Walkingstick

OLYMPUS DIGITAL CAMERA
The brown, twig-like walkingstick can only be easily seen when silhouetted against the background of the green leaves that it eats.

Common Name: Northern Walkingstick, Common American Walkingstick – Probably the most appropriately named of all invertebrates … for all intents and purposes, this insect looks like a stick that is, incongruously, walking on spindly legs. Even though it is named northern, its geographic range extends south to Florida and west to New Mexico.

Scientific Name: Diapheromera femorata  –   The generic name has no established etymology, but may imply that the use of chemical signals (pheromones) by these insects, “dia-” meaning consisting of. The species name is from femur, the Latin word for thigh and the name of the longest human leg bone. The plural form is femora so multiple femurs are implied.

Potpourri: Walkingsticks are not hiking poles although the two terms have been used synonymously with the former applied the latter but not vice versa. The walkingstick is a twig-like insect with legs to get from place to place, metaphorically like the poles that help the hiker do the same. Along with leaf insects that look like leaves, the walkingstick is one of the most compelling arguments for evolution according to Darwinian survival of the fittest. There can only be one reason why the complexities of a living, breathing, ambulating animal would be packed into an unlikely longitudinal and tubular package that looks like a stick except to fool a predator and thereby survive to procreate. Those that looked more like twigs over time became lunch less often, having off-spring that looked even more like twigs – and so on.  Ultimately, the near perfect foil as indistinguishable from the perch on which it stands.

Historically, walkingsticks were placed in the order Orthoptera, which was an odd assortment of insects with a life cycle characterized by partial metamorphosis – egg, nymph, and adult with no pupation ― the nymph looks like a small adult.  This taxonomy was impractical and unwieldy as it grouped cockroaches, mantids, grasshoppers, and crickets which have the straight wings for which the order was named (orthos is straight and ptera are wings in Greek) with the much more distinctive walkingsticks which are mostly wingless. Orthopteroid is still used as a descriptive unofficial category to refer to these insects with the addition of earwigs and termites, which, like mantids, are evolved from cockroaches. [1] The new stick insect order is Phasmatodea, from phasma, the Greek word for apparition or ghost. Insects that are almost invisible due to their extreme crypsis like the walkingsticks and the mostly Australasian leaf insects (there are none in the Americas) have long evoked a sense of the supernatural and were considered an anomalous category for which numerous names were variously applied, such as Cheleutoptera, Ambulatoria, and Spectra (from the Latin word for ghost). [2] Absent the theory of evolution, their presence would suggest that a twig or a leaf had suddenly come alive like the argumentative  apple trees of Oz or the methodical ents of Middle Earth; a specter was the only available. explanation from the animist perspective. The taxonomy of the phasmids, as they are commonly called, is far from settled but the walkingstick insects are tentatively placed in several suborders and the walking leaf insects are for the present in the family Phylliidae. This will surely change.

Walkingstick Nymph AT 130816
The small, green walkingstick nymph is vulnerable to predation

Walkingsticks are an extreme case study of adaptations that must be made by all living things to survive in an ecological niche, humans excepted as makers of the dominant “niche” at the expense of other species. Life starts for the hatchling walkingstick as a nymph emergent on the forest floor in mid-June elongated in form like the walkingstick it will become, but diminutive in stature and bright green in color. Rather than a stick, it might be mistaken for any of the green larvae that course about the forests and are the provender for many a predator. The nymphs walk to the nearest thing they can find and climb to the leaf level under the cover of darkness to begin a life of eating and growing. That is if they get that far. If a cement post is first encountered as a tenuous refuge, it won’t be for long, as the now obvious green shape is on display for any passing birds, the nemeses of walkingsticks. Their only defense at this stage is sacrificial amputation. Since a predator is likely to grab the first thing that it can, this would likely be one of the six conveniently extended legs. Walkingstick nymph legs are detachable to allow the body with the remaining five legs to survive; the removed leg regenerates within a few days to restore full mobility.   Of the approximate one thousand species of walkingstick, almost all are found in tropical climates, where foliage is dense and escape more probable … there are only ten species  in North America. [3]

Walkingstick nymphs that are either more fortuitous or perhaps have a better sensor suite find there way up a tree and out on a branch that supports multitudes of leaves extended by petioles from twigs. While not all that selective, they prefer black cherry (Prunus serotina) or hazelnut (Corylus americana) trees, choosing oaks (Quercus spp) as an alternative. As obligate herbivores, nymphs settle on an individual leaf to strip the preferable cellulose-rich plant tissues leaving only the vascular vein plumbing as a matrix of thin piping in its wake, skeletonizing the leaf. Those that remain concealed and can continue eating will molt (technically called ecdysis), shedding their green skin for the brown colors of  the twigs they traverse. The molting process  is repeated four more times going through each phase at about a two week interval as transitional forms called instars. The maturation to adulthood takes a long time that varies according to temperature … cooler weather promotes the growth of the nymphs. The period varies with annual and geographic weather differences that have been measured by field testing to vary from  74.7 days in one year followed by 84.9 days in the next. The unit degree-day is used as a direct measure of the time at temperature requirement for an individual plant or animal. It provides a metric for phenology, the study of natural phenomena that progress according to climate. It is determined relative to a standard mean temperature, mostly 65ºF in the United States, and can be applied above or below that level for cooling or heating. The northern walkingstick nymph requires 1835 degree days to reach adulthood―roughly 120 days if the average temperature is 80ºF. Over time this results in broader distinctions in growth and reproduction. Northern walkingsticks that are really in the north (New England and southern Canada) have a biannual life cycle while southerly northern walkingsticks reproduce annually. [4]

OLYMPUS DIGITAL CAMERA
A male walkingstick clamping to the abdomen of the female in preparation for copulation.

At some point in (almost) every animal’s life, there comes a time (puberty in humans), when growth is complete and sex becomes the predominant concern. Species survival mandates procreation. For the most part, this involves the combination of genes from a male and a female to literally engender some variation that enhances survivability in a changing world. Male walkingsticks are generally smaller than the females and in some species they attach themselves and ride on the back of the female until a time of her choosing. The tail of the male is configured into a clasper to latch on to the abdomen of the female to assist in positioning (Cheleutoptera, one of the alternate order names, is derived from chele, the Greek word for claw) so that mating can proceed with the transfer of a spermatofore. Many phasmids have the ability to produce fertilized eggs absent the participation of the male, a not uncommon characteristic called parthenogenesis. [5] Self-fertilization has the advantage of guaranteed progeny for short term survival  with the disadvantage of decreasing genetic diversity for long term survival.

The result of a tryst or non-tryst fertilization is between one hundred and one thousand eggs, dropped from the treetops to the duff below. Oviposition occurs one egg at a time beginning in late August and ending in October eventually covering the ground with as many as 400 eggs per square meter. The eggs of phasmids are rugged, coated with a layer of calcium oxalate, and sculpted and have the colors, shapes, and textures of various plant seeds … the eggs of the northern walkingstick look like the seeds of legumes.  The hard outer shell protects the embryo through the winter to survive until spring when the nymph emerges. It probably evolved to prevent wasp predators from penetrating the eggs. The eggs also have a rounded protuberance called a capitulum at one end that is similar in appearance to an oily appendage on many plant seeds called an elaiosome.  The unusual egg  characteristics contribute to egg dispersal. Ants collect seeds with elaiosomes and take them back to their nests to remove the nutritive lipids and proteins, leaving the reproductive portion intact, contributing to plant propagation. Bloodroot, spring beauty and trilliums, among others, use ants to distribute their seeds in this way, a mutualistic arrangement called myrmecophily. Ants fooled by the capitulum of the walkingstick eggs are unwitting accomplices in moving them to a safe location for the winter. The hard-shelled eggs also protect against damage when passing through the digestive system of avian predators after having consumed gravid female walkingsticks. Excised eggs were fed to one of the known bird walkingstick predators and its fecal pellets collected three hours later. Twenty percent of the eggs were still viable and two were successfully hatched into nymphs. [6] The ability to hitch a ride on the backs of ants or in the belly of a bird may explain the wide geographic dispersal of northern walkingsticks, which typically spend their entire lives in the same tree.

One would think that evolution would surely have stopped with the “intelligent design” of an insect that looks like a stick with eggs that look like seeds. The contest between prey and predator is a continuum, however, as faster cheetahs foster even faster impalas. Birds are a highly evolved class of animals with about 200 million years of refinements largely to enhance their search for food, particularly high protein insects. Their keen eyesight can discern the slightest variation in a background pattern as impetus to take note and action. As the winds blow, the boughs move with graceful undulations that transmit through the interconnecting branches to the leaves and twigs. A walkingstick standing still on a moving twig is a sitting duck for a watchful drake.  Crypsis, the ability of an animal to avoid detection, is widely employed by both predators and prey to stealthily creep to within striking distance for the former or blend into the background for the latter. In most cases this takes the form of colors and shapes, the tiger stripes that mimic tall sunlit grasses and chameleon lizards that change color to become incognito are well known examples. Most backgrounds don’t move so the companion behavior pattern of those seeking to hide is to freeze, as toads do instinctively when threatened. But twigs move, so most walkingsticks do too. Studies of walkingsticks swaying in concert with the amplitude and periodicity of the wind-generated movement of twigs on which they perch have concluded that they are synchronous. [7] When the grim reaper beckons, having rhythm matters. Some walkingsticks have taken this one step further, employing chemical weapons in the form of irritating sprays that deter not only birds, but also mammals and even other insects. [8]

The long, thread-like, jointed legs of the stick insect are ungainly, splayed out in all directions in support of the cumbersome cylindrical body like a tethered Macy’s parade balloon. They are not  the strut-like simple appendages that they seem, as walking on uneven surfaces with different textures at odd angles is a gravitational challenge. Each leg is terminated in a two-part foot pad with a toe that is adhesive and a heel that is what may be called stick-slip, having adjustable adhesive qualities. The physics of mobility require each foot to  stay where it is placed firmly yet yielding to removal without undue force when motion proceeds. To do this effectively is evidently an important feature of walkingstick behavior, as the heel pads incorporate three features to ensure it sticks when pressure is applied, but slips freely when needed. The surface is covered with rounded individual hairs that flatten like a squash ball when squashed, resulting in a larger surface areas. As the weight or pressure is increased a second. shorter group of hairs is engaged to broaden the base. At the most compressed and adherent state, the hairs bend over to add a third dimension to the contact surface. The net result is a lot of friction for minimal applied force that can be readily and rapidly applied and released so the walkingstick can walk. [9] The modes and methods of the stick insects is of interest to science as a possible mechanism to enhance footwear performance just as the overall carriage of the ambulating animal may be useful in designing all-terrain robots. [10]

 By all accounts, the walkingstick measures up as a success in the forest survival struggle. It is well hidden in its camouflaged exoskeleton, it reproduces with large numbers of protectively coated eggs, it eats voraciously but not too exclusively, it grows fast, and  it gets around. So why is there not a plague of walkingsticks on occasion like the biblical locusts? They are both orthopteran insects  so they are closely related. The swarming behavior of locusts is technically called kentromorphism, brought about by environmental factors that promote density dependent behaviors. It is the gregariousness of species that overwhelms an ecosystem. Walkingsticks are mostly solitary and they can’t fly, so large scale tree or crop devastation has not been an historical problem. They only spread at the rate of an eighth of a mile a year, even taking ants and birds into account. However, on a more regional scale, there have been numerous occasions where serious environmental degradation has been experienced. One report from 1874 noted the denuding of 25 acres of white oak and hickory trees in Yates County, New York. The density of walkingsticks was such that “they cluster upon a limb or fence-rail so thickly that they pile up upon one another, and one cannot enter the wood where they are, without having numbers on his clothing”. The worst recorded outbreak was in Ogemaw County, Michigan in 1936, when 2500 acres of oak trees were “completely stripped by the middle of July.”  [11] However, unless they gain the mobility of flight through some future mutation, walking sticks will remain interesting, if esoteric, insects.

 

References

  1. Marshall, S. Insects, Their Natural History and Diversity, Firefly Books, Buffalo, New York, 2006, pp 58-64.
  2. Latreille P. Histoire Naturelle, Générale et Particulière des Crustacés et des Insectes. 1802-1805, Books 1–14, Paris. http://www.insecta.bio.spbu.ru/z/nom/Spectra.htm
  3. Milne, L. and Milne, M. National Audubon Field Guide to Insects and Spiders, Alfred A. Knopf, New York, 1980, pp 445-446.
  4. Harrington, L. and Sannino, D. “Diapheromera femorata”, Animal Diversity Web. at http://animaldiversity.org/accounts/Diapheromera_femorata / University of Michigan Museum of Zoology, 2011.
  5. http://phasmidstudygroup.org/phasmids
  6. Suetsugu, K. et al “Potential role of bird predation in the dispersal of otherwise flightless stick insects” (PDF). Ecology. 29 May 2018 Volume 99 (6) pp 1504–1506.
  7. Bian, X. et al “The swaying behavior of Extatosoma tiaratum : motion camouflage in a stick insect?” Behavioral Ecology, Volume 27, Issue 1, January 2016, pp 83–92 at https://academic.oup.com/beheco/article/27/1/83/1742619
  8. Dossey, A. et al. “Developmental and Geographical Variation in the Chemical Defense of the Walkingstick Insect Anisomorpha buprestoides“. Journal of Chemical Ecology. 10 April 2008 Volume 34 (5). pp 584–590.
  9. Pys.Org University of Cambridge “How stick insects honed friction to grip without sticking” 19 February 2014 at https://phys.org/news/2014-02-insects-honed-friction.html
  10. Dean, J. et al. “Control of Walking in the Stick Insect: From Behavior and Physiology to Modeling”. Autonomous Robots. November 1999, Volume 7 (3) pp 271–288.

11. Baker, E. “The worldwide status of stick insects (Insecta: Phasmida) as pests of      agriculture and forestry, with a generalized theory of phasmid outbreaks”. Agriculture and Food Security. 1 December 2015 Volume 4 (22) at https://agricultureandfoodsecurity.biomedcentral.com/articles/10.1186/s40066-015-0040-6

One thought on “Walkingstick

  1. A plague of swarming walking sticks!?! Now that would be newsworthy! And I wouldn’t doubt it with everything else going on in the world. Great detailed blog on this cool creature. I learned quite a bit, although I’m still pretty sure they arrived here from Middle Earth through the Wardrobe door accidentally left open (to mix my two fave fiction authors). William

    Like

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s