Desert Demon, the Solifuge

You can often tell what opinion the human race has of another animal by the number of aliases we assign to it. Just like the law-abiding citizens of the Old West, we tend to give nicknames to creatures we consider unsavory or downright mean. Such is the case with the arachnids of the order Solifugae. Depending on your geographic location, you may be familiar with them as “camel spiders,” “sun spiders,” “wind scorpions,” or “solpugids.” Not that their reputation is entirely undeserved, but these are fascinating and enigmatic invertebrates.

Two things are immediately apparent about a solifuge that are cause for alarm. The first is the animal’s sheer speed. They don’t get the name “wind-scorpion” for nothing, as they do “run like the wind,” and on only six of their eight legs. They are more agile than an NFL halfback, too. Attempts to capture them can leave you face down in the sand while at the same instant the solifuge is crossing the county line. I recall chasing a small one around the floor of an outdoor dining hall at a camp in eastern Oregon to the point of dizziness (me; the solifuge was fine).

The second thing about a solifuge you notice is the size of its jaws. Known to scientists as “chelicerae,” this pair of mouthparts can take up nearly one-third the body length of some species. They easily have the largest jaws for their size of any terrestrial animal (invertebrate at least). Each chelicera consists of a fixed upper portion and an articulated bottom joint combining to form the equivalent of a nutcracker or pair of pliers. Armed with teeth and filled with muscle, they are formidable weapons. These are non-venomous animals, but they do so much mechanical damage to their prey so quickly that they don’t need venom.

There are roughly one thousand species of Solifugae known globally, in 140 genera, and twelve families. Only two families (Eremobatidae and Ammotrechidae) occur in North America. There are about 100 species in the southwest U.S., half the North American total. The order reaches its zenith of diversity in the Middle East.

The source of much recent misinformation about “camel spiders” has come chiefly from United States servicemen and women stationed in the Persian Gulf, during both the first Gulf War in 1991 and the present conflict. Camel spiders are abundant, conspicuous arthropods there, but contrary to popular reports the animals do not reach the size of dinner plates (North American solpugids rarely exceed one inch in body length), they don’t literally run screaming across the dunes at 25 mph (they make no noise, and can only sprint at about 53 cm/second for short bursts), and they certainly don’t eat the stomachs of camels or the faces of sleeping soldiers. A widely-circulated image of a pair of camel spiders and a thorough de-bunking of the commentary that accompanies it can be found at Snopes.com.

Reality is usually far more interesting when it comes to arthropods, and camel spiders are no exception. They are highly adapted to the arid environments they thrive in. They are covered in fine hairs that help insulate them from the desert heat, with sparse, longer setae that act as sensors which help find prey by touch. There are also rows of sensory organs on the underside of the hind legs. These stubby, hammer-shaped appendages are called “racket organs” or “malleoli.” They are basically chemoreceptors, literally sniffing out information about the substrate the animal is traversing. Solpugids can even detect subterranean prey at a shallow depth, through the malleoli and tapping movements of the pedipalps.

The pedipalps, which in solpugids are easily mistaken for the first pair of legs, are long, stout, and tipped with “suctorial” organs that are useful to the animal when it needs to ascend vertical surfaces, or pin down struggling prey.

The first pair of legs, immediately behind the pedipalps, are very slender, and also used as sensors, waving constantly along with the pedipalps. A cornered camel spider may rear up, waving both pairs of appendages menacingly, and opening its jaws.

Given their overall aggressive nature, one wonders how camel spiders are able to reproduce without killing each other first. Indeed, the “attack phase” during courtship can easily be mistaken for an attempt at cannibalism by a male intent on mating. The female repels his advances, flees, or assumes a submissive posture. The male then grasps her mid-body and massages her with his jaws while stroking her with his pedipalps and first pair of legs. He may lift and carry her a short distance, or simply continue courting at the initial spot of contact. He eventually secretes a droplet of sperm from his genital opening, cradles it in his jaws, and uses his chelicerae to force the sperm into the female’s genital opening. Mating rituals vary among the different families of camel spiders, but the basics are consistent.

Look for camel spiders mostly at night. Some are active by day (hence “sun spider” as an alternative name), but most North American species are nocturnal. They can be seen around outdoor lights where they prey greedily on insects that have fallen to the ground. By day, flip over boards, flat stones, and cow patties, as solpugids often seek refuge under such debris. Be sure to return the object to its original position to afford shelter to other organisms. Some species actively excavate burrows where they weather the daytime heat.

Search for virtual solpugids online at informative websites like The Arachnid Order Solifugae, and Solpugid Productions. The Biology of Camel-Spiders, by Fred Punzo (Kluwer Academic Publishers, 1998) is the best book reference to date.

Given their frenetic lifestyle, solpugids are not recommended as pets. They need their entire adult lifespan to find mates and reproduce. Enjoy them where you find them; and be glad you aren’t a prey-size animal yourself.

Hilltopping

One of the more interesting types of insect behavior is something called “hilltopping.” While hiking along a ridge in Tucson Mountain Park on Friday, March 5, I witnessed this firsthand.

Hilltopping describes the mating strategy of male insects of some species that will fly to the highest point in their immediate landscape. This may be a mountain, a butte, or even a small hill, provided it is sufficiently elevated above the surrounding terrain.

Once at the summit, the insects pursue one of several courses of action. Males of some species will defend a small territory such as a perch on a boulder, shrub or tree that affords the best view for spotting approaching females of the species. Competing males will be driven off. Butterflies in the family Lycaenidae, such as the Great Purple Hairstreak, Atlides halesus, and the Gray Hairstreak, Strymon melinus, both pictured below, defend perches.

Another method employed by males is “lekking,” in which several males congregate in an area where they display to females. Females then choose what they perceive as the fittest male specimen and mate with him. A “lek” is the term for the “stage” on which the males “perform.”

Still another form of hilltopping is when a male “patrols” a route along the summit of a hill or ridge, hoping to intercept a passing female. Many butterflies in the family Pieridae exhibit this behavior, also known as “scramble competition polygeny.” Males don’t have territories, but will actively compete when two of them spy the same female. The battle may consist of spectacular vertical flights, the two males spiraling around each other until one ceases to ascend further, conceding defeat. On my own hike, I noticed several males of the Desert Orangetip, Anthocharis cethura patrolling along the ridge, together with what I believe were Sleepy Orange butterflies, Eurema nicippe. Also seen at the summit of the ridge was a male West Coast Lady, Vanessa annabella, that repeatedly returned to the same patch of ground.

Besides butterflies, major hilltopping insects include several species of Hymenoptera (bees, wasps, ants) and Diptera (true flies). Flesh flies in the family Sarcophagidae, such as the one pictured below, were present on my afternoon sojourn.

To see the most variety of species, I would recommend spending all day at the very top of a likely hill, ridge, or mountain. The fauna will change as the hours pass and conditions change. Here in the desert at least, some species hilltop for only a few hours in the morning, then disappear. Other species are present all day, or may be most active in the afternoon.

Especially at this time of year, early spring, hilltops may be the only places to find an abundance of insects. Not only will you see hilltopping species, but other species that come to prey on them. I’m sure that is the only reason that dragonfly was up there, perching on a cactus of all things.

NOTE: I highly recommend reading the works of Dr. John Alcock for more about hilltopping insects. His books include Sonoran Desert Spring and Sonoran Desert Summer, both being collections of outstanding literary essays.

Tarantula Hawks

I consider wasps my favorite insects. The honest truth is that I first took interest in them because nobody could call me a sissy for collecting an insect that could fight back! The more I learned, though, the more fascinated I became with their behaviors and sheer diversity. The solitary “spider wasps” of the family Pompilidae are one such example.

Here in Arizona, spider wasps reach the gargantuan proportions of the so-called “tarantula hawks” in the genera Pepsis and Hemipepsis. The largest females exceed two inches in body length. The standard appearance of these colorful insects is an iridescent blue-black body with bright orange wings, though some species have black wings with bluish reflections.

Thankfully, despite their size and high voltage sting, the females are generally placid, not easily aroused as they seek nectar at flowers. Milkweed is decidedly their favorite source of nectar, but I have also seen them on blooming creosote bush (as this female Pepsis chrysothemis demonstrates), blue palo verde, eucalyptus, and various mesquites and acacias. Females will also drink water at the edges of puddles and ponds.

Males are often more numerous at nectar sources than the females, and are recognized by slimmer bodies, longer antennae, and long, flattened hind legs. The males can also gather in multi-species “bachelor parties” during the hottest hours of the day, and also overnight and during inclement weather. They gather on certain trees in large, loose clusters. The odd female may join them, though mating does not seem to take place at such congregations. The males of at least some species perform a behavior called “hill-topping,” whereby they perch on trees at the highest point in the surrounding landscape, the better to spot and intercept passing females as they fly below.

Once mated, the females go about the business of earning their name. They search feverishly on foot, with flickering wings and quivering antennae, for the burrow of a tarantula spider. This behavior frequently takes place in the cooler morning, evening, or overnight hours, and is not often observed.

An occupied burrow discovered by the wasp usually results in the spider chasing the wasp back aboveground, and the battle is on. Initially it is a series of strategic maneuvers, each animal well aware of the agility and weapons the opponent possesses. Most of the time the female wasp manages to eventually sting the spider on its underside, striking a nerve center that renders the hairy arachnid paralyzed. She then sets about dragging the helpless victim to the nearest underground cavity, often choosing the spider’s own burrow. The tarantula hawk then lays a single egg on the still-living but comatose spider. The larva that hatches will then consume the spider as it grows ever larger during the long feast. The grub eventually pupates, and emerges as an adult wasp months later. Given the abundance of the wasps, there must be a very large tarantula population in the Sonoran Desert.

Tarantula hawks are diverse, too. I have personally collected seven species in Tucson alone: Hemipepsis ustulata, Pepsis grossa, P. chrysothemis, P. mexicana, P. thisbe, P. cerberus, and P. mildei.

My good friend Justin Schmidt, an outstanding research scientist, ranks tarantula hawks near the top of his “sting scale,” but has learned something very interesting. He concludes, both from personal experience and chemical analysis, that the venom of these wasps causes excruciating, short-term pain in humans (and likely other mammals), but does virtually no damage to tissues, nerves, or any bodily functions. You are in utter agony for about three minutes, and then you generally can resume normal activity.

Personally, I think it pays to heed the bright aposematic (“warning”) colors of these wasps, and to let them be. I’ve also learned that, once in my net, the wasps adopt a threatening posture with wings flared and abdomen curled, while liberating a distinctive fragrance at the same time.

My own fascination with these wasps only continues to grow, and I encourage everyone who encounters tarantula hawks to react calmly and enjoy watching them live out their lives. After all, few insects are so large and colorful.