2019 by Amanda Whispell

Physiological Color Change in Argia apicalis

on the theory that the bright coloration is functioning as a sexual signal and that the ability to change color—and darken at times of vulnerability—likely evolved in these species as a complex solution to the opposing demands of signaling (sexual selection pressure) and camouflage (natural selection pressure).  I am currently studying the mating-associated color change exhibited by Argia apicalis  (Odonata: Coenagrionidae) males, and the adaptive advantage it may offer to them.

The overarching focus of my research has been how trade-offs between sexual and natural selection underlie the diversification and evolution of sexual signals and signaling behavior.  I have been researching the how these trade-offs may have led to the evolution of damselfly (Insecta: Odonata) species capable of physiologically changing color and I have been studying this color change premised         

Argia apicalis - male

© Amanda Whispell

Argia apicalis - Immature male

© Amanda Whispell

Argia apicalis - Mature male

© Amanda Whispell

In Odonata (dragonflies and damselflies), the gradual development of adult coloration during their post-eclosion, teneral period is a well-known phenomenon, and these morphological color changes occur slowly and irreversibly.  Physiological color change, in contrast, is always rapid and reversible, and has only been studied in a few insect species. While the change exhibited by most species is primarily temperature sensitive – changing to dark-phase (DP) coloration when below a certain temperature threshold, and then returning to their bright-phase (BP) when the temperature rises above it – this is not always the case. 

A. apicalis - Male after 20 minutes in a cooler

© Amanda Whispell

A. apicalis - Male kept at an ambient temperature of  27°C 

© Amanda Whispell

A. apicalis males possess the unique ability to change color, from BP to DP, in response to copulation, and given the past research into the selective advantage of physiological color change, the ability to change color in direct response to copulation could provide them with a significant adaptive advantage.

Since many Odonata males are brightly colored, they may be trading reduced survival for increased mating success. Rather than make this compromise, the ability to change color allows A. apicalis males to retain their BP coloration for inter-/intraspecific interactions, and adopt DP coloration to protect themselves from predation while copulating and ovipositing. 

A. apicalis - Pair captured in tandem

© Amanda Whispell

A. apicalis - Solitary in BP

© Amanda Whispell

A. apicalis - Mating pair in DP

© Amanda Whispell

In order to study the color change I found a site on the edge of a stream in the Van Nest Wildlife Refuge in Mercer County, New Jersey.  I set up an observation site near the part of the stream that had the highest density of damselflies (image on the right).

I did field observations at this site for one year (2012) and tagged over 500 males and 300 females. I photographed all color changes, mating-associated behaviors, and I also photographed them when solitary in order to have photographs of BP males against which I could compare the photographs of DP males.  I also recorded the ambient temperature at the time that each photograph was taken.

Unfortunately, since the damselflies were free to fly wherever they wanted I didn't get the amount of data that I wanted. So, for the observations during 2013 and 2014 I constructed a large in situ insectary on the side of a small lake at Duke Farms so I would have more control over how far my research subjects could go.  

One day each week I would go out and collect wild damselflies--both male and female--from sites along the Raritan River in New Jersey and bring them back to the insectary to tag and release them.  I would then spent the following six days (or how ever many days until there were no females remaining) sitting inside the insectary observing their behavior and photographing all damselflies at each chance I could.