The Nature Writers of Texas

The best nature writing from the newspaper, magazine, blog and book authors of the Lone Star State . . .

Friday, June 15, 2001

Invertebrates 7
Pseudoscorpions are rarely noticed, except by kids on a field trip
Burr Williams, Midland Reporter Telegram, 6-15-01

At the Sibley Nature Center second graders found some unusual critters. To teach observational skills I often have students look for insects. I expected butterflies, grasshoppers, and ladybugs.

The second graders did indeed catch ladybugs. They also found about ten species of creatures that live under rocks, piles of brush, dead mesquite trunks, and trash. Pillbugs (isopods) appeared in every child's collecting jar. The second most common "bug" were brown shield beetles (stinkbugs). Why were they hiding in the dark? Normally they are on mesquite blooms and young mesquite beans.

Earthworms and june beetle grubs came next, in every third jar. Various beetles that nectar or oviposit in flowers outnumbered the ladybugs. Ladybug larvae were also plentiful as well.

The most bizarre creatures were pseudoscorpions, tiny arthropods with the pincers of scorpions, but no tail at all. Three pseudoscorpions can fit on the little fingernail of an adult human! After the children left I investigated all of the field guides at Sibley for information about them. At first I could find no information about them, other than one drawing with a three line description. The books mentioned that they "are found under bark, debris, and stones." Luckily for my search, we collect old natural history books (including college texts) and in a 1959 invertebrate zoology text more information surfaced.

Pseudoscorpions are cosmopolitan, worldwide, and reasonably easy to find (if you look for them!) At only 4 to 8 millimeters in size, they are easily overlooked. Their pedipalps (the scorpion-like pincers) have glands with a poison that dissolves the internal tissues of their prey (tiny soil mites that are no bigger than a dot from a ball point pen).

After poisoning and immobilizing its prey, the pseudoscorpion rips open the exoskelton and inserts its chelicerae (fangs). Imagine the fangs as miniature straws for sipping the delicate ambrosia of dissolved mite guts! As they eat, their pedipalps wave about in a defensive matter above their head. Like crawdads, pseudoscorpions are fleetest when backing up. Doing a 180 degree turn is instanteous. Pseudoscorpions constantly groom, and under a powerful microscope a person can see the "spit" it uses on its tactile hairs.

Pseudoscorpions breathe through tracheal spiracles on the abdomen, but excrete through glands on the third pair of legs. Such a creature does not have an anus! Invertebrate physiology can be bizarre -- for example, do not forget that true scorpions "hear" with their legs, the tiny "tactile hairs" registering sound vibrations that alerts the functionally blind scorpion to the presence of potential prey.

Other sense organs pseudoscorpions have in common with other Arachnida are indirect eyes and slit sense organs. Slit sense organs are narrow slits on the exoskeleton. Some are chemoreceptors. Some are kinesthetic organs, reacting to air pressure and humidity. Chemoreceptor smell organs are on the pedipalps.

Indirect eyes do not form images, but just register light and dark. Their structure is different from eyes. Instead of light hitting photo-receptor cells, the light first hits a tapetum (a membrane) which reflects the light to photoreceptor cells.

Pseudoscorpions perform a mating dance, that ends when the male deposits a spermatophore. In the males of a few species strange horns are purposefully extended from the back, releasing strong pheromones to attract a female. Each species performs a different mating ritual. The spermatophore is a globule of sperm on a stalk. He then grabs the female and pulls her until she is over the spermatophore, whereupon she gently settles down, her genital orifice enveloping the spermatophore. Females are "easy" to recognize, for they have a bright reflective zone on the abdomen behind their legs.

A month later the female builds a nest of silk-lined dead leaves. She lays eggs, but they remain attached to her in a sac at the genital opening. In some species of pseudoscorpions, females colonially brood. Others defend a territory marked by the smell of a pheromone. Up to fifty babies emerge from the sac, after first molting after hatching in the tiny silken sac. The young crawl to the nest and then remain through three more molts. Afterwards, the young disperse, traveling in search of new territory.

Pseudoscorpions often travel about by phoresy. They grab any passing invertebrate and hang on with their pincers. Some grab flying insects and travel great distances. Other just ride beetles for a few feet. One ubiquitous species (the storeroom pseudoscorpion) travels with humans as a familiar, accompanying explorers, migrants and refugees alike. Young pseudoscorpions reach maturity in a year, and individuals can live at least three years.

Ah yes, observational skills... I would grade that batch of youngsters as quite proficient, because they have seen pseudoscorpions and I bet most adults have not!

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