Today’s contribution includes collages of moth photos by reader Aaron Hunt; the theme is “polymorphism”, or variability in appearance among individuals within a species. The photos were taken on Block Island, off Rhode Island. You’ll have to enlarge the photos by clicking on them (preferably twice in succession with a pause between clicks). Aaron’s narrative is indented. Especially note the last species group, which has wing patterns mimicking salticids (jumping spiders).
The theme of this batch of images is identification. Moth identification presents a variety of challenges, not least of which are the very incomplete state of knowledge of many taxa and the enormous species diversity of the order. The more than 13,000 described species of moths found in North America north of Mexico represent less than a tenth of the global described fauna. Perhaps a thousand more Nearctic species, and probably several tens of thousands globally, remain undescribed, In principle, most moths can be identified to species from wing pattern alone, but reliable sight recognition of unfamiliar species at higher taxonomic levels takes an enormous amount of experience. At the species group level, intraspecific variability in markings often visually overwhelms the modest consistent differences between species. Identification of higher taxa is easiest using a combination of structural characters of the head, labial palpi, and antennae in combination with the subtle, basic wing pattern elements that are most resistant to change over evolutionary time scales.
Most of the images shown here are collages showing numerous individual moths to illustrate variation and differences in pattern and color. Dimensions of individual photographs in each of the four large collages range from 500×500 to 900×900 pixels, so each collage is much too large to show up here at full resolution. Each has also been compressed into a smaller jpeg to reduce file sizes. At the end of the text corresponding to each collage [JAC: Pictures are beleow the text] is a link to the image (hosted in my Google drive) at full resolution in its original png format. There, you’ll be able to view the individual moths included in the collages in much greater detail than can be shown on this page.
Macrochilo orciferalis (Noctuoidea: Erebidae: Herminiinae) — This specialist of dune habitats is bivoltine on Block Island, with adults on wing mainly in June and August. Both sexes are highly variable in forewing maculation, with several characters seeming to vary independently and with no sexual dimorphism in markings apparent. (Males, which have bipectinate antennae, are much commoner at lights than females, which have simple antennae.) One photographer on the coast of New Brunswick across from Prince Edward Island reports seeing mostly dark (like those at bottom right and one below top left in my collage) or striped (like four individuals in my collage) moths of this species, whereas I see mostly lighter specimens on Block Island. One wonders what mix of environmental and genetic factors underlie color and pattern variation in this species and what selective forces sustain it. Click here for the collage at full resolution.
Hypsopygia olinalis (Pyralidae: Pyralinae) — This species is quite common on Block Island, where it is univoltine, flying June through early August. (I have seen a handful of fresh specimens in August and September that are either extreme stragglers or overly precocious offspring of the main generation; either way, I imagine any offspring they produce fail to complete development so far out of season. Still, this phenomenon illustrates how species can adapt to changing climate; if Block Island’s climate warms enough in the coming decades, some populations not quite able to complete a second generation each season will become able to do so with a slightly longer growing season.) Adults vary dramatically in coloration from pale greenish yellow to a dark brick red. This variability helps to make the species easy to confuse with a few similar congeners with overlapping ranges in eastern North America. I photographed all the moths in this collage on a single sheet one night last June. Unfortunately, I just now noticed that I missed a duplicate, so I got good photos of only 19 individuals, not 20 — see if you can spot the moth shown twice. Click here for the collage at full resolution.
Phyllocnistis Vitaceae feeders (Gracillariidae: Phyllocnistinae) — At least two or three species form long, narrow leaf mines in plants in the grape family (Vitis and Parthenocissus) in eastern North America. Species boundaries need to be sorted out in this species complex of minute moths. On Block Island, mines in the Virginia creeper around my yard yield at least two generations of moths that regularly come to lights. Adults in the summer generation (large photo) are shining white with faint black and yellow markings in the distal half of the forewing; those in the fall generation (small photos at right) are somewhat to much more strongly and extensively marked. This species complex offers a good example of seasonal dimorphism as well as confounding external similarity of same closely related species.
Acleris (Tortricidae: Tortricinae: Tortricini) — A number of species in this large, mostly Holarctic genus are highly polymorphic. The group has been popular with collectors since the dawn of modern taxonomy, with European lepidopterists naming up to dozens of color forms for some species from the early 19th century to the early 20th century. Very similar color forms often occur in two or more polymorphic species; genitalic dissections finally established species boundaries in the early- to mid-20th century. Block Island is mercifully short on polymorphic Acleris, with only two similar species present. The island’s population of flavivittana exhibits four highly discrete color morphs, two of which closely resemble the two local color morphs of robinsoniana. A non-polymorphic species found on Block Island, inana, very closely resembles one of the local morphs of flavivittana and somewhat less closely the corresponding robinsoniana morph, which happens to be the predominant of the two. At least inana is univoltine, though its flight window overlaps with those of second generation flavivittana and robinsoniana. Another species found on Block Island, maculidorsana, resembles inana. All four of the aforementioned species are pictured in the collage below, and in a sensible arrangement, but I’ve left identifying the specimens as a challenge to the reader. For the answers and the full resolution version of the collage, click here.
Euxoa detersa (Noctuoidea: Noctuidae) — With 182 species currently recognized in North America north of Mexico, Euxoa is one of the most speciose genera on the continent. Euxoa is primarily a genus of arid and semiarid habitats of the northern hemisphere and is nearly absent from tropical and humid subtropical habitats. Nearly all New World Euxoa are found in the western US and Canada, where some species are among the most abundant medium-large moths. About 30 occur in eastern Canada, but most of them are species of boreal forests found across the continent. Only five species are found in eastern deciduous forests, and only three aridland species occur on the Atlantic Seaboard; the genus is entirely absent from most of the Southeast US. Despite being very well studied, Euxoa presents a daunting identification challenge in western North America. Many species are highly variable in pattern and color, and some closely related species covary in maculation across habitat gradients. Characters of wing maculation recur across and vary greatly within phylogenetic species groups, making subgeneric taxonomy of little use in narrowing down identification possibilities using only wing markings. Just a few Euxoa species occur on Block Island, and telling them apart is straightforward. However, the island’s population of Euxoa detersa, which flies in abundance in dune habitats throughout September, provides a striking example of the remarkable variability so many Euxoa species exhibit. The image shown here depicts 49 individuals of this species from Block Island and is cropped for size from a larger collage of 121 individuals. Click here for the uncropped version at full resolution.
Tebenna sp. (Choreutidae), Eoparargyractis plevie (Crambidae: Acentropinae), Chalcoela iphitalis (Crambidae: Glaphyriinae), and Eucosma annetteana (Tortricidae: Olethreutinae: Eucosmini) — These species are all mimics of jumping spiders (Salticidae). Jumping spiders are highly visual ambush hunters and among the top predation threats to small moths. Salticid mimicry has evolved a few dozen times in more than a dozen moth families globally, producing very strong convergence in wing maculation in many completely unrelated groups of moths. It has evolved in other insect orders as well, including numerous times in planthoppers (Hemiptera: Fulgoroidea). Salticid-mimicking moth lineages are easily mistaken for each other and often are very different from their closest relatives in superficial appearance. Salticid mimicry in Lepidoptera is little-studied, but the few species studied have been found to produce aggressive displays in jumping spiders, resulting in markedly lowered rates of predation success.