Nemoria arizonaria

The family Geometridae contains over 21,000 species and can be found all across the globe.^[3] The Geometrinae subfamily contains 2350 species. Many of these species can be identified by the similar emerald green coloration observed in N. arizonaria, which led to the creation of the common name emerald moth. Comparative analysis of Nemoria’s biogeographic history suggests that the genus originated in South America.^[3] Such a close proximity to the United States’ southwest region can explain the present day localization to this area. It has been suggested that Nemoria was introduced many times - the genus is the largest of the New World Geometrinae and is estimated to be around 7.5 million years old. Nemoria is most commonly known for the phenotypic plasticity of its larvae stages in many species. There are now 15 recognized species of Nemoria.^[3]

Nemoria arizonaria is typically restricted to canyon habitats of elevations around 4,000 to 8,000 feet. Within the United States, N. arizonaria seems to originate from Arizona, New Mexico, Mexico, Southern California and the Davis Mountains of Texas, although the summer (aemularia) form has only been found in Arizona.^[4] The moth can commonly be found resting on windows and screens throughout the Southwest United States.^[5]

The species has two seasonal forms: the summer form and winter/spring form. The summer form can be identified by its white costa. Until recently, this form was thought to have been a separate species from N.arizonaria and was given the name Nemoria aemularia.^[4] In 1988, Noel McFarland discovered that N. aemularia adults could be reared from N. arizonaria eggs – proving that N.arizonaria and N. aemularia were actually the same moth, but due to seasonal dimorphism could result in substantially different phenotypic forms^[4][6]

Seasonal dimorphism is one type of polyphenism observed in the species, the other of which occurs at the larvae stage.^[4]

Larvae born in the spring feed on oak catkin (flower) and resemble catkins in appearance,^[7] while those that feed on oak leaves express a different phenotype, specifically one that resembles a twig.^[8] In fact, diet alone regulates the expressed phenotype.^[5][8] The larvae enter the pupa stage after a few weeks and soon develop into adults. The adult dies shortly after mating and laying egg.^[8]

As discussed above, larvae feed either on catkins or oak leaves and twigs depending upon the season.^[4][9] Adults, on the other hand, feed on nectar. Studies on Lepidoptera have found that feeding behavior is in fact triggered by sugar-receptor communication with chemosensilla, and that both starch and sucrose compete for taste receptor sites along the sensilla. When starch and sucrose were artificially added to bind to sensilla receptor sites, Lepodoptera stopped food-sucking behavior all together. Even though they were still hungry, their artificially occupied receptor sites signaled otherwise.^[10]

N. arizonaria is emerald colored, possessing a wide post-medial line - one of the broadest amongst all North American Nemoria species.^[4][5] The species also has a thin, yellow colored terminal line on the wing, with a slight red color between veins.^[4] A white fringe outlines its one- inch wingspan.^[5] A distinguishing feature between the two seasonal forms is the presence of purple-red markings on the costa of the forewing in the summer form. In this form, the abdomen contains reddish brown markings on the first few segments on the males, while female abdomens have pale red markings.^[4] Male and certain females also possess small white spots on the abdomen.^[4]

The timing of birth affects the phenotype of these caterpillars: N. arizonaria born in the spring feed on oak catkins and thus develop a cuticle that resembles catkin flower. Those that are born in the summer must eat oak leaves since catkins are no longer abundant at this time of year. These larvae develop to mimic oak twigs instead, acquiring a smooth grey-green appearance.^[9] Because catkins contain more nutrition than leaves and twigs, larvae that feed on catkins are larger before they pupate.^[5][9] Those that feed on catkins appear golden with many small projections, a fuzzy coating, and brown dots along its back that mimic catkin stamens.^[9][11] Since these larvae feed on pollen, their heads and mandibles are smaller than those that feed on leaves and twigs, possibly because large mandibles are not necessary for catkin consumption3. Subsequent rearing experiments have shown that only larval diet influence the developmental trigger.^[8][9]

Although genotypically similar, these eggs, upon hatching, begin feeding on oak leaves rather than the out of season catkins and develop jaws to accommodate feeding.^[8][9] It has been found that behavioral changes of larval mandibles occur in both H. buttivitta and H. subrotata depending upon usage, however research has not yet proven that this occurs in N. arizonaria.^[12]