San Francisco State University Department 
of Geography

(Geography 316:  Biogeography) 

The Biogeography of   

Mono Lake alkali fly (Ephydra hians)
by  Mono Simeone,  student in Geography 316, Fall 2000


Kingdom: Animialia 

Phylum: Arthropoda  

Class: Insecta

Order: Diptera  
Family: Ephydridae

Genus: Ephydra

Species: Ephydra hians 

A male Ephydra hians. Source:  Wirth, 1971.


Description of Species Distribution
Habitat Evolution
Natural History Threats
Breeding and Growth Other Interesting Issues
Food Bibliography

Figure 1: Location of the Mono Lake Alkali fly.  Source:  Mono Basin National Forest Scenic Area


Description of Species:
    Ephydra hians, also known as the “Alkali-fly”, is one of four species in the genus Cirrula and contains five subspecies.  The alkali-fly habitat stretches across the northwest part of the United States (Cash 1994).  The natural history, breeding, distribution, and threats to the alkali-fly discussed in the following pages are that of Ephydra hians at Mono Lake, California.  (figure 1)    
    Adult alkali-flies are 4-7mm long with a dark brown body.  The thorax segment is a metallic bluish or greenish reflection and the wings are a smokey brown color (Wirth, 1971).

    Larva of Ephydra hians consists of eight abdominal segments, three thoracic segments, and a membranous cephalic region (Cash, 1994).  During the first instar the body, all twelve segments, measure 1-3.5mm long and grows in each subsequent instar of 3.5-5.5 and 5.5-12 mm long respectively (Herbst, 1990b).  

Fig.2: Ephydra hians. Scanning electron micrography of the third larval instar.  The thoracic segments (t) are legless, while each of the eight abdominal segment have a pair of prolegs (p).  The caudal (8th abdominal) segment contains the anus (a) and a branched respiratory siphon (s).  Source: Cash, 1981.

 The alkali-fly habitat can be defined as benthic-littoral consisting of soft and hard substrate (MBEIR, 1993).

    Tufa, beach rock, bedrock, and mudstone all consist of hard substrate.  By far hard substrates, especially tufa, are much higher in density of larvae and pupae than soft substrate of mud, sand, and silt. The hard substate offer:  protection from waves and currents, secure holding, predators, shifting sands, and the presence of more nutritious food (algae) (Little et al. 1989).

    Salinity in the lake has a direct effect on algae, which directly has an influence on alkali-fly growth and development rates.  Higher salinity reduces algae and forces the larvae to use more energy to forage, leaving little osmoregulatory to occur.  Osmoregulatory allows the larvae to balance the amount of saline in their body, if too much saline enters the body without being disposed, the fly will die (Herbst, 1992).  The combination of salinity and alkalinity is extremely difficult for most species to adapt to.  In fact the only other life form that is apparent in all other lakes that contain Ephydra hians, is the brine shrimp Artemia (Wirth, 1971).

    In waters of high alkalinity and salinity, the survival rate of alkali fly tends to be greater (Fig.3)(Herbst, 1986).  Mono Lake alkaline concentration is caused by carbonate and bicarbonate ions like most alkaline lakes.  The carbonate and bicarbonate ions of Mono Lake produce forty percent of the dissolved solids.  Worldwide, no lake contains a higher level of alkalinity and supports an insect population (MBEIR, 1993). 

Fig.3: Saline water habitat for the distribution of Ephydra.  Each corner of the environment table is inhabited primarily by a single species.  Source: Herbst, 1981.

Natural History:

    The alkali-fly life cycle is typical of many insects.  Developing from an egg to larva before pupating and metamorphosing.  Female alkali-flies walk down a substrate and lay their eggs usually on algae mats at depths of about 3 meters in Mono Lake.  Larvae will then go through three stages ranging from 25 to 30 days (Herbst, 1986).  Once the larvae have had time to develop and are ready to pupate they will attach to a hard substrate such as a tufa formation.  Tufa is a pumice rock formed by a volcanic eruption caused by a fault that runs underneath Mono Lake (Stine, 1992).  Clinging on the tufa to protect its self from turbulent water, the larva then form a puparium or a sac that will surround itself.  During the pupa stage, the larvae will be in a non-feeding, usually immobile, transformation stage that lasts 1-3 weeks (Herbst, 1986).  Once the adult alkali-fly reaches the surface and its wings dry it will spend the remainder of its life feeding and breeding on the lakeshore.  The life span of an adult alkali-fly is about two weeks, however, some will managed to live over the winter months because the cold temperatures extend their life span (MBNFSA, 2001). 

Life cycle of the alkali fly.  Source:  Mono Basin Virtual Visitor Center.


Breeding and Development:    

    Breeding occurs daily and a female will likely lay 10 eggs over a two-week period.  Mating between the alkali’s are random with no mating displays (Herbst, 1992).  Eggs are laid by the female literally walking down emerged substrate and attaching the eggs to an algae mat.  Tiny hairs on the adult form an air pocket around the fly allowing it to descend under water, up to 15 minutes, and attach the eggs on a benthic filamentous green algae (Ctenocladus circinnatus) (Foley, 1989) or in tufa crevices. The eggs, which can remain at that depth because of their weight, are oval-shaped and are 0.6-1 mm in length.  When hatched the larvae grow in three stages or “instars”, labeled first, second, and third instars (Herbst, 1986).

The head.  Source: Cash, 1994 The anus.  The perianal pad (pp).  Source: Cash, 1994.

    Although the stages are basic, some of the larvae anatomy is fascinating.  The “prolegs”, “respitory siphon”, and “lime gland” are all adaptations to the high levels of salinity and alkalinity at Mono Lake.  The prolegs are crucial to the survival of larvae and act like claws.  The prolegs allow the larvae to cling to the undersides of mostly hard substrate to prevent strong currents from dislodging them (Cash, 1994).  Once dislodged the larvae might float to the surface or settle in silt or mud subjecting them to desiccation or predation (Herbst, 1990a).  The respiratory siphon appears on the eighth or caudal segment extends into two retractable respiratory tubes. The respiratory tubes can be extended or retracted and is thought to be used to breath at surface water (Cash, 1994 et al.).  A white stripe that appears through the transparent body of the larvae is called the lime gland. The lime gland acts like a kidney, by removing carbonate ions from the blood.  Inside the glands carbonate is mixed with calcium to form a limestone.  This is a physiological adaptation to the high concentration of carbonates in the lake water.  The limestone is stored probably because the fly has a need for calcium in the diet and Mono Lake has a very low level of calcium (MBEIR, 1993).  Tufa formations are contributed to when the larvae reach the pupa stage and excrete the limestone onto the hard substrate (MBNFSA, 2001). 

    When in the pupa stage the larvae will seek a safe place to anchor, usually on tufa or algae mats, which provide a safe place for metamorphosis.  Survival of the pupa depends on the depth at which it is stationed.  Un-emerged and fully developed adults found at a depth of 5 meters have been found in a decomposed condition, suggesting that deep-water pupae fail to survive (Herbst, 1993).  In water less than 3 feet deep, larvae and pupae are most abundant and are rarely discovered below the thermocline (Herbst, 1990a).  Over the next 1-3 weeks, depending on water temperature, their skin hardens and forms the puparium or casing. The hard brown case holds the fly in a non-feeding, immobilized stage that enables the pupa to mature into an adult fly, much like that of a caterpillar into a butterfly (MBNFSA, 2001).  At the end of the pupa stage the adult alkali-fly will exit the puparium and was thought to float immediately to the surface encased in an air bubble. However, a diving expedition in 1989 revealed some individual flies walking about the bottom for a period of 15 minutes (Foley, et al. 1989). 


    The feeding niche of the Mono Lake alkali-fly is described as that of a scraper-gatherer; herbivore-detritivore (Herbst, 1986).  Eggs that hatch feeding on algae mats grow so fast that they shed their skin twice before becoming full-grown larvae (MBNFSA, 2001).  Consumers of benthic algae and detritus, they in turn become food for many birds in the area (Herbst, 1990).  The main food sources are benthic algae Nitzshia frustrulum, green algae Ctenocladus circinnatus, blue-green algae Oscillatoria, and some protozoa (Herbst, 1986).  Because nutrient supply is greater in the shallow waters algae growth is limited by photic zone (Biogeography-316, 2001).  The volume of algae growth is directly related to the size and population of the Mono Lake alkali-fly.

     The alkali-fly is one of four species in the genus Cirrula and each species can be found in the United States, Canada, and Mexico.  In the United States, Ephydra hians is mainly located in California, Nevada, and Utah.  California’s Mono Basin, specifically Mono Lake, is the largest habitat for Ephydra hians (Mathis, et al. 1995).  The population density of the Mono Lake alkali-fly is among the highest of any saline aquatic ecosystem known (Herbst, 1988).  Other locations of Ephydra hians occur in Minnesota, North Dakota, and Washington. 

Map of Distribution:

Fig.4: Distribution of Ephydra cinerea (filled circles), E. currani (open squares), and  E. hians (filled squares).  Source Wirth, 1971

     The Mono Lake alkali-fly is believed to be evolutionary adapted to acidic and alkaline water sources.  The respiratory siphon during the larvae stage was probably used for breathing oxygen at the waters surface.  Today, the larvae are able to dissolve the oxygen that is in the lake while submerged.


    The Mono Lake alkali-fly is in no immediate danger from other species and is the most abundant invertebrate around Mono Lake.  The high level of salinity and alkalinity cannot support other insects or competitors to the alkali-fly.  In addition, the high levels also reduce the parasitism and diseases (MBEIR, 1993).  At other lakes when salinity is low, beetles, damselfly larvae, and dolichopodid larvae prey on the flies. 

      The primary predators of the Mono Lake alkali-fly are birds.  The third instar and pupa stage contain the highest level of calories in the flies.  Also, it is during these stages when many are dislodged by currents and sent to the surface, where they are easily accessible and helpless (MBEIR, 1993).  Many are dislodged that it is estimated a metric ton of larvae and pupae can be found at the surface during the summer on Mono Lake (Herbst, 1992).  The California gull that nests on the islands of Mono Lake feed on this rich source of food.  The fly is used in about fifteen percent of the gulls diet, and varies with lake level (Herbst, 1993).  Pupae are also subject to the parasitoid Urolepis rufipes when dislodged and sent ashore (Herbst, 1990a). 

    Human interaction with the lake level has a large effect on the Mono Lake alkali-fly.  Streams of the Sierra Nevada have been diverted by the Los Angeles Department of Water and Power over the last 50 years.  While streams provide water and power to Los Angeles, the amount of freshwater left to contribute to Mono Lake has dropped.  The decrease in freshwater has led to an increase in salinity to the lake.  The impact of higher saline concentration has a devastating effect on algae growth, which is the necessary food for larvae, and therefore directly influences the growth of Mono Lake alkali-flies.  During the years in which the volume of the lake was reduced by 50%, the salinity doubled.  In addition, reduction in lake level prompts areas of hard substrate, such as tufa, to become exposed.  Such areas are critical to the habitat and growth cycles of the larvae and pupae. When hard substrate is exposed habitat is lost.  Loss of habitat and energy spent removing excess salt from their bodies the Mono Lake alkali-fly population fell by over 50% (MBNFSA, 2001).  

Fig.5: Predicted population of Mono Lake alkali fly at different lake levels.  Source: Herbst, 1993.

Other Interesting Issues:
 How did the Mono Lake alkali-fly get its name?

      Ephydra hydropyrus hians is the full name and was discovered by a scientist named Say in 1830.  Say discovered the fly in an area called Mono Basin in California.  The area just east of Yosemite was home to Kuzedika or Mono Lake Paiutes.  The Paiutes called the pupae “kutsavi,” and during the summer would harvest it and use is as a main source of food.  Trade between other tribes in the area become popular and neighboring Yokuts called the Paiutes “Monoche” and their food “mono”.  It is believed that during a U.S. Cavalry chase of Indians to the east side of the Sierra Nevada they came upon Mono Basin area.  They guides being Yokuts, taught them the word mono for the area (MBNFSA, 2001).

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 Biogeography 316. 2001. Lecture on aquatic biomes.  SFSU Geography Department.

Cash, Clark; Bradley, T.J. 1994.  External morphology of the alkali fly (Ephydra hians) Say at
            Mono Lake, California (USA) in relation to physical habitat.  Journal of Morphology 219(3), March: 309-318.

Foley, C.; White, B.  1989.  Occurrence of Ephydra hians Say (Diptera: Ephydridae) in deep water in Mono Lake, California.  Bulletin
            Southern California Academy of Sciences 88(1): 40-41.

Herbst, D. 1986.  Comparative studies of the population ecology and life history patterns of an  alkaline salt lake insect: Ephydra hians Say
            Ph.D. thesis, Oregon State University, Corvallis. 206pp.

Herbst, D.  1988.  Comparative population ecology of Ephydra hians at Mono Lake and Albert Lake.  Hydrobiologia 158: 145-166.

Herbst, D.  1990a.  Distribution and abundance of the alkali fly at Mono Lake, California in relation to physical habitat”.  Hydrobiologia 197

Herbst, D. 1990b.  Coordination proceeding – special title (Rule 1550b), lake level.  Superior   Court of the State of California for the County of
            El Dorado.  Volume 21.  South Lake Tahoe, CA.

Herbst, D.  1992.  Mono Lake benthic ecosystem research:  aquatic productivity component of the environmental impact report.  Report for the
            State Water Resources Control Board.

Herbst, D.  1993.  A population model for the alkali fly at Mono Lake:  depth distribution and changing habitat.  Hydrobiologia 267, September:

Herbst, D.  1999.  Biogeography and physiological adaptations of the brine fly genus Ephydra (Diptera: Ephydridae) in saline waters of the Great
            Basin.  Great Basin Naturalist 59(2) April: 127-135.

Little, P. et al.  1989.  Brine fly spatial distribution in Mono Lake.  Unpublished contract report to Los Angeles Department of Water and Power.

Mathis, W.; Zatwarnicki, T.  1995.  World catalog of shore flies (Diptera: Ephydridae).  Memoirs on Entomology International 4: 237.

Mono Basin Environmental Impact Report (MBEIR) 1993.  (October 5th, 2001).  Environmental Impact Report . Available:

Mono Basin National Forest Scenic Area (MBNFSA).  (November 5th, 2001).  Available:

Stine, S. 1992.  Distribution of “substrate types” at Mono Lake.  Report to Jones and   Stokes Associates.

Wirth, W.  1971.  The brine flies of the genus Ephydra in North America.  Annals of Entomological Society of America 64: 357-377.

*Special thanks to Barbara Holzman (save the mullet people!), Sam  Pedone (finally, Im done!), and Catherine Huybrechts (nice pumpkin pie) and Elise Watkins (I've never met anyone who misses more classes and still manages an “A”).  Text and graphic help from Karmen and Terry :)

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  date:   Dec. 2001

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