Predator Satiation

Polistes carolina/perplexus with Magicicada prey | Shaw Nature Reserve, Missouri

I’ve probably used the term predator satiation more often during the past couple of weeks than I have during the entire rest of my life.  Students of ecology know this as an antipredator adaptation in which prey occur at such high population densities that they overwhelm predator populations.¹  This ‘safety in numbers’ strategy reduces the probability that any given individual will be consumed, thereby ensuring that enough individuals survive to reproduce.  With St. Louis currently experiencing the appearance of Brood XIX of periodical cicadas, I’ve gotten lots of questions recently from many coworkers and friends wanting to know more about these cicadas.   Often the first question is “What is their purpose?”  My standard reply begins with a statement that they, like all living organisms, are the products of natural selection, which then presents an opportunity to explain how natural selection might result in such massive, temporally synchronized, multiple-species populations.  A few eyes have glazed over, but I think most have found my answer interesting, often even leading to further questions about where they lay their eggs, what is their life cycle, why are they so loud, how do they “do it” and select mates, etc.  Of course, as an entomologist with a strong natural history orientation, I’m always anxious to introduce people to ecological concepts, and right now the periodical cicada is providing a conspicuous, real-life example of such.

¹ Also called “predator saturation,” although this term might be misconstrued to mean that it is the predators that are over-abundant.

First the eyes...

A few weeks ago, right at the beginning of their emergence in the St. Louis area, my friend Rich Thoma and I observed predator satiation in action.  While hiking one of the trails at Shaw Nature Reserve, we heard the unmistakable shriek and cellophane-sounding wing flapping of a just-captured male cicada.  Tussling on the ground ahead of us was the cicada in the grasp of a Polistes carolina/perplexus wasp, which was repeatedly stinging the hapless cicada on the underside of the abdomen.  The shrieking and wing-flapping grew less frequent as the stinging continued, until at last the cicada lay quiet.  As we approached, the wasp spooked and flew off, but we knew it would be back—we parked ourselves in place while I setup the camera, and before long the wasp returned.  It took several minutes of searching from the air and on the ground before the wasp finally relocated her prey, but once she did she began voraciously devouring it.  As the wasp was searching, we hypothesized that our presence had altered the visual cues she had memorized when flying off, resulting in some confusion when she returned, and thus the long period of time required to relocate her prey.

...then the legs!

We watched for awhile—first the eyes were consumed, then the legs.  As it consumed its prey, Rich remarked that he bet he could pick up the wasp and not get stung—likely the entirety of its venom load had been pumped into the cicada.  Both of us declined to test his hypothesis.  We also wondered if the wasp would butcher the cicada after consuming part of it and bring the remaining pieces back to the nest.  We had seen a European hornet do this once with a band-winged grasshopper, consuming the head, then cutting off the legs from the thorax and flying away with it before returning to collect the abdomen as well.  No butchering took place this time, however, the wasp seemed content to continue eating as much of the cicada as possible—a satiated predator if there ever was one!

Leg after leg is consumed.

One eye and all six legs down, time to start on the abdomen.

Copyright © Ted C. MacRae 2011

Do you think I’m tasty?

As I hiked the upper stretch of the Shut-Ins Trail at Sam A. Baker State Park in southeastern Missouri, I encountered this 2-inch long millipede slowly making its way across the rocks.  Many millipedes, of course, produce hydrogen cyanide (HCN) as their primary method of defense against predation, and the bright yellow markings of this individual were an obvious sign that this particular species is no exception.  The wrinkled dorsal surface and black coloration with yellow wedge-shaped posterolateral markings identify it as a species of Pleuroloma (BugGuide), and of the four species known from North America (Shelley 1980) only the widespread Pleuroloma flavipes (literally meaning “yellow legs”) occurs as far west as Missouri (Shelley et al. 2004).  A similar pattern of coloration is seen in a number of related genera, e.g. Apheloria, Boraria, and Cherokia—all belonging to the order Polydesmida, presumably functioning across the group as an aposematic (warning) signal to predators that they should be left alone.  Another feature shared by the members of this group is the lateral expansion of the dorsal segments into “paranota,” giving the species a much more flattened appearance than other millipedes with the more typical cylindrical shape.  While all millipedes exhibit diplosegmentation (embryonic fusion of paired body somites and associated legs, spiracles, and ventral nerve cord ganglia), members of the Polydesmida have taken this condition to its culmination with no evidence of external sutures (Myriapoda.org).

The bright coloration of this species was an interesting contrast to the cryptic invisibility of the copperhead snake I had seen just a few moments earlier during the hike—opposite strategies with identical goals.  Defense compounds are, of course, widely employed by many plants and animals; however, only millipedes and a few insects have developed the ability to utilize HCN, a highly toxic compound that halts cellular respiration in most animals through inhibition of the mitochondrial enzyme cytochrome c oxidase.  Evidence suggests that Pleuronota flavipes and other millipedes can tolerate HCN because they possess a resistant terminal oxidase that makes their mitochondria insensitive to the effects of HCN (Hall et al. 1971).

Perhaps some of you will be interested in this recent checklist of the millipedes of North and Central America (Hoffman 1999).

Update 6/13/11: My ID as Pleuroloma flavipes must be considered tentative, as Rowland Shelley has sent me an email with the following comment:

It could be Pleuroloma flavipes Rafinesque, 1820, or it could be Apheloria virginiensis reducta, I can’t really tell from the photos.

 

REFERENCES:

Hall, F. R., R. M. Hollingworth and D. L. Shankland. 1971. Cyanide tolerance in millipedes: The biochemical basis. Comparative Biochemistry 34:723–737.

Hoffman, R. L.  1999.  Checklist of the millipedes of North and Middle America. Virginia Museum of Natural History Special Publication No. 8, 584 pp.

Shelley, R. M. 1980. Revision of the milliped genus Pleuroloma (Polydesmida: Xystodesmidae). Canadian Journal of Zoology 58:129–168.

Shelley, R. M., C. T. McAllister, and S. B. Smith. 2004. Discovery of the milliped Pleuroloma flavipes in Texas, and other records from west of the Mississippi River (Polydesmida: Xystodesmidae). Entomological News 114 (2003):2–6.

Copyright © Ted C. MacRae 2011

Eye to eye with a copperhead

I don’t know what it is about Osage copperheads (Agkistrodon contortrix phaeogaster) that makes every encounter with one so special. They are perhaps the most common of Missouri’s five venomous snake species, and I’ve seen them more often than I can count. Still, every time I see one I simply must stop and marvel. This particular individual was seen a few weeks ago at Sam A. Baker State Park in Missouri’s southeastern Ozark Highlands. You might say it was “sloppy seconds”—I had actually gone to the park to look for timber rattlesnakes (Crotalus horridus), a juvenile of which I had seen during last year’s Annual-Birthday-First-Bug-Collecting-Trip-of-the-Season™ trip. I did not see any rattlesnakes this time, as access to the rockpilish cliffs along Big Creek where I saw the juvenile last year was blocked by high water, but I was quite pleased to find this copperhead underneath a log while we were there.

Copperheads are marvelous photographic subjects. Beautiful, rarely seen by those who don’t know how to look for them, and with an air of “danger” about them. Yet they are among the most docile of all snakes, venomous or otherwise. They don’t use aggression or warning sounds when threatened like cottonmouths (Agkistrodon piscivorus) or rattlesnakes, nor do they dash for cover like most non-venomous species. Instead, they rely on their cryptic, dead-leaf coloration to make them invisible. It works—even I, my eyes tuned to see just about anything after a half-century of clambering through the brush, didn’t immediately notice this individual when I first rolled over the log under which it had taken cover (although I did immediately notice the little red-backed salamander, Plethodon cinereus, at the other end of the area covered by the log). I suspect I’ve walked right by many more copperheads than I have seen, completely unaware of their presence.

Their docile nature also invites extreme close-ups that I wouldn’t dare attempt with a rattlesnake or cottonmouth—at least not without a much longer lens than my 100mm. These photos make it seem that I was right on top of the snake, although at a maximum magnification of around 1:2 there was still a reasonable amount of working distance (I did, however, keep my hands well back of the front of the lens—just for good measure). Still, in all my copperhead experiences, I have never seen a copperhead actually try to strike unless I touched it (not what you think!).

Eventually it’d had enough of our gawking and began to look for new cover.  As it uncoiled, I could see it’s still greenish but not too yellowish tail, indicating that it was still a youngster, though perhaps a little older than the first copperhead I tried to photograph.  We watched it as it crawled into the loose, dry leaves… and disappeared.


Copyright © Ted C. MacRae 2011

Pardalophora phoenicoptera – Orange-winged grasshopper

For some reason, I’ve found myself increasingly fascinated with certain grasshoppers—not just any grasshoppers, but band-winged grasshoppers (family Acrididae, subfamily Oedepodinae).  And not just band-winged grasshoppers, but band-winged grasshopper nymphs.  It began last year when I found adults and nymphs of Trimerotropis latifasciata in the Glass Mountains of northwestern Oklahoma.  I believe it has something to do with the combination of their frequent association with the same habitats where I look for my beloved tiger beetles and their marvelously cryptic coloration.  Adults themselves are cryptic enough—that is, until they flash their brightly colored hind wings, but the nymphs are positively invisible until they move.  Moreover, many species show a wonderful range of intraspecific diversity in their crypsis—Ronald Reagan may have thought every redwood tree looked the same, but when you’ve seen one band-winged grasshopper nymph, you most certainly have not seen them all.


These two band-winged nymphs were seen at St. Joe State Park (St. Francois Co., Missouri) in the vast central “sand flats” of the park (actually waste areas of crushed limestone tailings left from lead mining operations during the previous century).  At first I assumed they each represented a different species, but based on comments at BugGuide I take both of them to represent Pardalophora phoenicoptera (orange-winged grasshopper)—distinguished from Xanthippus by having only one notch in the pronotal crest and unusual amongst most grasshoppers in that the winter is passed as a nymph rather than egg.  This leads to well-developed nymphs at the beginning of spring and adults much earlier in the season than many other grasshoppers.  These photos were taken on April 28, and the size of the wing pads suggests they are not quite full-grown yet, maybe 3rd or 4th instars.  Acridoid aficionado David J. Ferguson has found this species in the Ozarks on rocky/gravelly hilltops (e.g., “cedar glades”) and on gravelly or stable sandy slopes in sunny openings in Oklahoma. He places the species (particularly the green ones) high on his favorite hopper list, and I’d have to say I agree with him (so far).

One of these days, I’m going to find and photograph the king of all green oedepodines—Trimerotropis saxatilis!

Update 6/8/11: Dave Ferguson has kindly confirmed the ID, writing:

…yes these are identified correctly.  Assuming 5 instars, they look like 4th (where there are 6 instars, numbers 4 and 5 look a lot alike).

Copyright © Ted C. MacRae 2011

They’re baaaaack… finally!

For almost a month I waited—waited for that spaceship-sounding drone from the trees; waited for their bodies to drip from the vegetation and their skins to litter the yard; waited for their delightful shrieks every time I jostle a tree branch. I had seen them mass emerging from the ground in southern Missouri in late April, but just 100 miles north in my hometown it seemed they would never show. Cold, rainy springs must not be to their liking, as it was not until the sun finally broke through and temperatures climbed into the 80s that they finally made their appearance in St. Louis—nearly a month after that mass emergence event further south had me looking and listening daily for one of North America’s most spectacular natural history events, Brood XIX of the periodical cicada!

Finally, on May 21 I saw the first adults of the year at Shaw Nature Reserve not too far from my house (not intending to claim this as the date of their first appearance in my area!). They were not yet singing, but the adults were everywhere, many sitting right next to the skins they had just emerged from the previous evening. I had to travel on business through the southeastern U.S. that following week, and it was while visiting the beautiful MSU campus in Starkville, MS that I got my first taste of their late-afternoon synchronized, pulsating song. Upon my return to St. Louis at the end of the week, the eery drone filled the air as soon as I stepped out of the airport. It had been 13 years since I’d heard that sound, but euphoric recall instantly transported me back to 1998 and 1985 and my experiences with these marvels of evolution.

I don’t know that there is anything I can say about the periodical cicada that hasn’t already been said—repeatedly—by the numerous, more erudite sources that are following this event as if it were the approach of Haley’s comet. I don’t even know for sure which species are in my area and how to tell them apart. All I do know is that the constant droning of their singing is both maddening and amazing—a spectacle to behold for what it is, knowing that it will be the year 2024 before I have my next chance to witness it.

As I write this, I’m sitting in my hotel room in Salem, AR, where the cicadas are even more abundant than around my home in the woods—several stops to check building lights for beetles have ended in frustration because the cicadas were so numerous that they virtually swamped the space on the walls under the lights. There do seem to be two species here—a smaller one with a completely dark underside and the raspy, screeching sound that I am familiar with, and a larger one with the abdominal segments light along the posterior margins and a softer trill that almost reminds me of the song of an American toad. Maybe there are other species mixed in that I have not discerned, but I’ll not concern myself with that. Instead, I will continue to marvel at the extraordinary event unfolding before me, watch it as it cycles out, and chuckle at the complaints of the masses bemoaning their temporary inconveniences.

Copyright © Ted C. MacRae 2011

Wrong lens

During the past couple of years, as I’ve transitioned from strictly a net-wielding entomologist to one that also carries a camera, I’ve had to start making choices about whether to keep the camera in the backpack or hold it at the ready, and if the latter which lens to keep on it. They are situational decisions, influenced largely by what I’m focused on (heh!) at the time—keeping the camera in the bag facilitates collecting, but it also tends to reduce the number of subjects I deem worthy of the setup effort required to photograph them. Conversely, carrying the camera out of the bag greatly impedes collecting but results in much more photographs having been taken. Even when I do decide to carry the camera at the ready, which lens should I have on it—the 100mm for tiger beetle-sized and larger, or the 65mm for tiger beetle-sized and lower? (Annoyingly, most tiger beetles are right at that life-sized threshold, and neither lens alone allows me to float above and below 1:1 for the full range of photos I like for them. As a result, I sometimes end up with extension tubes stacked under the 100mm lens to give me some extra range above its normal 1:1 limit.) I wish there was some way to have the camera with either lens at the ready (and not impeding net swings would be even better), but that just isn’t possible. As a result, I sometimes find myself with the wrong lens on the camera when I see something I want to photograph. If it’s important, I’ll go through the trouble to switch out lenses—hopefully quickly enough to avoid losing the photographic opportunity; other times I might just decide I don’t really need the photo that badly.  Then there are times when I feel a little adventurous and will just go ahead and take the photo anyway without switching lenses.

The following is an example of the latter—an eastern fence lizard (Sceloporus undulatus) photographed with the 65mm lens (minimum magnification 1:1). Not only is this the first time that I have succeeded in approaching one of these lizards closely enough to take a good photograph, but the short working distance of the 65mm required that I get extraordinarily close. He was on the side of a fallen log, and I approached from the other side crouching low, then slowly (slowly!!!) peered over the edge of the log until I had his head in focus. I got off just this one shot, as the flash caused the lizard to bolt for good. The angle could have been better, but I got the eye focused spot-on so it’s a keeper.

Sceloporus undulatus (eastern fence lizard) | Shaw Nature Reserve, Franklin Co., Missouri

I wonder if anybody else has ever photographed a 6-inch long lizard with a 65mm lens…

Copyright © Ted C. MacRae 2011

Mite on White

I found this velvet mite at Shaw Nature Reserve (Franklin Co.) in east-central Missouri on a trail through mesic upland forest. At 4 mm in length, this member of the superfamily Trombidioidea is a downright honker compared to most other mites. I suspect it belongs to the nominate family, but comments at BugGuide suggest a lateral view of the palps are necessary for a conclusive ID to family. Regardless of its identity, its screaming red color made it ideally suited to be photographed on a white background. On the other hand, its small size and refusal to ever stop crawling made it a frustrating subject to track through a 65mm lens (all photos shown uncropped).


Copyright © Ted C. MacRae 2011