with peer reviewing
shed nymphal skins of the emerged periodical cicadas
It may have been Heraclitus who said, No man sets foot in the same stream twice without the stream and the man having changed. Seventeen years after I studied cicadas for publication the first time (see earlier blog post on cicada arrival), I reported on the next generation. Peer reviewers remarked there were too many variables to know what caused change in cicada behaviors over time. But I found that comparing study sites in different places allows more control of variables than comparing the same sites in apparently very different times. So, I focused mainly on the emergence of 2021, comparing locations, and discovered what I had not noticed in 2004. For example, I could discuss two species of periodical cicadas I had not found (or not noticed) before in my neighborhood. . . .
It is probably instructive to consider what the peer reviewers said about my two papers' drafts, what they found wrong and in need of revision in my study of animal behavior (the editor took care of taxonomic nomenclature and bibliographic citations, so the peer reviewers and I did not need to examine those aspects of the papers):
Use the right vocabulary. One reviewer pointed out that for my insects, the transformation to adulthood was not called "metamorphosis," but "ecdysis". Another preferred changing wording throughout from "arrival" to "emergence". (And in my college biology lab, decades before, a teaching assistant marked "WC [word choice]" beside the title of my experiment about honey bees' "disregard" of a pheromone; he suggested, instead, the term "habituation".)
Describe precisely. Perhaps in part from sensitivity to historically obsolete supposition, one reviewer requested rewording about insect hatching: the insects hatch from egg nests, not exactly from branches.
Don't speculate. Unless one can find evidence from observation or text research, don't guess about causes for phenomena, at least not without acknowledging it's speculation.
Order information in appropriate sections. At least one reviewer requested, for clarity or accuracy, I move one passage from Results to Discussion, and another from Discussion to Introduction. (I recently found in the published version a passage I should have moved from Introduction to Results, and a site description I should have moved from Results to Study Sites.)
Choose a fitting statistical test. Some reviewers were sensitive about the choice of statistical test: some tests are better than others for conveying information from the data. (For example, see my first cicada publication for use of two different tests to portray and interpret the data.)
More recently, when I myself was a peer reviewer with a major entomological journal for an article on cicada behavior, I prioritized clarity: in methodology, data presentation, and verbal explanation. It was important to me that readers should not have to work hard to understand tables and charts, might replicate the experiment, and could be persuaded by the candor and reasoning of argumentation. (See my blog post about artificial intelligence, A.I., for more about feedback on writing.)
Published writers often wish they could correct errors or add realizations to their published texts. This website could allow me to make such improvements to my publications -- for example, movement of a paragraph from one section to another and correction of typos under Figures. Below are links to the published article, then my polished version pasted into this blog post.
. . .And news update: The recent popular science tome about animal senses, An Immense World by Ed Yong (2022), includes a chapter, "Rippling Ground: Surface Vibrations" that details an area of research that I recommend about emerging cicadas. (See my Discussion section, sub-section "Emergence Patterns," below.) Cicada nymphs underground may time their emergence by sensing the movement of other cicada nymphs nearby. Yong cites the same influential researcher, P. S. M. Hill, that I do, about animal sensation.
From The Maryland Entomologist, at ResearchGate:
or, more specifically:
Emergence Patterns and Species Distribution of the Brood X 17-Year Periodical
Cicada, Magicicada Davis (Hemiptera: Cicadidae), near Downtown Silver Spring,
Montgomery County, Maryland, 2021
Abstract: The identification of all three Brood X species is reported for the vicinity of downtown Silver Spring, Maryland, year 2021, with approximate timing of their first appearance and sample locations. Data describing the daily and seasonal emergence of Magicicada septendecim (Linnaeus) at different sites is provided, and contrasted with that for M. cassinii (Fisher). An apparent pattern of small-bodied adult cicadas emerging late in the season is also reported. Consideration is given to the role of weather and adaptive strategies in the emergence patterns, and discussion is offered on possible subtle changes to the future emergence and distribution of periodical cicadas in the study area.
INTRODUCTION
This report observes the emergence and distribution of the species of Brood X periodical cicadas, Magicicada Davis (Hemiptera: Cicadidae), around downtown Silver Spring, Maryland, bordering Washington, District of Columbia, in May and June 2021.
Brood X is comprised of three species: Magicicada septendecim (Linnaeus), the largest in size and the dominant Brood X species in the study areas (Dybas and Lloyd 1974, Simon 1996); M. cassinii (Fisher); and M. septendecula (Alexander and Moore). The ratios among these three species vary with the region, nationally. Magicicada septendecim females generally lay eggs in branches of maples, Acer L. spp. (Aceraceae), and oaks, Quercus L. spp. (Fagaceae); M. cassinii lays eggs in a variety of smaller tree species; and M. septendecula possibly lays eggs most often in hickories, Carya Nutt. spp. and walnuts (Juglans L. spp. (both Juglandaceae) (Dybas and Lloyd 1974, Williams and Simon 1995). (The three cicada species are depicted among Figures 9, 10, 14, and 15.)
The nymphs hatch from the egg-nests in summer and drop to the ground where they tunnel and attach to rootlets to feed. The nymphs do not disperse much after hatching (Gilbert and Klass 2006, Smits et al. 2010, Simon 2021)
After the nymphs emerge from the ground 17 years later, they undergo ecdysis to an adult, winged form. Upon separating from the nymphal skin, the eclosion process, they are almost completely white. The adults darken gradually to black, over approximately a day, passing through an intermediate blonde stage, and for about four days their bodies remain relatively soft, the teneral period.
Protandrous arrival, or males arriving before females, which is widespread in periodical cicadas and many other animals (note Morbey and Ydenberg 2001), was manifest in the study sites, in a pattern of emergence generally similar to what this researcher observed here 17 years before (Kriesberg 2020). (In the literature, the term “protandrous arrival” can refer to any life form exhibiting this sex ratio behavior, while “protandrous emergence” is most often applied to ectothermic taxa. In cicadas, the term “emergence” pertains particularly to the above-ground activities or life stage associated with ecdysis. Both terms “emergence” and “arrival” will be used in this report for cicadas, depending on context.) According to the periodical cicada form of emergence, it is advantageous for individual males to emerge earlier because females can generally mate only once, and advantageous for females to emerge after some predator satiation sets in, and once many males have become available. Individuals of both sexes have a better chance of avoiding predation if they emerge at the same time as many others (Karban 1982). Eventually, predators apparently become satiated from eating the cicadas, and predation declines (Karban 1981, Williams and Simon 1995), to possibly resume again in a few weeks (Williams et al. 1993) in the end of the season when periodical cicadas are largely disappearing.
Cicadas cluster in high density in certain places, or are patchy (Simon et al. 1981). Dybas and Lloyd (1974) remarked that, contrary to published concerns from 19th century cicada researchers, the cicadas adapt surprisingly well to the human disturbance of tree-cutting (see also Collinge [2010] on the effects of habitat fragmentation). Kritsky et al. (2005) report from a site observed in Ohio that nymphs failed to emerge from disturbed ground of a new housing development, but adults dispersed into that area.
A decrease in emergent body sizes of periodical cicadas in a population during a season is not discussed as typical, in the literature, perhaps because the phenomenon is so prevalent among insects. After working on this study, this researcher observed several other insect taxa with adult specimens in some cases much smaller late in the season than earlier. The following species exhibited individuals at least half the size in early fall as most individuals were in the spring and summer: Halloween pennant dragonfly Celithemis eponina Drury (Libellulidae); Pearl Crescent butterfly Phyciodes tharos Drury (Nymphalidae); Red-spotted admiral butterfly Limenitis arthemis Drury (Nymphalidae); Silver spotted skipper Epargyreus clarus Cramer (Hesperiidae). The adult forms of some bumble bee species may display a variety of sizes, depending perhaps on variation in nutrition during the larval stage (Fitzgerald, et al. 2022). Late in the season, the goal of this investigator, counting the adult cicadas of different sizes near the study sites, was to check assumptions and ascertain from a sample at a certain time in the season what proportion of the Magicicada septendecim were smaller than the usual, mode length.
Seventeen-year periodical cicadas, appearing regularly, and in great numbers in the eastern and midwestern United States in late spring, even in residential areas, offer an opportunity to study varied aspects of easily accessible wildlife. At a time of well-documented, worldwide biological loss (e.g., Carrington 2017), and environmental change, their super-abundance and brief, regular return may seem—to some observers—uniquely reassuring.
STUDY SITES
This researcher observed two sites daily near downtown Silver Spring, Maryland, in 2004 (Kriesberg 2020), and returned to them, adding a new Site 3 for 2021 (Figures 1, 2, and 3). This researcher also explored parks around downtown Silver Spring (Figure 4) to discover where the three Brood X species might be found.
Figure 1. The three locations where cicadas were counted daily in Silver Spring, Maryland, May and June 2021. The terrain sloped downward from Sites 1 and 2, approximately 102 m (335 ft), to Site 3, approximately 91 m (300 ft). Basemap obtained from MERLIN Online (2022). (For exact boundaries of the three Sites, see Fig. 1 map in published article.)
Figure 2. Portions of Sites 1, 2, and 3 (left to right, respectively). For the Site 3 image of the willow oak, right, the photo was taken from the upper edge of the wooded “alley” (Figure 3).
Figure 3. Wooded alley, part of study Site 3. Many Magicicada cassinii were found here.
Figure 4. Parks in the area of downtown Silver Spring explored for Brood X cicada species. The three Study Sites mapped in Figure 1 were in the vicinity of Bullis Local Park and Nolte Local Park (middle right), with Site 1 close to downtown. Often visited was Jesup Blair Local Park (lower center). Also shown is Sligo Creek (upper right). The terrain slopes downward from a high point of Jesup Blair Local Park (elevation ~110 m [~360 ft]) down to Sligo Creek (~67 m [~220 ft]). Basemap obtained from Google Maps (2021).
The vegetation densely bordering the alley of Site 3 was mostly vines, including many invasives: Chinese yam, Dioscorea oppositifolia L. (Dioscoreaceae); grape, Vitis L. sp. (Vitaceae); Amur honeysuckle, Lonicera maackii (Rupr.) Herder (Caprifoliaceae); English ivy, Hedera helix L. (Araliaceae); American pokeweed Phytolacca americana L. (Phytolaccaceae); Amur peppervine, (Ampelopsis brevipedunculata (Maxim.) Trautv. (Vitaceae); as well as two mature Norway maple trees, Acer platanoides L. (Aceraceae); a large mulberry tree, Morus L. sp. (Moraceae); and a huge black walnut tree, Juglans nigra L. (Juglandaceae).
Though the study area featured many trees, two very different trees were important for the Magicicada septendecim count at two Sites (Figure 1). At Site 2, a recently planted sapling, about 2 m (7 ft) high, American hornbeam, Carpinus caroliniana Walter (Betulaceae), supplied about half of the cicada count. Nymphs crawled from a nearby stump of a recently cut mature tree to gather densely and molt on and around this young tree. At Site 3, the mature, approximately 26 m (85 ft) high willow oak, Quercus phellos L. (Fagaceae) supplied most of the cicadas counted. Nymphs molted in the surrounding grass and ascending the large trunk of this shade tree.
METHODS
To track the daily emergence of the periodical cicadas, recently molted adult cicadas, white or near-white tenerals, were counted by sex each morning on street curbs and sidewalks and nearby foliage and tree trunks in an approximately seven square block area, divided into three study sites (Figures 1 and 2). The count was made from approximately 0800 to 0930 hours daily. Temperature and other weather conditions were noted at the start of each count. The researcher counted at Site 1 first, followed by Site 2, and Site 3 last. Counting only new tenerals was especially important to avoid double counting, if some cicadas did not move much from where they had emerged the day before. Cicada counts for analysis ended when a daily minimum yield of six cicadas was reached with no increase on subsequent days.
Many researchers gather nymphal skins, exuviae, to count emergent cicadas. This study, counting new adult cicadas from both residential and public property, including those emerging in dense vegetation, would have had difficulty reliably gathering all of each morning’s nymphal skins – especially for Sites 1 and 2. With such a methodology, new skins might have been inadvertently missed on the emergence day and included in a subsequent day’s count, and old skins from a previous day might fall to the ground from branches. For Site 3, the counting of new nymphal skins was especially useful when all the Magicicada septendecim tenerals were apparently eaten or otherwise missing before the researcher’s arrival for the morning count—the public lawn setting made it easier to find and collect all of that day’s new skins—and for M. cassinii when the emergence of the adults was also missed. But especially for Site 3 late in the season, all three species may have been emerging, possibly including small M. septendecim; it could be difficult to identify with certainty species by the skins alone (Dybas and Lloyd 1974 provided diagrams of intact skins to show differences among the three species).
On 6 and 10 June, the researcher also counted dead and dying Magicicada septendecim in an approximately two square block area between Sites 1 and 3 (Figures 1 and 11), carefully observing on 10 June not to double count any of those previously noted, tallying those smaller than 3 cm (1.2 in) and 3 cm or larger.
OBSERVATIONS and RESULTS
The adult cicada arrivals in the study area were delayed in 2021 compared to 2004, because of cold weather. This was reported, also, elsewhere in Maryland. On 25 May 2021, the Chesapeake Bay Program noted, “Some areas of the Chesapeake watershed are already seeing many cicadas from Brood X, while others are still waiting.… Temperature is a big factor.”
The one-day delayed beginning for the count of emerging periodical cicada adults in 2021 contrasted with 2004 (Kriesberg 2020) is not what this researcher expected, given, in recent years, the days-earlier arrival of the local annual (“dog-day”) cicadas (Cicadidae spp.), and the weeks-earlier arrival of local fireflies (Coleoptera: Lampyridae) (pers. obs.). Despite this writer’s experience observing these other insects, the 2021 periodical cicadas in the study areas did not arrive early. In fact, the first evidence detected by this observer of any cicadas in downtown Silver Spring—the finding of a cicada wing or an adult, indicating nymphs emerged days before—was on exactly the same calendar day, 10 May, for both 2004 (Kriesberg 2020) and 2021.
In each of the three study sites (Figures 1 and 2), mainly Magicicada septendecim were found. Starting 15 May, Sites 1 and 2 offered about 50 teneral M. septendecim. In Site 3 and to a lesser extent Site 1, M. cassinii were found in increasing numbers, particularly in late May and in June. Magicicada cassinii began and ended its emergence later than M. septendecim.
Site 3 was remarkable for its cicada emergence. On 10 May, five days before the Magicicada septendecim cicadas first appeared for counting at Sites 1 and 2, 75 M. septendecim nymphal skins were found around the willow oak tree of Site 3, but no evidence of any tenerals. Apparently, unless tenerals safely crawled away, almost all the tenerals were eaten, wings and all, by the many avian or other predators nearby, especially from the woods. Five days later, 15 May, with hardly any preamble, there was an explosion of emerging M. septendecim adults found at Site 3, mainly in the grass around the tree. A few M. cassinii, in increasing numbers with passing days, were noted at the bottom of the slope by the wooded, asphalt “alley” (Figures 1 and 3). By 20 May, the M. septendecim at Site 3 were emerging fewer than 20 per day, with 11 by 23 May. And this investigator was prematurely resigned to the Site 3 alley not being productive for finding cicadas.
Then, about a week later after two days of heavy rain, on 31 May, the alley (Figure 3) was full of recently cast M. cassinii nymphal skins—about 60 counted, with a few adults, also, that may have been two days old. This alley of Site 3, that seemed strangely almost devoid of M. septendecim or any other periodical cicada species during the previous week of study, revealed itself at the end of May to be a stronghold for M. cassinii.
The results of the daily counts are shown in Figure 5. Data from Sites 1 and 2 are amalgamated because they were in close proximity to each other and with similar habitats, and with a relatively small data set from each. See Discussion section for more about the emergence patterns.
Compare, also, Figures 5 and 6: Note the extent to which the number of emerging cicadas varies somewhat with temperature.
[For the daily emergence data corresponding to the graph of Figure 5, in table format, find Table 1 in the published paper ("Number of newly emergent adult Magicicada septendecim and M. cassinii found in daily morning counts in designated study sites"):
Figure 5. Magicicada septendecim and M. cassinii daily emergence tallied at the study sites, 10–29 May 2021. Day 1, 10 May, was a count of M. septendecim nymphal skins only. No complete, concluding count was made for M. cassinii. Note that for day 8, Site 3 count for M. septendecim is decreasing while Sites 1 and 2 are increasing. It is also intriguing to note that on day 10, at Site 3, populations of both M. septendecim and M. cassinii, in close proximity, had an uptick of emergence while the relatively distant proximate populations at Sites 1 and 2 together, were beginning a decline in emergence numbers. So, M. septendecim at the two graphed locations display similar seasonal patterns of emergence, with Site 3 starting earlier in the season. Magicicada cassinii did not show this pattern of emergence.
Figure 6. Daily reported temperatures during cicada count at the study area, 15 May–28 May. Compare Figures 6 and 5, temperature with Magicicada septendecim emergence, days 6 through 19. Cooler weather might have contributed to the plunge in Magicicada septendecim numbers Site 3, day 8 of count. But it only slowed the ascending numbers at Sites 1 and 2 on the same day. Then on day 14, warmer weather (22 °C [71 °F]) coincided with the second peak emergence for Sites 1 and 2. On days 13, 15 and 16, cold and/or rainy weather (from 19 °C [66 °F] to 14 °C [58 °F]) perhaps depressed this count, but the count for Sites 1 and 2 was not finished after day 16; with two days’ improved weather on days 17 and 18 (21 °C [70 °F]; 20 °C [68 °F]), there followed an uptick in emergence (Figures 5 and 6).
Figures 7 and 8 show more detail about the Magicicada septendecim emergence pattern at Sites 1 and 2 and at Site 3 by displaying the daily emergence of males and females. The disparity between numbers of males and females on most days is typical of a protandrous emergence pattern (Kriesberg 2020). No clear difference between daily or seasonal numbers of emergent males and females was found for the sample of M. cassinii. The Discussion section speaks of protandry and reports overall sex ratios for 2004 versus 2021; see also the Data Appendix.
Starting 19 May, this investigator noted the discovery of small M. septendecim, late arrivals for their species. (In June, very small M. cassinii also appeared.) Early in their appearance, apparently most small M. septendecim were male, some misshapen; late in the season, most small M. septendecim were female.
Distinguishing among the three Brood X species was made complicated by the wide range of sizes discovered for Magicicada septendecim as depicted in Figure 11. Among the Brood X species, the most immediate distinguishing feature of M. septendecim is its size: it is larger than the two other species. Magicicada septendecim is generally about 3.0 to 3.5 cm (1.2 to 1.4 in) in length from tip of abdomen to end of head (pers. obs. and University of Connecticut [2021]), while the two smaller species are often shorter than 3.0 cm (1.2 in) (note Figure 9 for M. septendecim and M. cassinii). When there are small M. septendecim, the deciding feature is the red or orange patch or shading between each eye and its wing insertion. Only M. septendecim has this coloration (Figures 10 and 15), while both the smaller species, there, are black (Dybas and Lloyd 1974, University of Connecticut 2021; Figures 13 and 14 for M. septendecula).
Figure 7. Total Number of male and female newly emergent adult periodical cicadas, Magicicada septendecim Brood X, found on successive mornings on 15–27 May 2021 at Sites 1 and 2 in a fixed route of a neighborhood of Silver Spring, Maryland. (This count begins five days later than in Figure 5.) A total of 645 recently emerged adult males and 592 recently emerged adult females were found (1,237 overall total cicadas).
Figure 8. Total Number of male and female newly emergent adult periodical cicadas, Magicicada septendecim Brood X, found on successive mornings on 15–20 May 2021 at Site 3 in Silver Spring, Maryland. (This count begins five days later than in Figure 5.) A total of 344 newly emergent adult males and 253 newly emergent adult females were found (597 overall total cicadas). This chart excludes the early outlier, 10 May emergence, and the last emergences with small numbers on 21–23 May. Note that on day 2, anomalously, the number of emergent females increased while the number of emergent males decreased.
Figure 9. A female Magicicada septendecim (left) alongside a male M. cassinii (right), near Site 3. 25 May 2021. Especially in June, some M. septendecim found were as small as typical M. cassinii. (Periodical cicadas generally show no conspicuous size difference between males and females, so the size difference here is indicative of the different species.)
Figure 10. Two male Magicicada septendecim, about 3.0 cm (1.2 in) or less in body length. May–June 2021.
Figure 11. Total number of larger (3 cm [1.2 in] or greater) and smaller (less than 3 cm) dying and recently dead adult periodical cicadas, Magicicada septendecim Brood X, found in a fixed route of two blocks (Thayer Avenue and Easley Street) in Silver Spring, Maryland, on 6 and 10 June 2021. Excluded from this graph are the 29 larger and small cicadas counted from Bullis Local Park walkways and parking lot on 10 June; there, the ratio was 16:13, almost 1:1. According to these samples, the proportion of small adult cicadas increased as the season ended. As the smaller cicadas effectively replaced the larger ones, the totals of these two particular counts were probably skewed in favor of the larger-sized, since if they emerged first, they should be dying first. Adults of this species are usually of the larger size. Late in the season, some cicadas discovered were perfectly formed miniatures.
This researcher discovered parks and other locales around downtown Silver Spring (Figure 4) where the three Brood X species could be found. Magicicada septendecim appeared throughout the Silver Spring study area. Magicicada cassinii was clearly favoring a few spots, in addition to Site 3, such as on Thayer Avenue and Easley Street and nearby parks, as well as along Sligo Creek; the researcher found M. cassinii also along streets on the north and west boundary of Woodside Urban Park. These cicadas at Woodside Urban Park were beneath large maple and mulberry trees, but at least one of the locations did not seem conspicuously toward the bottom of a slope – only about 3 m (10 ft) lower than Sites 1 and 2. A notable sight in one case was near the stumps of two recently cut down mulberry trees, without many places nearby for ecdysis, perhaps resulting in many M. cassinii nymphs dead on the sidewalk and in the street, which they had apparently attempted to traverse.
Then on 23 May, at historical Jesup Blair Local Park (Figure 12), this researcher finally found Magicicada septendecula. The researcher heard the distinctive long, uninterrupted male trilling in a tree nearby, then found the female cicada on a small hickory tree nearby. The cicada met the criteria for the species of black around the eye and both black and orange bands on the abdomen (Figures 13 and 14) – in contrast to M. cassinii whose abdomen is all black and M. septendecim whose abdomen is almost all orange. But what was distinctive of this discovery was the combination of hearing clearly the song and seeing the physical traits. No M. cassinii were seen nor heard at the park that day; on 2 June, however, some M. cassinii were chorusing. On 8 June, one M. cassinii was found in this park, many others were chorusing, and M. septendecim chorusing seemed to decline (three very small female M. septendecim were also found there). There was no clear evidence of any M. septendecula in June. So Jesup Blair Local Park is a site where all three species might be detected, depending on the time of season.
Dybas and Lloyd (1974) recalled studying part of Bull Run Park in Virginia in late spring 1962: “when we visited…the open stand of hickories, for instance, we would hear a strong chorus of septendecula…septendecula appears most prominently in…open-grown scattered large hickory and oak trees with a grassy understory, a habitat it shares with septendecim.” All this description, along with the “upland” preference for Magicicada septendecula, characterizes Jesup Blair Local Park, on a plateau, the highest elevation park around downtown Silver Spring, and even characterizes the specific tree and its surroundings within the park where the M. septendecula cicada was found. Possibly an M. septendecula vocalized from a small residential tree, and another rested on Grove Street (Figure 14), both near Bullis Local Park (Figures 1 and 4). And, perhaps to be expected near the large walnut tree at the Site 3 alley, among the M. cassinii nymphal skins was the remains of an adult M. septendecula.
The sound of Magicicada septendecula needs to be distinguished from M. cassinii. Magicicada septendecim, the most musical of the three, includes the “pharaoh” call (hence its nickname the Pharaoh Cicada). Dybas and Lloyd (1974) stated, “The only completely reliable criterion for distinguishing septendecula from cassini [sic] is male song.” Magicicada cassinii is a wind-up clock, and then a whining song (or vice versa), and is relatively brief (University of Connecticut 2021). Magicicada septendecula is so rare in this area that this researcher did not hear it chorusing in a group. With the ubiquitous sound of auto traffic, as well as background bird song, it is difficult to detect the solitary call of M. septendecula. To this researcher’s ear, the song of the M. septendecula is like a high-pitched chug-chugging of an old-fashioned wind-up toy, like a little robot, almost a ticking, and it continues for a long time, longer than most individual cicadas would vocalize.
Figure 12. Jesup Blair Local Park where Magicicada septendecula was discovered, 23 May 2021. The spacious park has old oak and hickory trees that have experienced several Brood X generations. A few days after this photo was taken, M. cassinii became audible in the park.
Figure 13. Likely Magicicada septendecula, female. Subtle orange bands on abdomen (visible in magnification), no orange patches near eyes. Jesup Blair Local Park, 23 May 2021.
Figure 14. Likely Magicicada septendecula, female. An abdomen pattern of mixed black and orange bands, no orange patches near eyes. Site 1, Grove Street, 28 May 2021.
Figure 15. Magicicada septendecim with diagnostic orange patch visible near eye.
4 June 2021.
DISCUSSION
Emergence Patterns
Timing of Emergence, Comparing Species:
There is variation in reporting whether Magicicada cassinii and M. septendecim start emerging at the same time, seasonally, or M. septendecim emerges earlier. The study of Brood X from Indiana by Young (1970) showed that of the eight sites where emerging cicadas were tallied, at perhaps half of them, and the only places where one species appeared earlier than the other, M. septendecim appeared before M. cassinii. Dybas and Lloyd (1974), on the other hand, studying in Ohio in 1965, did not report these two species appearing in sequence, explaining, “the species are tied together in time.” But apparently, many observers have noted that M. septendecim emerges before M. cassinii. “There is evidence that Decim emerge before Cassini” (Simon, in litt., 30 March 2021). Such was the case in this study.
Possible Cyclical Pattern of Emergence During Season:
This researcher observed a basic “M” shape pattern for the season’s daily emergences of adult Magicicada septendecim (Figures 5, 7, and 8). Young (1970) in Indiana, counted emerging Brood X M. septendecim every other day for six visits, and for the two of eight sites with the most data, between 200 and 300 cicadas at each, Young also reported emergence patterns that vaguely suggest the “M” shape. So, the plots of the emergence data appear to show a cyclical pattern.
This observer found this basic “M” pattern or sequential double chevron evident in a variety of populations and conditions: mapping only males or only females or both sexes together, independent of location, starting time for the cicada season, or emergence year 2004 versus 2021 (Kriesberg 2020). And this pattern was somewhat independent of weather (Figures 5 and 6). It seemed independent of timing for predator satiation, as well.
An explanation for this emergence pattern for Magicicada septendecim may relate to a combination of the daily temperatures along with the protandrous emergence and its somewhat conflicting strategies for avoiding predation. The best predictor for the M. septendecim emergence pattern seems to be daily temperatures (compare Figure 5 alongside Figure 6). But in some populations, the number of emergent M. septendecim may be disproportionate to the season’s daily temperatures; and, as shown in Figure 5, different locations may have somewhat different emergence patterns despite experiencing the same daily temperatures.
If protandrous arrival says that males arrive first, there must be a time in the season when females arrive second, and either stage of arrival could be explained, in part, as a strategy for avoiding predation (Kriesberg 2020). But the emergence patterns for males and females at Sites 1 and 2 combined, both in 2004 and 2021 (Figure 7 for 2021), and Site 3 in 2021 (Figure 8), show that, based on these samples, usually when the males emerge in relatively large numbers, the females emerge also, and vice versa. This may be an example of the safety in numbers phenomenon. Williams et al. (1993) pointed out that “males…emerged more synchronously than did females.” So, the first, big peak of emerging cicadas tended to be mainly males, with some females; a subsequent, smaller peak tended to be mainly females, with some males, and females may also emerge in proportionately larger numbers at other times later in the season (Figure 7).
Williams et al. (1993) explained, “avian predators appear to be satiated for several weeks.” Reptiles and amphibians may show satiation sooner than birds, the latter which may have young to feed. As a possible example of the start of such predator satiation in an amphibian: a large, captive Fowler’s Toad Anaxyrus fowleri (Hinckley) (Anura: Bufonidae) in 2004, readily eating periodical cicada nymphs in its diet, refused to choose them as prey after eating them four successive days, choosing other prey items instead (pers. obs.). But the timing of predator satiation’s beginning and end may be of doubtful relevance for the cicada emergence pattern, since in these samples the second peak was observed after 10 days at Site 3, yet after 14 days at Sites 1 and 2 (Figure 5). (It is unclear that predators would be satiated at different times at nearby locations.) Different sites apparently have different schedules.
This “M” emergence pattern seems not evident in this sample of Magicicada cassinii (Figure 5). And Young (1970), reporting Brood X M. cassinii from Indiana, counting every other day, in 1970, from a study site with more than 500 total M. cassinii, provided data forming a single convex chevron pattern, “∧”. This researcher’s data hint at the possibility of a similar single chevron pattern for M. cassinii. Whiles et al. (2001), reporting Kansas Brood IV M. cassinii emergence of 1998, characterized their discovery of M. cassinii as a “somewhat protandrous emergence pattern” in sex ratios. The absence of a second high point of nymph emergence in the season may be typical for M. cassinii. Perhaps the relatively small numbers of M. cassinii in most populations precludes a double-peak emergence as part of a strategy for individuals to elude predation; there would be too few individuals in the emerging populations of this species to support a second strong emergence.
Substrate Nymphal Vibrations Possibly Prompting Later Emergence:
Various studies help predict the timing of first emergence for periodical cicadas based on spring temperature (Kritsky et al. 2005, Raupp et al. 2020); and discussions of annual xylem flow influencing cicada emergence relate to the evolution of periodicity (Williams and Simon 1995, Karban et al. 2002). But explanations for how nymphs emerge later in a season are more gradually garnering attention. As a mechanism for cuing cicada emergence, in addition to temperature, perhaps underground nymphal vibrations play a part. The zoological analysis by Hill (2008, 2014), the study of adult cicadas by Alt and Lakes-Harlan (2018), and the study of Magicicada nymphs by Gibson (2015), considered cicadas’ receptors for detecting substrate vibrations, which might cue nymphs to emerge proximate to each other. Such behavior could be selected consistent with the safety in numbers survival strategy (defined in Lloyd and Dybas 1966, Williams and Simon 1995, Holzapfel and Bradshaw 2002). Nymphs joining the emerging crowd later in the season might detect each other from underground.
Conducting further research of Magicicada emergence behavior, reporting on different populations and settings and their emergence patterns, especially M. cassinii, could enhance the literature on these species’ natural histories.
Decrease in Size of Emerging Cicadas:
This researcher also wondered what might account for so many small Magicicada septendecim, and smaller than typical M. cassinii, noted toward the end of the season. Beasley et al. (2017) hypothesized that a number of factors, including urbanization—with disturbance and pollution—could produce “stressors” that impact periodical cicada nymphs. (During the past 17 years, within a few city blocks of Site 1, ongoing construction of new apartment buildings and road repair created much noise; and a new apartment building had been constructed, inhabited, and maintained on the border of Site 1). Beasley et al. (2017) also indicated that the species shows plasticity in body size, and Lloyd and Dybas (1966) and Williams and Simon (1995) discussed flexibility in nymphal growth rate relating to the evolution of synchronous emergence.
Beasley et al. (2017) reported a mixed message about the impact of urbanization, recalling research that showed, instead of detrimental effects on cicadas, apparent benefits. The higher temperatures in cities, and the horticultural fertilization of trees, are associated with larger sizes of the cicada species (note White and Lloyd 1985), which could help both males and females in their reproductive roles (also note Koyama et al. [2015] and Moriyama and Numata [2019]). On the other hand, Beasley et al. (2017) reported the xylem sap flow of urban trees in some places is disrupted by “xylem cavitation,” and there may also be urban noise that could inhibit nymphal growth. (Cooley et al. [2016] reported the contradictory hypotheses of beneficial vs. detrimental effects on periodical cicadas in the same habitat as “urban oasis” versus “suburban stress.”)
Koyama et al. (2015) generally found that periodical cicadas were larger in habitats of warmer annual mean temperatures, findings about cicada size consistent with those by Beasley et al. (2017).Yet according to Verberk et al. (2021) and the Temperature-Size-Rule, which happens to agree with Bergmann’s rule, terrestrial ectotherms such as many insects may tend to smaller size with warmer temperatures: “At high temperatures or low oxygen, animals may preferentially allocate resources towards development and away from growth.” Beasley et al. (2017) concluded, “continued monitoring of periodical cicadas in urban habitats, including a more fine scale assessment of habitat conditions, is needed to understand how urbanization could affect cicadas over long time scales and in earlier developmental stages.”
Smaller cicadas appearing toward the end of the season may simply be, in part, the “runt-of-the-litter” phenomenon, with those needing most growth waiting until the end to emerge. Whether the incidence of relatively small adult cicadas is widespread and increasing, perhaps related to climate change, or whether it could be mapped alongside other environmental factors, might be questions for community scientists and telephone apps.
Overall Sex Ratios Compared, 2004 vs. 2021:
Finally, this investigator was also curious whether the overall sex ratio might change in sequential generations, so compared combined data for Sites 1 and 2 from 2004 with those from 2021. Partly because tree cover changed in the intervening years, the author could not sample the exact same route or boundaries for the sites in both studies, in gathering an adequate sample. But data came from almost identical locations using the same methodology for counting (Kriesberg 2020). A two-proportion z-test of the data (Essa 2016), at a significance level of 5%, shows that a significantly greater proportion of adult male cicadas emerged in 2021 than in 2004. The seasonal sex ratio for the 2004 sample was significantly, disproportionately, female. There are confounding variables (some discussed in Frank 1983), so we cannot be sure what factors may have most contributed to the 2021 increase in the proportion of males.
One mechanism for maternal sex selection could be described, according to theory, as “selection favors mothers that produce sons when in good condition but daughters when in poor condition” (summarized in Wade et al. 2003). The logic would be that sons need sufficient fitness to compete for mates, and the “condition” might refer to nutrition available to the mothers, which in the case of cicadas, could be the xylem flow available to the females when they were nymphs. One might speculate that global warming could also contribute to a change in the sex ratios of the periodical cicadas (consider Moiroux et al. 2014, Edmands 2021). Various ectothermic taxa are changing sex ratios in response to changing temperatures (Edmands 2021). But, according to Kuznetsova and Aguin-Pombo (2015), the suborder Auchenorrhyncha, to which cicadas belong, has sex chromosomes (indicated also for Magicicada by Karagyan et al. 2020). Taxa with sex chromosomes, according to Edmands (2021), generally do not exhibit temperature-dependent sex determination. Williams et al. (1995) contend that sex ratios for emerging adult periodical cicadas are generally 1:1, “although temporary biases may occur.” It might be interesting to test this hypothesis about sex ratios in other sites. The overall sex ratio for the 2021 Site 1 and 2 data, alone, measuring the adult M. septendecim emergence, was not significantly different from 1:1. (See Data Appendix.)
Species Distribution
Simon et al. (1981) studying periodical cicadas in stunted scrub oak environment of Long Island, New York, and Morton (1987) studying birds in an island ecology of Panama, wondered about the way their subject animals perceived the world, and whether people could envision the environment in a similar way. Morton, studying introduced wrens (Song Wren and White-breasted Wood-Wren, Cyphorhinus phaeocephalus and Henicorhina leucosticta respectively), found that they chose, for nesting, the borders of trails unfortunately frequented by their predators, possibly because to the wrens, the trails looked like streambeds, their favored nesting environment. Similarly, one might speculate that Magicicada cassinii chose the alley site (Figure 3), perhaps also in part because it resembled a floodplain such as along Sligo Creek, which the species apparently prefers for chorusing and egg-laying.
Dybas and Lloyd (1974) and Young (1970) reported Magicicada cassinii as generally favoring the floodplain and sites along streams. But this investigator found that by June, M. cassinii apparently were present wherever periodical cicadas could be detected, though in lesser numbers than M. septendecim at their height. Notably, M. cassinii were chorusing in Jesup Blair Local Park and along busy Georgia Avenue nearby, relatively high elevation areas near downtown Silver Spring. Dybas and Lloyd (1974) examining cicada nymphs in Iowa, June 1963, and adults in Kansas, June 1964, were also surprised to find M. cassinii in “upland” habitats mixed with M. septendecim.
This researcher wonders whether environmental pressures favoring particular cicada features, if present, could yield, over generations, a relative increase in the number of Magicicada cassinii. Moriyama and Numata (2019) reported that in urban Osaka, Japan, starting a few decades ago, one cicada species, Cryptotympana facialis (Walker), prevalent in the south, began to supplant a native Osaka species, Graptopsaltria nigrofuscata (Motschulsky). Hypotheses explaining the incoming species’ greater fitness and ability to extend its range included a warming trend favoring its hatching during a rainy season, and nymphs more adaptable to the compacted soil of the urban area.
Larson et al. (2019) explained that “insects are responding to climate change by altering the seasonal timing of adult emergence” (note also Moriyama and Numata 2019), and there is a “potential for climate change to influence species boundaries between closely related insect species.” In the study areas, Magicicada septendecim generally began flying in mid-May, and M. cassinii about a week later. In 2021, the later date was about 5 °C (9 °F) warmer than earlier in the spring: 18 May was 17 °C (62 °F) and 23 May was 22 °C (71 °F). Temple (2021) explained that if species behaviorally related to each other respond differently to evolving temperature changes, the species might become out of sync with each other. Though some relevant cicada behavior may not be keyed to temperature, warming might adjust the adult arrival time and place of species in relation to each other.
It seems that in the reproductive process of Magicicada cassinii and M. septendecim, the females are ultimately patchier than the males: males and females of each species met and mated in close proximity, as in Bullis Local Park, but the females of both species apparently then went to particular places to lay their eggs, since the nymphs of M. cassinii generally emerged in locations different from the nymphs of M. septendecim.
In the last days of June, with the periodical cicada season ended, one can note the locations and quantities of egg-nests based on the browned leaves and withered branches tattering or marking (flagging) the trees; hence one could anticipate the disposition of emerging cicadas 17 years in the future. It is possible “nymphs do not fall straight to the ground” (Smits et al. 2010), and there might be high nymph mortality in certain locations. But since the nymphs do not disperse much after hatching (Gilbert and Klass 2006, Smits et al. 2010, Simon 2021), it might be possible to estimate roughly where and in what abundance the nymphs would emerge in year 2038—and from tree selection, which species might emerge, where.
Jesup Blair Local Park had many oak trees displaying browned branches, and relatively few branches of hickory; blackgum, Nyssa sylvatica Marshall (Cornaceae); and American sycamore, Platanus occidentalis L. (Platanaceae) apparently with egg nests. Sites 1 and 2 generally had relatively few egg-nests in evidence in the tree canopy; those two sites might yield fewer teneral cicadas in the next emergence than in this recent one. Site 3 had many egg-nests from the willow oak tree, hardly any evidence of them in the alley. Female cicadas may choose egg-laying sites in tree branches based on the amount of sunlight available there (Yang 2006).
Beyond recognizing tree species, this investigator could not visually detect any local pattern for where the females laid eggs, except for one observation. Neighborhood planners coincidentally experimented with a traffic route in part of Site 1. Grove Street had most car traffic blocked and the street given over to pedestrians and bicyclists. Subsequently, trees along Grove Street had a relatively large number of browned leaves from cicada egg-nests. So, many nymphs might emerge along Grove Street in 2038. And, based on the tree species with egg-nests, Magicicada cassinii may once again dominate Site 1 when their time for chorusing arrives.
Two Silver Spring parks this investigator found most suitable for Magicicada were Bullis Local Park and Jesup Blair Local Park. Both featured alternating sunny (Yang 2006, Sheppard et al. 2020) and shady settings with open ground and a wide variety of tree species (Maier 1980), including various tree heights and ages. Such habitats are recommended for cicada study, next emergence.
ACKNOWLEDGMENTS
The author thanks Allen Hirsch (CEO, CryoBioPhysica Inc., Chevy Chase, MD), who suggested focusing on patterns of emergence; David Kriesberg (instructor in engineering design, A. James Clark School of Engineering, University of Maryland, College Park, MD), for assistance with research; two anonymous peer reviewers for constructive critique; John R. Cooley (Assistant Professor in Residence, Department of Ecology and Evolutionary Biology, University of Connecticut, Hartford, CT) for insights into methodology in the field and the cicada emergence and distribution; and Gene Kritsky (Dean of Behavioral and Natural Sciences at Mount St. Joseph College, Cincinnati, OH), for information on the study of cicada phenology.
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DATA APPENDIX
Two-Proportions Z-Test procedure derived from Essa (2016)
Purpose: To investigate whether overall sex ratios of emerging adult cicadas, in the same sites 17-years apart, were comparable and not significantly different (null hypothesis).
Comparing the totals of Sites 1 and 2 in the two generations, a greater proportion of males emerged in the 2021 generation.
· In 2004: 548 males of 1248 (n1) total adults = 0.44 (p1) or 44% of adult emergent cicadas.
· In 2021: 645 males of 1237 (n2) total adults = 0.52 (p2) or 52% of adult emergent cicadas.
Are these two ratios significantly different?
· P (overall proportion) = (548 + 645) / (1248 + 1237) = 1193 / 2485 = 0.48 or 48%.
· Subtract the smaller percentage (44%) from the larger percentage (52%) and divide by the Standard Error to obtain the z score.
· Z = p2 – p1 / √ (square root) of: P (1-P) / (1/n2 + 1/n1)
· Z = 0.52 - 0.44 / √ of: 0.48 (1 - 0.48) / (1/1237 + 1/1248) = 0.08 / √ 0.0004 = 0.08 / 0.02 = 4.00 ∴ Z = 4.00
· Significance at alpha level 5% would be z less than -1.96 or greater than 1.96.
· One would reject the null hypothesis if the z score falls at the outer edges of the bell curve or normal distribution.
· Result: In the 2021 sample, a significantly greater proportion of adult male cicadas emerged than in the 2004 sample. The two proportions (sex ratios) were significantly different.
Purpose: To compare the 2021 overall sex ratio at Sites 1 and 2 to a 1:1 ratio, to see if the experimental ratio for this emergence did not differ significantly from a 1:1 ratio (null hypothesis).
· Divide total males and females (n2 = 1237) in half (618.5) and compare that 50% number with the sample number of males (645) or of females (592). (To determine closeness to 50%, data for either sex would be informative. For the following statistical test, the number of males is used.)
· P = 645.0 + 618.5 / 1237.0 + 1237.0 = 1263.5 / 2474.0 = 0.51 or 51%
· Z = p2 – p1 / √ (square root) of: P (1-P) / (1/n2 + 1/n1)
· Z = 0.51 - 0.50 / √ of: 0.51 (1 - 0.51) / (1/1237 + 1/1237) = 0.01 / √ 0.25 / 0.0016 = 0.01 / 12.50 = 0.0008 ∴ Z = 0.0008
· This sum is not negative, less than -1.95. And neither is it more than 1.95. So, the test result is not statistically significant.
· Result: The null hypothesis is accepted; this 2021 sex ratio is not significantly different from 1:1.
See the Discussion section for context and interpretation.
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