San Francisco forktail (Ischnura gemina)

Synonyms: Celaenura gemina
KingdomAnimalia
PhylumArthropoda
ClassInsecta
OrderOdonata
FamilyCoenagrionidae
GenusIschnura (1)
SizeLength: 24 - 28 mm (2)
Wingspan: 28 mm (2)

The San Francisco forktail is classified as Vulnerable (VU) on the IUCN Red List (1).

One of the rarest damselflies in the United States, the San Francisco forktail (Ischnura gemina) belongs to the order Odonata, a group of insects which includes dragonflies and damselflies. Although the current global population of the San Francisco forktail is unknown, the rarity of this species places it at high risk of extinction in the wild (1).

The Latin name for this species is gemina meaning ‘twin’. This is because the San Francisco forktail is incredibly similar in appearance to the black-fronted forktail (Ischnura denticollis) (2). The male and female San Francisco forktail have a distinct colour pattern, with the male being black with striking bright blue streaks on either side of the thorax (3).

The female San Francisco forktail occurs in two different colour forms. The most commonly observed female form is rather cryptically coloured, generally appearing greenish to dullish brown when mature. However, some females display a similar colour pattern to that of the male, although this type of female is especially difficult to spot, and is believed to be rare among the San Francisco forktail population (3).

The San Francisco forktail has an incredibly limited range, as it is endemic to the San Francisco Bay area of California in the United States. The total range of this species is estimated to be less than 500 square miles (1).

The San Francisco forktail requires permanent freshwater marshes or other open aquatic habitats for mating and reproduction (4). This species has been known to inhabit temporary urban pools found at construction sites, and has also been sighted at the base of steep hills where freshwater has seeped down and accumulated (1).

The San Francisco Forktail is an active predator, and will feed on invertebrates and any other animal that it can capture. The larval San Francisco forktail is aquatic and will lie in wait until an invertebrate comes near, sometimes staying motionless for hours. It will then lash out, catching the prey from the water. The adult San Francisco forktail feeds on small invertebrates, and will also take spiders from their webs (5).

The San Francisco forktail displays unique behavioural traits that are not seen in other members of the genus Ischnura. The most notable difference is that the San Francisco forktail will often be observed sitting on horizontal surfaces, while similar species prefer resting on vertical ones, such as blades of grass. This is thought to be an adaptation to the warmer climate (6).

Another distinctive behaviour seen in the San Francisco forktail is its ability to fly from early March to mid November, with other species only flying for shorter time periods of about one month. The extended flight of the San Francisco forktail is believed to have evolved in response to the foggy weather conditions that are often observed in San Francisco (6).

The male and female San Francisco forktail possess quite different behavioural traits. For example, the male is often located in sunlit areas, on low vegetation near open water, while the female San Francisco forktail is frequently found foraging in grasses, or resting in shrubs. The female typically only moves to open water in order to mate and lay eggs (3). The juvenile is commonly seen to inhabit short dry grasses close to water until they mature (6)

The maturation times for the male and female San Francisco Forktail are slightly different, with the male maturing earlier. The average time for a female to mature has been recorded at around seven to ten days, whereas the male matures between five and seven days of age. The male also has a longer lifespan than the female San Francisco forktail (6).

Once sexually mature, the male may grasp the female and initiate mating. The male will guard the female until the eggs have been laid to prevent a rival male mating again with the female (3) (6). The female San Francisco forktail will lay its eggs in plant stems, and is capable of producing multiple broods during the breeding period. This species over winters as larvae (5).

The most common threat to the San Francisco forktail appears to be the development of residential and commercial sites. The removal of shallow ponds due to urban construction instantly results in habitat loss, which rapidly increases the chances of local extinction (1).

Development in Glen Canyon Park has resulted in extensive habitat destruction, including a reduction of the watersheds by 80 percent. Unfortunately, this has resulted in the localised extinction of the San Francisco forktail from Glen Canyon Park, highlighting the strong negative impact that urbanisation may have on this species (4)

Another potential threat to the existence of the San Francisco forktail is hybridisation with other damselfly species. In areas where other species of Ischnura share a similar habitat, the San Francisco forktail has been observed mating with Ischnura denticollis, resulting in the production of a mixed hybrid species. This will result in decreased population numbers of the San Francisco forktail,therefore increasing its probability of local extinction (4) (5).

Restoration projects in areas such as Glen Canyon have included the attempted reintroduction of the San Francisco forktail. Unfortunately, reintroduction has so far been unsuccessful, as maintaining open water is difficult in this diverse habitat. In an effort to maintain suitable areas of open water, the San Francisco Recreation and Park Department have started to remove plant growth from around the drainage channel in Glen Canyon every two to three years. This successful method of maintaining open water has shown promising survival rates, hopefully allowing the San Francisco forktail to survive within the park (4).

More information on conservation in California: 

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:
arkive@wildscreen.org.uk

  1. IUCN Red List (January, 2011)
    http://www.iucnredlist.org/
  2. Biggs, K. (2009) Common Dragonflies of California, Revised 2009 Edition. Azalea Creek Publishing, California.
  3. Hafernik, J.E. and Garrison, R.W. (1986) Mating Success and Survival Rate in a Population of Damselflies: Results at Variance with Theory? The American Naturalist, 128(3): 353-356.
  4. Hannon, E.R. and Hafernik, J.E. (2006) Reintroduction of the rare damselfly Ischnura gemina into an urban California park. Journal of Insect Conservation, 11: 141-149.
  5. U.S. Army Combat Support Training Centre (2006) Camp Parks Master Plan Redevelopment - Biological Assessment. Installation Management Agency, Army Reserves Office. Available at:
    http://eul.army.mil/RPX/campparks/ProjectDocs/BiologicalAssessment.pdf
  6. Garrison, R.W. and Hafernik, J.E. (1981) Population structure of the rare damselfly Ischnura gemina (Kennedy) (Odonata: Coenagrionidae).Oecologia, 48(3): 377-384.