scientists, this insect has been described, but remains only vaguely understood. We can only conjecture about the evolutionary forces and contingencies forging an insect that survives and prospers in the deadly pools of tar seeps.              

Perhaps on a mission somewhere in our solar system (12) or beyond, scientists will discover unimaginably extreme environments teeming with exotic (to us) life forms. For now, if you want to see living, carnivorous creatures that seem more like something from science fiction than bona fide scientific reality, oil flies reside quietly at the La Brea Tar Pits on Wilshire Boulevard in Los Angeles, California (4). These tiny, mysterious things thrive in an inhospitable, extreme environment, in the shadows of Sabre-toothed Tigers and other outlandish inventions of evolution.

  1. W. H. Thorpe. 1930. The Biology of the Petroleum Fly (Psilopa petrolii) Transactions of the. Entomological Society of London 78:331-334. 
  1. W. H. Thorpe. 1931. The Biology of the Petroleum Fly. Science 73(1882):101-103, 23 January 1931. https://www.science.org/doi/10.1126/science.73.1882.101
  1. W. H. Thorpe. 1932. Petroleum Bacteria and the Nutrition of Psilopa petrolei. Nature 130(3281):437. https://www.nature.com/articles/130437a0
  1. La Brea Tar Pits and Museum, Los Angeles, California. https://tarpits.org/
  1. Dana R. Kadavy et al. 1999. Microbiology of the Oil Fly, Helaeomyia petrolei. Applied and Environmental Microbiology 69(4):1477-1482. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC91210/  
  1. Dana R. Kadavy et al. 2000. Natural Antibiotic Resistance of Bacteria Isolated from Larvae of the Oil Fly, Helaeomyia petrolei. Applied and Environmental Microbiology 66(11):4615-4619. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC92357/
  1. Jong-Shik Kim and David E. Crowley. 2007. Microbial Diversity in Natural Asphalts of the Rancho La Brea Tar Pits. Applied and Environmental Microbiology 73(14):4579-4591.
  1. L. O. Howard. 1899. An Insect Breeding in Crude Petroleum. Scientific American 80:75-76.
  1. D. L. Crawford. 1912. The Petroleum Fly in California. Pomona College Journal of Entomology 4:687-697. https://www.biodiversitylibrary.org/page/12457001#page/318/mode/1up  
  1. C. O. Easterley. 1913. The Oil Fly of Southern California. Psilopa petrolii. Bulletin of the Southern California Academy of Science 12:9-11
  1. S. F. Peckham. 1894. On the Nitrogen Content of California Bitumen. American Journal of Science 48(285):250-255. https://typeset.io/papers/on-the-nitrogen-content-of-california-bitumen-4xegdf0mpq  
  1. Mark Kaufman. 2024. NASA Excitedly Confirms It Will Fly a Sci-Fi Craft on Distant Moon. Mashable.com, 20 April 2024.  https://mashable.com/article/nasa-dragonfly-mission-titan-saturn

###