Mygalomorph Spiders: Part 1, Tricky Assassins

I’m going to go out on a limb and say that you’ve probably seen a spider before. Well, that is, unless you’ve been living your entire Earthly existence on the barren, ice-blasted wastes of the Antarctic interior…in which case, I don’t have conversations with murderous, shape-shifting aliens, kthnxbai. Spiders comprise the largest and most successful order of arachnids, with nearly 46,000 species living everywhere from rainforest canopies, to scorching desert sands, to pitch black caverns that wind miles underneath the ground. They are exceedingly varied in their lifestyles and habits. Some can glide like a frisbee from high up in trees. Some slink along the surface of standing water, and nab minnows like some kind of mini, eight-legged, catfish noodlin’ Jesus of Nazareth. Some specialize in the most dangerous prey…other spiders…and net them with a toxin-laced phlegm that they spray from their mouthparts.

Yes, with all their diversity and ubiquitousness in both ecological and human cultural settings, spiders are unquestionably the flagship arachnid group (although many other fascinating orders exist; see the first entry in my lesser-known arachnids series for information on the Class Arachnida in general). Whether we love them or hate them, spiders, as a group, are familiar to us, and many people are aware that spiders come in a huge range of shapes and sizes. For example, it is no secret that black widow spiders (various Lactrodectus species) are among the more dangerously venomous species in North America….or, that the fuzzy, hamster-sized tarantulas (family Theraphosidae) are the biggest spiders on Earth. But what is much less widely known is that far more than size and venom differ between tarantulas and black widows.

While it’s perhaps instinctual to think of spiders as existing in a big monolith of vague “spideriness” with minor tweaks on a common, eight-legged theme, spiders actually divide up into distinct groups with important biological differences, the result of deep evolutionary rifts that stretch back hundreds of millions of years. The truth is that there are several very distinct branches of the spider family tree.

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Evolutionary Anachronisms

The avocado.

You may think you have a good relationship with the avocado. The buttery fruit of this plant may regularly accompany your turkey sandwich, sliced and fanned out across the bread. Or, it may serve as a hearty dip in the form of guacamole. More recently, avocado is seemingly being utilized in a greater variety of ways, being deep-fried, thrown in macaroni and cheese, and finding its way onto burgers, Subway sandwiches, and even into ice cream. You’d expect that with all this attention, our green-fleshed, knobby-skinned friend, the avocado, would be content with its current role in human culinary efforts.

However, the avocado may very well be lonely.

Despite all our affection, this loneliness stems from the avocado potentially having eyes for another. I mean this, of course, in the sense of the concept of co-evolution (which I examined in a previous post), which is directly tied to the reproductive role of the fruit itself. The main function of a fruit (a botanical fruit, typically meaning the structure derived from the reproductive tissues of the flower housing the seed(s)), is to move seeds away from the parent plant and into areas that promote growth and survival. Many types of adaptations in fruits exist to help achieve this goal of seed dispersal; from the wind-catching dual blades of maple tree fruits (known as samara), to exploiting the appetites of the ubiquitous (and notably mobile) animals in the neighborhood. This latter evolutionary strategy involves the development of fruits that enrapture animal taste buds and provide irresistible caloric value, allowing consumed seeds to travel safely inside the gut of an unwitting, far-traveling chauffeur until being excreted away from the crippling shade of the parent plant. This is called “endozoochory.”

Most endozoochorous fruits have evolved to be eaten by fairly specific animals. Predictably, fruits adapted to be taken by songbirds are going to have different physical attributes than those associated with insects or elephants. You can try and fit a peach pit through the body of a sparrow, but you aren’t going to get very far. Similarly, expecting tiny, thin-walled seeds to withstand an elephant’s battery of grinding teeth isn’t realistic either. The suite of fruit traits evolved for dispersal by a given group of animals roughly categorize into “seed dispersal syndromes.” By interpreting these syndromes, we can often get a good idea of what the primary dispersing animal, the other partner in a co-evolved relationship, is likely to be.

In light of this, it becomes obvious that despite our love of the avocado (specifically, domesticated cultivars with lots of flesh; wild avocado fruits have a thinner layer of green deliciousness surrounding that pit), it is not “meant” for human consumption and seed dispersal. Any attempt to chew up the whole fruit and swallow the massive pit is bound to land your asphyxiating ass in the cemetery. However, the situation for avocado’s seed dispersal isn’t much better in its wild Neotropical range. Many smaller animals (like monkeys) that partake in avocado consumption are “pulp thieves”, ingesting the oily layer and tossing the seed at the base of the parent tree. In fact, no native animal is known to consistently and effectively disperse wild avocado. Why then does the avocado make a big, energetically expensive fruit that doesn’t cut the mustard on dispersal? Also, who is the true “buyer” of avocado’s product?

The answer to both those questions may be that the avocado’s chief dispersal agent is extinct. Kaput. Gone. Effectively an “ex-animal.” This would mean that the avocado fruit is an evolutionary anachronism, equipped with traits fine-tuned by evolution for interaction with a species that has quite suddenly disappeared, leaving the once perfectly capable seed vessel under-appreciated and inadequately used.

“Fruit’s almost ripe, guys. Come and get it! Hello? ….Guys?!”

It’s a very serious case of being all dressed up with nowhere to go.

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