Wednesday, August 31, 2011

Wasp Wednesday: Wasp Workshop

I recently had the privilege of leading a wasp identification workshop at the ultra-modern Eulett Center in Adams County, Ohio. The ten participants arrived Friday evening, August 26, at the nearby rustic lodge known as Rieveschl Chalet. Both facilities are run by the Cincinnati Museum Center, though The Nature Conservancy also occupies offices in the Eulett Center and the two organizations co-manage the Richard and Lucile Durrell Edge of Appalachia Preserve System.

All my “students” were enthusiastic and energetic in the field and in the lab. I gave an introductory lecture on selected wasp families at the Chalet on Friday night, and we looked forward to seeing actual living specimens in the rural and prairie habitats the next morning.

There are never any guarantees that one will actually find wasps just by looking, but my hosts Chris Bedel and MarkZloba and myself scouted out some nearby areas ahead of time on Friday. That paid off. The participants got to see the Katydid Wasp, Sphex nudus, nesting in the dirt floor of an old barn. We even witnessed one of the wasps fly in with her prey, a Carolina Leaf-rolling Cricket (Camptonotus carolinensis).

We also discovered caterpillars of the Catalpa Sphinx moth, Ceratomia catalpa, covered in the cocoons of the parasitic braconid wasp Cotesia congregate. These and other wasps we observed will be topics in future “Wasp Wednesday” posts, and images shared on my Flickr Photostream.

We convened at the Eulett Center for lunch, then went back out for a more field time at another location. There we collected cuckoo wasps (family Chrysididae), and observed a mating pair of thread-waisted wasps, Eremnophila aureonotata. Inside a barn we saw many old nests of the Pipe Organ Mud Dauber, Trypoxylon politum, and an active nest of the Northern Paper Wasp, Polistes fuscatus. When wasps were not to be found, the sharp eyes of the group spied many other insects including bizarre caterpillars like the “Monkey Slug.”

The late afternoon was spent attempting to learn the complicated anatomy of wasps, and introducing “keys” as a way to identify wasps to family, genus, and sometimes species. Traditional keys are dichotomous, meaning that they are composed of a series of couplets. One reads each couplet and decides which of the two lists of characters corresponds to their specimen, then proceeds to the next indicated couplet. Eventually, this process yields a name instead of another couplet. Interactive keys are a product of internet technology whereby the user checks boxes that correspond to their specimens, then hits the search button to whittle down the possibilities.

Chris Bedel, in addition to having a wealth of knowledge about the local flora and fauna, is a fabulous cook. Dinner on Saturday night was healthy and delicious: Pasta with artichoke hearts, cherry tomatoes, black olives, chicken, and spices, plus a mixed greens salad, garlic bread, and brownies and ice cream and strawberries for dessert choices.

After dinner we played a game I invented called “Wasp/Not Wasp,” whereby students view two images on one PowerPoint slide. They must determine which is the wasp, and what the other insect imposter is. I complicate matters by sometimes showing two wasps, or two non-wasps. The whole thing seemed to be a hit, and most of the time everybody got the answers right.

I concluded the evening with a short lecture on wasp sleeping behavior.

Sunday morning found us afield again, this time in the Lynx Prairie unit of the preserve system. Mark Zloba had set up an enormous malaise trap days before, to help secure wasp specimens. A malaise trap is a tent-like structure designed to intercept flying insects. The “bugs” then try to fly over the barrier. Instead they are funneled to the highest point (at both ends of the trap in this case) where they drop into a container with a killing agent. This trap is often one of the few ways one can capture certain kinds of Hymenoptera, and indeed the diversity just in the one sample was amazing.

Before we knew it, the morning was over and it was lunchtime. After a leisurely meal, the students slowly departed to resume their normal lives, as if having an interest in nature and insects means you have a “normal” life. I had a great time seeing friends I haven’t seen in decades, meeting Facebook friends, and making new friends that I know will be lifelong colleagues.

Special thanks go to Stephen Pelikan who drove me out to the Eulett Center from the Cincinnati International Airport. Chris and Mark were exceptional hosts that made me feel instantly at home. They made modest demands, and allowed me ample time to explore on my own.

Do consider attending Advanced Naturalist Workshops like this, or simply visiting the Eulett Center on your own. They always welcome visiting naturalists, researchers and scientists who give them ample warning.

Sunday, August 28, 2011

Spider Sunday: Crevice Weavers

Perhaps no spiders are more often misidentified than the “crevice weavers” in the genus Kukulcania, family Filistatidae. They are quite common throughout the southern United States, where they are frequently confused with everything from brown recluse spiders to tarantulas.

There are currently five species and one subspecies of Kukulcania recognized in the U.S. Here in Arizona we have both K. arizonica and K. hibernalis (the “Southern House Spider”). Indeed, houses are often where you find them. Look for their sprawling, lacy webs issuing from the exterior of window frames, under the eaves, and similar situations.

Filistatids in general belong to a larger group of arachnids called “cribellate” spiders. These spiders possess an extra silk-spinning organ called a cribellum, and a comb on the outside of the metatarsal segment of each fourth leg. This row of short, stout, curved hairs is called a calamistrum and is used to “fluff” the silk that issues from the plate-like cribellum. This “carding” of silk is accomplished by very rapid vibration of the fourth leg as it rests on the third leg. This is in contrast to most other cribellate spiders that employ a slow rocking motion. The web that results is therefore not sticky, but the threads so random that they easily entangle potential prey.

The odd web is just the beginning of weird for Kukulcania crevice weavers. The genders differ so dramatically in appearance that one can be forgiven for assuming they are different species. Mature females are a lovely, velvety black or dark gray in color, with a body shape and lumpy eye arrangement reminiscent of a tarantula. Mature males are beige or pale brown in color, with small bodies and extremely long legs. They are mistaken for brown recluse spiders much of the time, but note the differences. Male Kukulcania have eight eyes, grouped atop the crown of the head as they are in the female. Recluse spiders have only six eyes, grouped in three pairs across the front edge of the carapace. Also notice the extremely long, elbowed pedipalps of the male crevice weaver. No recluse spider is that….well,…well-endowed.

We’re not finished, it gets stranger. All filistatids known thusfar have their jaws (chelicerae) fused. That means that each one of the pair is incapable of moving independently, like in most other spiders. They are also built to lose their legs if necessary to escape a predator. Actually, many kinds of spiders possess the ability to break off parts of their legs without losing much blood in the process. This phenomenon is known as “autospasy,” and in the case of crevice weavers the weak joint is at the juncture of the tibia and patella segments.

Interestingly, female Kukulcania continue to molt after they become adults. Mygalomorph spiders (tarantulas) are the only other spiders known to do this. It seems to be related to longevity: both spiders take a long time to reach sexual maturity, and are capable of living several years. Like the cellar spiders I addressed in an earlier blog entry, filistatids are “haplogynes,” meaning that the females have largely unmodified, if not non-existent external genitalia. The paired openings to her reproductive tract are under what is more-or-less a slot-like orifice.

The best time to observe Kukulcania species is after dark. Females will venture to the lip of their retreat in anticipation of prey. Males will be wandering in search of love and romance. See if you can’t find one or both of the sexes on the exterior of your own home, garage, shed, or barn.

Wednesday, August 24, 2011

Wasp Wednesday: Pimpla sanguinipes

I should know by now that nothing comes easy in entomology. Here I figured I could just whip out a short piece on an ichneumon wasp I imaged in southern California back on March 26, but no-o-o-o, it looks like even the name I have for it, courtesy of Bob Carlson via, might be outdated. Meet Pimpla sanguinipes, or whatever alias it goes by these days.

Actually, it looks like the appropriate name for this member of the family Ichneumonidae is Coccygomimus sanguinipes. Bugguide lists Coccygomimus as a “synonym” for Pimpla, so maybe it still *is* Pimpla.

The wasp I found at the salt marsh preserve in Carpenteria is further defined by the subspecies name erythropus. I do wonder what all these references to “blood” mean, though the wasp does have reddish-orange legs.

Dr. Carlson pointed out the complexity of all this in a personal communication with Vasile Bagazzoli, a volunteer editor at Bugguide:

"erythropus and sanguinipes might really be two separate species, judging from the fact that they occupy different habitats: sanguinipes in arid areas and erythropus in forested areas. Townes even had differences in punctation for the two. His concept of subspecies was very dubious, and he did not strictly apply the concept in a geographical sense and named many sympatric subspecies, many of which I relegated to synonymy in the 1979 catalog. But this case was different, and I might have just elevated the two taxa to species level but did not. I don't remember if I had a reason for not doing it, but maybe it was because there was a dearth of specimens in the National Collection from which I could form an opinion."

Dr. Carlson is retired from the Systematic Entomology Laboratory at the National Museum of Natural History in Washington, DC. He also studied under the “grandfather” of ichneumon taxonomy, Henry T. Townes.

Ok, back to the actual wasp, and what *it* does for a living. The species ranges west of the Rocky Mountains from Idaho to New Mexico and west to the Pacific coast including British Columbia and southern California.

Females seek out moth caterpillars in which to lay their eggs. Their hosts include tent caterpillars (Malacosoma spp.), buck moths (Hemileuca sp.), the Virginia Tiger Moth (Diacrisia virginica), the Douglas Fir Tussock Moth (Orgyia pseudotsugata), Western Tussock Moth (O. vetusta), Coddling Moth (Cydia pomonella), Genista Broom Moth (Uresiphita reversalis), Gooseberry Fruitworm (Zophodia convolutella), Barberry Geometer (Coryphista meadii), the Oak Looper (Lambdina punctata), and two other geometer moths (Eucaterva variaria and Prochoerodes forficaria). Several of these moths are abominable pests, so this generalist parasite is a welcome friend to agriculture and forestry.

The physical dimensions of this wasp don’t fit its superhero reputation. Females range between 10.5-12.5 millimeters in length, males around 8.5 mm. The black body, orange legs, and short, stout ovipositor help to identify the species fairly easily. We just can’t decide what name to call it.

Sunday, August 21, 2011

Spider Sunday: The Cross Spider

’Tis late summer, now, in most parts of North America, anyway, and orb weaver spiders are becoming more conspicuous as they mature into large adult specimens and spin bigger webs (soon to be revealed by falling autumn foliage). Among the most abundant of these spinners is the “Cross Spider,” Araneus diadematus.

The Cross Spider is a European immigrant, just like most of us human residents of the U.S. and Canada, so the species feels most at home in northern climes. It is recorded from Newfoundland and Nova Scotia to British Columbia and south to northern California, Michigan, Ohio, Pennsylvania, New York and Rhode Island. It is plenty accustomed to people, too, so it is a regular occupant of gardens and yards in urban areas.

Araneus diadematus gets its popular English name not from an angry disposition, but because it usually sports silvery-white dots that form the pattern of a traditional Christian cross on its abdomen. This is a relatively consistent marking, but as with most orb weavers, there can be exceptions. The spiders usually hang head-down in the very center (hub) of their webs, but sometimes an individual spider may be more reclusive, and connect herself to the web via a bundle of “signal threads” that run from the hub to her hiding place in a rolled-up leaf or other nearby retreat.

The reaction of homeowners to the presence of this and other species of orb weavers runs the gamut from curiosity to consternation. No species of orb weaver is known to be dangerously venomous to people or pets, so there is no reason to fear them. The spiders themselves will literally shake at the close approach of a person or other large animal, vibrating their web and no doubt startling the inquisitive visitor. Should that tactic fail, most orb weavers drop abruptly from their web, anchoring a dragline to the hub so they can climb back up once danger passes.

This species happens to include some real celebrities. No, seriously. “Anita” and “Arabella” were two female Cross Spiders sent into space on Skylab 3 in 1973 to study the effects of zero gravity on web construction. Prior to that, several specimens were used as guinea pigs in the study of how psychoactive drugs affect spiders’ ability to spin webs. Those experiments were first conducted by a German scientist beginning in 1948, then repeated by NASA scientists in 1984. For an absolutely hilarious send-up of that research, you must see ”The Wood Spider” video on YouTube. I take no responsibility for laughter-induced fatalities.

An adult female Cross Spider has an average body length of about 13 millimeters, though gravid females certainly appear larger. Like the story of Charlotte’s Web, each spider’s life from egg to adult spans only a year. Enjoy their handiwork and pest-controlling services while you can.

Thursday, August 18, 2011

Welcome Birds & Blooms Readers!

I am honored to have this blog profiled online by Birds & Blooms magazine as their "blog of the week" today.

Please let me know what kind of entries you would like to see here in the coming months. Perhaps you want to know more about certain kinds of butterflies, or have always wondered what that strange animal is that behaves like a hummingbird but looks like it has a lobster tail (psst: it is a sphinx moth).

Your garden or backyard is a fascinating urban habitat for many insects and arachnids that you probably never see, but that play a significant role in pollinating your plants, keeping pest insects in check, and making rich compost for you to use in fertilizing your flowerbeds.

I look forward to making this your "go-to blog" for solving your garden "bug" mysteries. Thank you for visiting, I hope you will come back again.

Wednesday, August 17, 2011

Wasp Wednesday: Beewolves

I was mistaken the first time I thought I saw a beewolf. It turned out that I was seeing a cuckoo bee in the genus Nomada. The real deal, the beewolves of the wasp genus Philanthus, are truly complex for solitary wasps. The most common species in North America is P. gibbosus, found from the Atlantic to the Pacific, and Canada to Mexico.

The European beewolf, Philanthus triangulum, which preys on honeybees, gained fame as one of the research subjects of Nikolaas Tinbergen, a Dutch animal behaviorist (ethologist) who shared the 1973 Nobel Prize in Physiology or Medicine. Tinbergen coined the name “beewolf,” and demonstrated that the female wasp was able to locate the buried entrance to her burrow by using subtle landmarks as cues. Our P. gibbosus does so as well. Meanwhile, we can’t remember where we parked our car.

A single female P. gibbosus usually excavates in bare, coarse sand, sandy-clay, or sandy loam soil, often removing small pebbles as she goes. A typical burrow extends for 15-24 centimeters, beginning as an oblique tunnel that eventually becomes horizontal. Some burrows may reach a length of a meter or more, but these may represent expansion of an existing nest by the succeeding generation. Individual cells at the end of very short tunnels radiate from the main burrow along its length.

Beewolves earn that name. The females target small bees, and even other wasps, as food for their larval offspring. Not all beewolves you see around flowers are there for nectar. The females will actively stalk bees that are busy gathering nectar and pollen themselves. A distracted bee could be a dead (well, paralyzed) bee.

Philanthus gibbosus is a well-studied insect, and has been recorded taking 35 species of bees and wasps as prey. The majority of victims are “sweat bees” in the family Halictidae, but also yellow-faced bees in the family Colletidae; and a couple members of the family Andrenidae. There are also records of aphid wasps in the family Crabronidae, genus Pemphredon falling prey.

A victim is stung immediately between its front legs, disabling a nerve center and rendering the bee paralyzed. The wasp then carries the bee beneath it, held in the wasp’s middle legs. It takes several bees to feed one larval beewolf wasp.

Male beewolves are highly territorial, perching on low twigs or leaves where they can intercept a female or chase off a competing male. Males also scent mark twigs and foliage, employing brushes of hairs on the underside of the abdomen to “paint” an odor that communicates their individual ownership of a small area. Their possession of a territory is usually short-lived, however, as competing males frequently displace resident individuals.

Interestingly, Philanthus gibbosus is known to engage in burrow sharing, whereby sibling females may occupy their birth nest for a short time before dispersing. There is also evidence that sibling females may expand their birth nest for at least one generation, depending on the condition of the burrow. This tendency toward sociality might be more of a population-level phenomenon than a behavior typical of the entire species.

One or more males may also spend nights, and periods of inclement weather, in the burrows of females. Males tend to be return to the same burrow each night.

Invincible as they might appear, beewolves are not immune to their own enemies. P. gibbosus is plagued by a host of adversaries, especially “satellite flies” in the genera Metopia, Senotainia, and Hilarella. The female flies follow female beewolves back to their nests, then deposit larvae at the lip of the entrance. The larvae crawl down the tunnel and become parasites of the larval wasps. The velvet ant Dasymutilla nigripes is likely a parasite as well; and the cuckoo wasp Hedychrydium dimidiatum is another suspect parasite. Adult beewolves can be killed by crab spiders lurking in flowers, and by robber flies that take advantage of the slow flight of prey-laden female wasps.

Beewolves are members of the family Crabronidae, and the subfamily Philanthinae. Worldwide there are about 136 species, about 30 of which occur in North America. The majority are rather diminutive insects, only ten millimeters or less in body length. They usually sport ornate patterns of black and yellow or white, the males with all-pale faces and no “tarsal rake,” the spines on the front legs that help the females dig.

Source: Evans, Howard E. and Kevin M. O’Neill. 1988. The Natural History and Behavior of North American Beewolves. Ithaca, NY: Comstock Publishing Associates (Cornell University Press). 278 pp.

Sunday, August 14, 2011

Spider Sunday: Arizona Blonde Tarantula

Without a doubt, one of the iconic animals of the Sonoran Desert is the tarantula. What would a Western, or desert horror movie be without a shot of one of these giant spiders moving leisurely across the landscape? Maybe a tumbleweed, too, though a Russian Thistle moves faster than a strolling tarantula.

Tarantulas belong to the family Theraphosidae, and only the genus Aphonopelma is native to North America. There are 54 currently recognized species north of Mexico, and determination to species level is complicated by the fact that these are primitive animals in the evolutionary sense, with simple genitalia that don’t allow for easy comparison. That said, the most common species here in southern Arizona is the “Arizona Blonde,” Aphonopelma chalcodes.

It is mature male spiders that are most often seen crossing roads, especially at night, during the summer monsoon season. The male is easily recognized by his relatively lanky appearance, with a smaller abdomen and longer legs than the female. His front legs also have a spur on the underside of the tibial segment. The spurs are used to hold the female’s fangs at bay during mating. His coloration also differs from that of the female. His legs are entirely black, while the female has black femur segments with the remaining leg segments brown.

Both sexes live in burrows in the ground. Females rarely venture out, lest they become even more vulnerable to tarantula hawk wasps, and other predators such as coatimundi. An occupied burrow will have a thin curtain of silk over the entrance during the daytime. The spider will sit at the lip of its tunnel during the night, the better to ambush any unsuspecting insect or other small animal that comes within striking range. During the winter months the spiders plug their burrows with soil. Fat reserves sustain the spider during those lean months.

Tarantulas begin life as surprisingly small spiders, like the one below. Margarethe Brummermann found this specimen under a cow patty near Sunsites in the Dragoon Mountains. It takes a minimum of seven years for a tarantula to reach sexual maturity, and that is for captive specimens that receive regular meals and are otherwise provided with optimal conditions.

The black patch on the abdomen of the spiderling indicates special hairs that tarantulas use in self-defense. Rather than striking at an adversary with their fangs, our North American species literally kick themselves in the rear. Be careful if you see a tarantula doing this. It is dislodging short, microscopically barbed hairs that easily become airborne. These hairs, if they contact mucous membranes of the nose or eyes, cause extreme irritation. Some people may also have an allergic reaction, including a rash or other inflammation of the skin. Even a molt (shed “skin”) of a tarantula can bring about an allergic reaction in those people who are susceptible.

Tarantulas are among the very few spiders that continue molting after they reach adulthood. This process helps replenish those defensive hairs, and shed mites, other parasites, and dirt. Captive female spiders like the one shown above have been known to live as long as thirty years, but “wild” specimens likely don’t survive nearly as long. Males, after reaching adulthood, abandon their burrows in search of mates. After successfully breeding they usually die.

Members of the genus Aphonopelma are collectively distributed west of the Mississippi River and as far north as Missouri, Colorado, Utah, Nevada, and the southern two-thirds of California.

Friday, August 12, 2011

Life Cycle of a Leaf Beetle

Leaf beetles of the family Chrysomelidae seem to be particularly diverse here in southern Arizona. One of the most common species is Leptinotarsa lineolata, a relative of the familiar Colorado Potato Beetle. A visit to one arm of the Sonoita Creek State Natural Area in Rio Rico, Arizona on July 31 found this beetle in profusion, and all life stages but the pupa present.

The adult beetles are about 7-8 millimeters long, but colorful. The head and thorax are metallic green, while the elytra (wing covers) are ivory with black streaks. The beetles and their larvae feed exclusively on Burrobrush (Hymenoclea monogyra). The beetles become numerous after the onset of monsoon rains, usually in early or mid-July. They quickly set about reproducing.

Once mated, the female beetles lay eggs in rows along the edge of a leaf near its tip. The large numbers of eggs and their bright yellow color make them very conspicuous.

Larvae hatch from the eggs in about seven days and commence feeding on the burrobrush leaves. The grubs go through four instars (intervals between molts) before pupating. I assume the large larvae I imaged are fourth instar. The larvae probably sequester the toxins in the plant to use in their own chemical self-defense. That would explain the bright black, white and yellow markings reminiscent of Monarch butterfly caterpillars. Such toxic insects tend to advertise their unpalatable nature through “warning colors,” a phenomenon known as aposematism.

Well, no wonder I couldn’t find the pupal stage. An unpublished study by Ross Arnett and Richard L. Jacques in 1971 followed L. lineolata through its entire life cycle, and they found that pupation occurs in the soil. Adult beetles emerged from the pupae roughly ten days later.

According to Michael Plagens’ account of this species, the beetles are capitalizing on disturbed riparian zone habitats where their host plant thrives in the wake of cattle grazing. Burrobrush is toxic to mammals, so livestock leaves it alone while eating competing vegetation.

Leptinotarsa lineolata ranges from Texas to California, and south into Mexico. There is no missing this insect if you are at all observant and in the right habitat at the right time. Check them out!

Source: Jacques, Richard L., Jr. 1988. The Potato Beetles: the Genus Leptinotarsa in North America (Flora & Fauna Handbook No. 3. Gainesville, Florida: Flora & Fauna Publications. 147 pp.

Wednesday, August 10, 2011

Wasp Wednesday: Western Cicada Killer

According to my friend Margarethe Brummermann, this has been the year for cicada killer wasps in southern Arizona. I have spent much of my summer traveling to other parts of the U.S. though, so I’ve missed a lot of the action around Tucson. Plus, cicada killer abundance tends to be very localized. Couple this with the spring drought and the fact that I live in the city itself where there is little greenspace, and it is no wonder I have seen few insects. However, a visit to Tohono Chul Park back on June 17 did allow me a glimpse of a female Western Cicada Killer, Sphecius grandis.

As its scientific name implies, this is a large, magnificent animal. Separating this species from the nearly identical Pacific Cicada Killer is not easy, but I have observed a semi-reliable trait to tell the two apart. The abdomen of the Western Cicada Killer has pale bands along the entirety of its length, whereas the Pacific Cicada Killer has the bands terminating well before the tip of the abdomen. The Pacific Cicada Killer also tends to have much more of a contrast between the dark reddish brown and pale yellow coloration.

The only truly reliable means of discerning the two species is by the punctation of the first two abdominal segments, as illustrated in the online key to cicada killers by Charles Holliday. This requires extremely close examination of the wasp, though, and a live specimen won’t hold still for that.

Sphecius grandis ranges throughout much of the western United States, from Kansas and Nebraska south to Texas and west through Nevada to California, Idaho, Oregon, and southeast Washington. I am hoping that I can find this species in Colorado next summer.

Like the majority of cicada killer species, the Western Cicada Killer likes to nest near other members of its kind. This was graphically demonstrated to me last summer when John Rhodes of the Sonoran Arthropod Studies Institute invited me to his neighborhood where a number of S. grandis were nesting in the bank of a dry wash. Ironically, the date was June 17, 2010, exactly one year to the day prior to my next sighting at Tohono Chul. John’s place sits in the lower foothills of the Santa Catalina Mountains and the male “Cactus Dodger” cicadas, Cacama valvata, were singing loudly from seemingly every branch of mesquite and palo verde trees.

The one thing the cicadas could not dodge were the cicada killers. Just standing in the wash and looking up and down it I could see female wasps flying in with their prey. It is a jaw-dropping sight to see a large wasp, carrying an equally large prey insect, droning along like it has no burden at all. That would be like you or I running full speed while carrying a sofa.

The wasp has perhaps her greatest challenge in shoveling her prize through the entrance to her burrow. Once inside she will cart the cicada, paralyzed but alive, into a previously-prepared cell near the bottom of the tunnel. This will be the cicada’s death chamber, as it is destined to become food for one of the female wasp’s larval offspring. She will cache one or two cicadas in each of the several underground cells before sealing the entire burrow and beginning a new nest.

Cicada killers do visit flowers to refuel on nectar, but they are not commonly seen on blossoms. They will also sip sap oozing from wounded trees and shrubs. Male cicada killers are territorial, and will perch where they can oversee a nesting aggregation of females. They aggressively challenge all intruders but, since they have no stingers, it is all bark and no bite.

Enjoy watching these wasps if you have them on your property. Their nesting activity lasts at most about a month, and as long as you don’t walk barefoot through the area they occupy, risking stepping on a female wasp, you need not fear.

Sunday, August 7, 2011

Spider Sunday: Desertshrub Spiders

The deserts of the American southwest are harsh, demanding environments. Consequently, they host a unique spider fauna. Among the more common of these endemic arachnids are the desertshrub spiders in the family Diguetidae. They might easily be mistaken for funnelweb weavers or labyrinth spiders at first glance, but with a little practice one can recognize both the spider and its web fairly easily.

Diguetia is the only genus in North America, with seven species occurring north of Mexico. They have collectively been recorded from Utah and Colorado to California, and Texas. D. canities is the most widely distributed species, and has been separated into three subspecies.

These are mid-size spiders, mature individuals topping out at 5-10 millimeters in body length. Their color and pattern is very reminiscent of spiders in the family Agelenidae, and their webs might reinforce that misconception. The webs are horizontal sheets, slightly dome-shaped, with a central tubular retreat in the vertical plane above the sheet. There is also a tangle of threads on all sides of the sheet, above and below. The spider travels on the underside of the sheet to attack prey caught in the tangled threads.

My experience has found that populations of these spiders are rather localized: where you find one web you will find several others nearby. They are mostly located less than two feet off the ground, frequently amid the spiny pads of prickly pear cacti (Optuntia spp.). The retreats are shrouded in plant debris, remains of prey insects, and may include the camouflaged egg sacs of female spiders. These conical “houses” are often the first thing you notice, before the rest of the web comes into focus.

The desertshrub spiders are relatively primitive in the evolutionary sense, and are classified as “haplogynes.” This means the female’s genital opening lacks a hardened plate, and the male’s pedipalps (used in mating) are rather simple. Tarantulas and cellar spiders are more familiar examples of haplogyne spiders.

Next time you are out in the desert, keep an eye out for the webs of desertshrub spiders. The snares are roughly four to six inches in diameter, so are not too easily overlooked.

Wednesday, August 3, 2011

Wasp Wednesday: Black & Yellow Mud Dauber

Few wasps in North America are as ubiquitous and conspicuous as the familiar Black and Yellow Mud Dauber, Sceliphron caementarium. Even when the wasps themselves are not obvious, their mud nests are often are. Homeowners consider the amorphous clod-like nests to be eyesores, if not a hazard when the female wasp is at work on one. Take a closer look, and you will be amazed by the wasp and her legacy of insect lodging.

Sceliphron caementarium has an extraordinarily widespread geographic distribution, and varies markedly in its appearance from north to south. Northern specimens tend to be mostly black, with dark, violaceous wings. Western specimens tend to be more yellow, with yellow wings.

Not only does this species occur from Canada to Central America and Peru, it has been introduced to the West Indies, Bermuda, Hawaii, the Marshall Islands, Cook Islands, Society Islands, New Caledonia, Australia, Fiji, Samoa, France, and Germany. Once again, international commerce has played a role in the wasp’s seemingly jet-setting lifestyle. Females may build nests inside of shipping containers that then make landfall at various ports.

Why does the Black and Yellow Mud Dauber succeed wherever it finds itself? The female wasps are generalist hunters of spiders, and spiders can be found everywhere as well. The wasp first finds a place to build her nest, usually in a sheltered situation such as beneath a rock overhang. Once she establishes a suitable location, she flies off to a patch of mud and rolls up a ball with her jaws and front legs. She then flies off with this pea-sized load and plasters it to the construction site she chose earlier.

Once she has laid down a layer of mud on the surface of the substrate, she begins fashioning a three-dimensional cell. Each subsequent load of mud makes a “rib” that reaches across half the span of the cell she is building. The arcs from either side meet at the middle, dovetailing nicely with each other and the adjacent ribs on the same side. Once the cell is completed, leaving an opening at one end, she may plaster more mud over the ribs, obliterating the initial artistic appearance of the cell.

A finished cell is then provisioned with paralyzed spiders captured by the wasp. She uses her sting to subdue her prey, but does not kill it. A comatose spider won’t spoil before her larval offspring has a chance to feed on it. Many spiders are harvested and packed into the cell. The wasp usually lays an egg on the first spider to go into the cell. Orb weavers and crab spiders seem to make up the bulk of prey, but the wasps are opportunists and will not hesitate to take other kinds of spiders.

Most female mud daubers make more than one cell, the next one placed immediately beside the previous one. The whole series of cells may then be covered in mud, making it look like some mischievous teenager hurled a clod onto a wall. Not very pretty, but an effective fortress against parasites.

And do mud daubers ever have parasites! Chief among them are the cuckoo wasps of the family Chrysididae. These little metallic jewels wait for the hard-working mud dauber to go off on the hunt again, then enter the nest and lay an egg inside. Should the mud dauber catch the intruder, the cuckoo wasp roles into a tight ball, its dense cuticle impervious to the stings and bites of its irate host. Chrysis angolensis is an African species now well-established in North America, but there are native species that attack the mud dauber, too.

Other enemies of the Black and Yellow Mud Dauber include ichneumon wasps in the genera Acroricnus and Osprynchotus, as well as chalcid wasps in the genus Melittobia, velvet ants in the genus Sphaeropthalma, and bee flies in the genus Anthrax. “Satellite flies” in the family Sarcophagidae, genus Amobia, are also a threat.

Look at an old mud dauber nest and you can decipher what happened to the offspring. A large hole chewed out at the end of a cell means an adult mud dauber successfully emerged. Small holes along the length of the cell mean some kind of parasite came out instead.

Once the new generation of wasps have emerged, the nests can serve as shelters for other insects or, ironically, retreats for spiders. Other wasps use the ready-made mud cavities for their own nests. Mason wasps like Ancistrocerus tuberculocephalus will partition an old mud dauber cell into at least two cells of its own.

Carpet beetle larvae will clean up any old spider and insect parts left over from all the tenants, and eventually the old nest may weather away, or be torn up by a mammal hoping for a juicy wasp larva snack.

It is impossible to address the many fascinating aspects of mud dauber biology in a single blog, so expect me to revisit this species in the future. Meanwhile, go see if any mud daubers have been busy on your own property. Chances are, they have been.