Ischnura gemina

Geographic Range

This nonmigratory species is restricted to the San Francisco Bay area in California.

• Biogeographic Regions
• nearctic nearctic
  • native native

Habitat

Ischnura gemina are active on sunny, warm days near any water body, such as a lake, river, pond, hotsprings (up to 120 degrees F), cold glacial streams, swift rapids, or very salty lakes. They are very rare in polluted water. The eggs and larvae are aquatic and nymphs are found on submerged vegetation and on the bottom of these bodies of water. Climber nymphs are usually found on debris in the water and move about at a slow pace in dense vegetation.

• Habitat Regions
• freshwater freshwater

• Aquatic Biomes
• lakes and ponds
• rivers and streams
• coastal coastal
• brackish water brackish water

Physical Description

Ischnura gemina are known for their large compound eyes each separated by more than their own body width, muscular mandibles, very slender, elongate ten-segmented abdomens, short legs used mostly for perching, and tiny filiform antennae. Sight is their most specialized sense. Unlike the dragonfly, their two pairs of large wings stand straight up over their 7-8 cm long bodies. The female adult has a black humeral stripe, lacking the pale spot located at the base of the median ocellus that the males exhibit. Females also have specialized ovipositors whose primary function is to cut holes in the stems of submerged plants in order to receive eggs. Adults are called "darning needles" or "mosquito hawks" due to their size, which decreases with advancing season.

Each adult female usually has 11 to 14 nymphs, whose life cycles take up to one year. The eggs and larvae have caudal and rectal gills. Ischnura gemina nymphs are identified by their three leaflike tracheal gills at the tip of their abdomen and their usually robust green or grey color. Nymphs are usually 10-15 mm long, not including their gills. The bodies of the nymphs bear small spines and are often covered with algae and debris. They have five labial setae or food getting devices armed with stout teeth.

• Other Physical Features
• ectothermic ectothermic
• heterothermic heterothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• sexes colored or patterned differently

Development

Odonate eggs hatch in 12-30 days, depending on temperature and moisture. Metamorphosis is incomplete and odonate nymphs usually go through 10-15 instars.

Reproduction

Flight time occurs from May to November. Mating is a very specialized and time-dependent process and occurs at rest or during flight. Females mate only once per clutch laid. Each adult female usually has 11 to 14 nymphs. The male is nonterritorial, the oviposition site rendezvous is widely dispersed and there can be multiple matings per day. In studies, females mated a second time with the same male in only 7.6% of cases and a third time only 0.9% of the time. Three percent of males and 5% of females had the same distributions for mating frequency. The sex ratio in studies was 50%. Mating and reproduction were directly related to the number of sunny days in which the male was sexually active.

When mating, the male grips the female around her neck with two of his pairs of claspers at the end of his abdomen. The female then arches her abdomen forward and under to come into contact with the sperm. Sperm is stored in the male's accessory genitalia on the underside of his thorax. Ischnura gemina oviposits in tandem, which is unusual in that the male possesses a "scoop" on his penis. This scoop's sole purpose is probably to displace the sperm. Post-copulatory guarding prevents sperm from being displaced by a rival mate. When the time has come to deposit the eggs, the female inserts them in plant tissue above or below water, sometimes in masses of green algae. The pre-reproductive period is 5-7 days.

• Key Reproductive Features
• seasonal breeding seasonal breeding
• sexual sexual
• fertilization fertilization
  • internal internal
• oviparous oviparous

Lifespan/Longevity

The cycle from egg to adult takes about one year. Females usually survive longer than males.

Behavior

From early morning to evening, I. gemina can be seen flying back and forth along shores of marshes, lakes, ponds, or rivers at speeds of 25-35 km/hr. Most emerge in early morning or late afternoon and consistently light up when they fly in the dark. A complete flight period lasts only a few weeks. The burrowers of the species lie covered in sand or silt bottoms by raking out the material so that they slowly sink downwards.

• Key Behaviors
• flies
• diurnal diurnal
• motile motile

Communication and Perception

Odonates have highly developed sight. The large compound eyes are used to capture prey.

• Communication Channels
• chemical chemical

• Perception Channels
• visual visual
• infrared/heat infrared/heat
• tactile tactile
• chemical chemical

Food Habits

Ischnura gemina are carnivorous insects that prey on mosquitoes , annelids , small crustaceans , and mollusks . They usually catch their food on their wing after stalking the prey by remaining motionless until the food comes within reach and is seized by rapid extension and contraction of the labium. The labium then holds the food in their mouth where it is crushed easily by the strong mandibles.

• Primary Diet
• carnivore carnivore
  • insectivore insectivore
  • eats non-insect arthropods
  • molluscivore molluscivore
  • vermivore

• Animal Foods
• insects
• mollusks
• aquatic or marine worms
• aquatic crustaceans

Predation

Predators of I. gemina include falcons, herons, gulls, sandpipers, grackles, red-winged blackbirds, bats, spiders, and wasps. Weather is responsible for considerable mortality. For example, cloudy, rainy days and windstorms drive damselflies out to sea or into large lakes. Females usually survive longer than males.

Ecosystem Roles

Adults and nymphs of Odonata eat many pest insect species, such as mosquitoes. The nymphs play an important role as food for fish.

Economic Importance for Humans: Positive

The San Francisco fork-tailed damselfly has no positive economic benefits to humans.

Economic Importance for Humans: Negative

This species also has no negative economic benefits to humans.

Conservation Status

This damselfly is not listed as endangered or threatened, but is considered rare in its range.

Other Comments

Respiration takes place through the thoracic spiracles with the caudal lamellae or gills acting as supplementary respiratory structures, unlike the rectum which is the primary respiratory structure in most Odonata .

Additional Links

Encyclopedia of Life

Contributors

Jana Marshall (author), Southwestern University, Stephanie Fabritius (editor), Southwestern University.

References

Biggs, K. 2005. "Damselflies (Zygoptera) of the Southwest" (On-line). Accessed March 02, 2005 at http://southwestdragonflies.net/damsels/swzygoptera.html .

Brackenbury, J. 1994. Insects:Life Cycles and the Seasons . New York: Sterling Publishing Co., Inc..

Brusca, R., G. Brusca. 1990. Invertebrates . Sunderland, Massachusetts: Sinaur Associates, Inc..

Corbet, P. 1999. Dragonflies:Behavior and Ecology of Odonata . Ithaca, New York: Cornell University Press.

Farb, P., The Editors of LIFE. 1962. The Insects . New York: Time, Inc..

Merritt, R., K. Cummins. 1984. An Introduction to the Aquatic Insects of North America . Dubuque, Iowa: Kendall/Hunt Publishing Company.

Pennak, R. 1953. Fresh-water Invertebrates of the United States . New York: The Ronald Press Company.