A Checklist of Little Monsters: Don’t Read This If You’re an Arachnophobe!

by H. Bruce Rinker, Ph.D.

For us scientists, checklists of flora and fauna are all-important tools that indicate our proficiency in the natural world.  Ornithologists often keep lists of birds they’ve seen or heard in the wild.  Entomologists sometimes keep lists of surveyed beetles.  Botanists may use their lists to hunt down rare orchids.  A Cuban colleague travels the world to document spiders – even looking in museum drawers to see if any have been misidentified in old scientific collections.  A first step in natural resource management is to know what you have available to manage!  Such lists verify the presence of species in the region: an inventory, if you will, of local biodiversity.

I, too, keep such lists.  One of the oddest is what I call my checklist of “little monsters.”

Before I tell you what’s on my list, let me try to answer a common question about arthropod abundance and diversity.  Why are there so many bugs and other creepy-crawlies?  In other words, why isn’t the world filled with eagles, pandas, tigers, and whales?  You know, the sexy backboned megafauna.

We have many hypotheses to explain high arthropod abundance and diversity, especially in the tropics: for example, long-term environmental stability, complex ecosystem structure, and powerful energetics.  Speculation about the basis for this invertebrate diversity may narrow, however, into one simple explanation – their small size and accompanying small niches.

Insects and their kin are necessarily small because of two physiological constraints on their body size.  First, their brittle exoskeletons can support only a limited mass of muscle before collapsing inward.  Second, unlike the circulatory system of a vertebrate that delivers both nutriment and oxygen throughout the body, the circulatory systems of insects carry only food molecules.  Oxygen is delivered separately and directly to cells via a complex system of minute tracheae.  The shell of an elephant-sized insect would shatter from gravity and stress, and its innermost tissues would die of oxygen starvation as soon as the animal exerted itself.  These two constraints keep such invertebrates small in stature despite their vast collective ecological importance.

The upper limit for body size of terrestrial arthropods seems to have been reached by the world’s largest species of beetle (titan longhorn), moth (white witch), wasp (tarantula hawk), scorpion (emperor scorpion), and spider (Goliath birdeater) – all found in Amazonia except for the African scorpion.  In his 1992 book, The Diversity of Life, Pulitzer prize-winning scientist E.O. Wilson wrote: “We don’t know with certainty why invertebrates are so diverse, but a community held opinion is that the key is their small size.”  Their Lilliputian size, allowing insects to divide the environment into little domains where specialists can co-exist, seems to have guaranteed them broad ecological and evolutionary success in the tropics – and in just about every other global environment.  Because of that ubiquitous success, Wilson recognized creepy-crawlies as “the little things that run life.”

With all this as background, let’s take a look at my checklist of little monsters.

One of my favorites is the tailless whip scorpion.  I’ve found them throughout Latin America.  Because of their nightmarish appearance, I thank the Good Lord that I’m not a bug.  Superficially resembling spiders, they run around at night on three pairs of skinny legs with a fourth pair as long feelers near their mouthparts.  With their flattened bodies, they hide in narrow crevices, under tree bark, and in rock fissures to ambush their prey with strong raptorial pedipalps armed with nasty-looking spines.  Ugly, but harmless.

Another organism on my checklist is that Goliath birdeater, a tarantula the size of a dinner plate in its adult form.  When these were first reported by 19th century naturalists, Europeans thought the accounts a grotesque fabrication.  Hairy and heavily fanged, these spectacular creatures explore the forests of Amazonia, looking for prey: insects, frogs, lizards, and –yes, occasionally – small birds and mammals.  On one of my expeditions into South America, a student, incurably arachnophobic, found one of these crawling timidly on his back while canoeing through thick riverbank vegetation.  I don’t know which was more frightened, my student or the spider.  Gently, I prodded it off his back and onto the vines that surrounded us.  Ugly, but MOSTLY harmless.

Other little monsters on my checklist include the human flesh-eating botfly and the infamous candirú, all of which I’ve encountered in the tropics.  Both are nasty parasites.  The former infected me years ago in the cloud forests of Ecuador but, using a native remedy, I quickly extracted the fly maggot from my arm without any serious consequences.  It resides in a jar in my office.  I collected the candirú, a relative of our catfish, in a remote oxbow lake in the Upper Amazon of Peru, the surface rippling with hundreds of them.  As I fished out two nice specimens with a collecting net, my friend and guide Guillermo exhorted, “Bruce, do NOT fall into the water!”  Its common name – urethra fish – tells all.  Ugly and unpleasant.

As much as we may despise them or take them for granted, the creepy crawlies serve important roles in the economy of nature.  In the intricate web of interrelationships called an ecosystem, all God’s creatures serve incalculable service to life across the planet, often inversely proportional to their overall size.  That should provide a humbling lesson for us humans, given our self-serving and invasive habits as a newcomer on the stage of evolution.