BY ALPER ÖZKAN (MSN/PhD)
d_ozkan@ug.bilkent.edu.tr
There are sentences that can break down one’s suspension of disbelief regarding reality. Here’s one: “Soldiers of [Tuberaphis] styraci defend the colony on the inner and outer surface of the gall and attack predators such as moth caterpillars […] by stinging with their stylet.” Let’s take a closer look at this.
Here we have aphids that are hunted down by ravenous caterpillars, which snatch up their victims in a flash and return to their silken retreats to consume them at their leisure. The aphids are far from helpless, though, since they produce a soldier caste, whose members stab at their enemies with their sharp, piercing mouthparts (that’s what the stylet is; normally, it is used to pierce plants). The paper this sentence is from also details how caterpillars so accosted are paralyzed and may die, and how soldier aphids are well on their way to developing a venomous bite. Fascinated, I do a Google search to see what else aphids are up to these days, and moments later I run headfirst into an account of an aphid that figured out the nest-infiltration trick that lycaenid caterpillars have going on, and spends its life drinking blood from ant larvae. The very same search reveals that even the benign, plant-eating aphids suck blood from their seniors when food runs low. The net effect on me was similar to that experienced by a parent who buys “Watership Down,” expecting fuzzy, friendly rabbits, and makes the mistake of watching it with his/her children—I mean, I thought aphids were supposed to be nice.
Oh, but I was aware that there were colonial aphids (and also that there were aphids that acquire wings when infected with a mutualist virus, and aphids that use a process analogous to photosynthesis to derive energy from the sun). There are a lot of colonial animals, in fact: wasps are primed for eusociality because of their rare haplodiploid sex determination system, which makes it especially beneficial for a wasp to care for her siblings’ well-being—and from there on it’s a straight road from the solitary wasp to the yellow jacket, the honeybee (bees are wasps, of course) and the fire ant (ants are also wasps). Termites are a whole different kind of beast, however: they are social cockroaches, which makes sense if you consider how even these household pests move in swarms, distribute themselves evenly across nest sites and get depressed when they are unable to mingle with their fellows. Aphids, too, are insects of the “all-consuming swarm” variety, and the ridiculous number of morphs that even a single species may contain probably helped them to produce specific castes for their needs (one species, Nipponaphis monzeni, even has a caste that explodes into glue to repair the communal nest, which earns it an honored place in the eusocial elite beside the suicide ant Camponotus saundersi and the suicide termite Neocapritermes taracua). It makes sense for them to have evolved sociality.
But then there are social spiders, which exist because evolution has it in for you. As one moves into the tropics, animals get larger and nastier, and the flimsy web of a single spider cannot hold them for long. One type of solution to this problem is to hunt smaller prey, or to catch larger morsels quickly, or to specialize in a specific kind of insect; another is to gather together, and coat a building or two in webs. There is evidence that at least one species, Anelosimus studiosus, produces extra-aggressive individuals that serve as a sort of proto-soldier caste, because evolution really has it in for you. Also, their young can fly, but people insist that it be called “ballooning,” either because of the technical definition (it isn’t flying if you aren’t doing it under your own power), or because a reality where spiders can fly is too horrific for the human mind to process. Curiously enough, the largest aggregations of social spiders may feature several different species, in case you thought one flavor of spider wasn’t enough for your needs.
Ambrosia beetles (not my favorite kind of bark beetle—not when there’s one that uses its infrared sensor to detect forest fires and lays its eggs in the still-burning wood), snapping shrimp and two species of mole rat have also evolved eusociality, and the latter are some of the more interesting critters in existence. I’ve covered them in a previous column, and they rival the platypus in sheer concentration of bizarre traits: no sense of pain in their skin, eusociality, hideously long lives for rodents, cancer immunity, being a cold-blooded mammal….
In contrast, though, the platypus has egg laying, ten sex chromosomes, a bird-like sex determination system, strangely primitive eyes, the ability to detect electrical fields, venomous spurs on the hind legs and a duck bill that hides its Illithid skull. There’s some stiff competition there.