American Rubyspot (Hetaerina americana)

American rubyspot
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Clade:	Pancrustacea
Class:	Insecta
Order:	Odonata
Suborder:	Zygoptera
Family:	Calopterygidae
Genus:	Hetaerina
Species:	H. americana
Binomial name
Hetaerina americana (Fabricius, 1798) [1]
Range of H. americana [2]

The American rubyspot (Hetaerina americana) is a damselfly of the family Calopterygidae. Males have a lustrous red head and thorax and a large red spot at the base of each wing.^[3] The abdomen of both genders is brilliant green.^[4] The female may have either green or copper colored marks on the thorax and possess faint amber wing spots with patterned bodies that vary from brown to green.^[3][5] The species has served as a widely used model system in studies of sexual selection, territorial behavior, and population genetics.^[6]

• 
  Female copper form
• 
  Female green form

The name highlights this damselfly's status as the most widespread of the North American rubyspots.^[7]

Hetaerina americana was first formally described by Fabricius in 1798. It is one of 37 sexually dichromatic species in the genus Hetaerina.^[3] Most rubyspots are concentrated in the neotropical region, with the greatest number of species found in South America.^[8] Members of the genus Hetaerina are difficult to identify because they closely resemble each other in body size, coloration, and habitat use. The most reliable way to distinguish species is through the shape of male caudal appendages, the structures at the tip of the abdomen used to grasp females during mating.^[8] H.americana itself shows considerable variation in the shape of these appendages, as well as in other traits such as the presence or the absence of the pterostigma and the relative extent of the red wing spot. It is because of this variability, that several names were applied to what are now considered the same species, including H.pseudoamericana, H.texana, and H.californica.^[6]

H.americana occupies a broad geographic range from northern Central America to southeastern Canada, inhabiting a variety of environments from temperate woodland to tropical forest.^[3][6] It has been recorded from across the United States except Washington and Idaho, and is frequently the abundant odonate species in areas where it occurs alongside other rubyspot species.^[8] Both males and females remain closely tied to running water throughout their adult lives, rarely moving more than a few meters from suitable stream habitat, and all mating activities take place at said streams.^[3]

H.americana operate under a lek mating system, where groups of males gather along streams and compete for small perching territories where females arrive to mate and lay eggs.^[9] Females do not receive any resources or parental care from their mates and typically deposit their eggs outside of their mate's territory.^[9]

Territorial males defend their territories only during the portion of the day when females are reproductively active.^[9] Individual territory sizes range from approximately 1 to 4 square meters, and resident males return to defend the same location each day until they are displaced or die.^[9] Resident males are stationary unless mating or fighting, typically choosing spots with direct sunlight and nearby rippling water.^[9] Non-territorial males tend to occupy areas with slower-moving water or spots with less sunlight.^[9] When a mature male enters the territory of a territorial male, the resident will immediately try to chase them out, whereas non-territorial males rarely start chases and instead to choose to retreat when confronted.^[9]

A complete mating sequence in H.americana involves eight sequential stages. First the male clasps the female from behind using his abdominal appendages, then the pair enters a precopulatory tandem flight. The male initiates copulation by fluttering his wings and pulling the female forward; copulation occurs, during which the male removes the females stored sperm from previous matings before depositing his own. Males remove 80-100% of the sperm stored by the female through this process.^[9] The pair makes the postcopulatory tandem flight, and the female is then released, submerging herself in water to look for a place to place her eggs. The female probes submerged vegetation with her ovipositor, and upon finding a preferred spot, she oviposits.^[9]

• 
  Male above, female below
• Mating wheel; sperm removal

Observational selection gradient analyses and controlled wing spot manipulation experiments confirm that the male wing spot is under direct, positive sexual selection through male competition for territories, rather than female preference.^[3][9] Males with larger natural wing spots hold territories for a greater portion of their reproductive lives compared to males with smaller spots.^[9]Higher overall mating rates are observed with large-spotted males because of their greater territory control, but wing spot size has no detectable effect on how often a male mates per day, ruling out direct female choice as the mechanism.^[9]

Further studies then conclude that the agonistic handicap model is consistent with all the data from H.americana.^[9] Under this model, a trait that has no direct effect on fighting ability can still evolve as an honest signal of fighting ability. In H.americana, experimentally enlarged wing spots caused higher male mortality, confirming that there is a real cost, while males with naturally large spots lived longer despite this cost.^[3][9] This makes the wing spot an honest signal that only males of genuinely high quality can sustain without a net loss in fitness.^[3][9]