Cleaning While Small

When a shower is not an option

A typical scene in the brood area of my desktop tiny pet ant, Pheidole adrianoi. The brood are mostly pupae, pharate pupae, and eggs. There are a few early instar larvae as well. The pale grey workers are recently emerged, not yet fully darkened. The workers average 2 mm in length.

In several of my essays on my tiny pet ant, Pheidole adrianoi, I have pondered the consequences of being very small—- tiny creatures experience vastly different forces in their world than we do in ours, forces that affect how they move through air, fall, drink, and move objects.  Here, I want to explore how such tiny creatures stay clean, because at their scale, the world is exceedingly contaminated (aka dirty).

When we want to clean ourselves, we probably take a shower or maybe a bath.  Shower heads are gadgets that break a stream of water (maybe hot water) into droplets and speed them on their way to splatter on your head and shoulders.  If a bath is your choice, you just fill the tub and get in, no big deal, perhaps after you have added some bubble bath.  There’s also the old-timey sponge or washcloth bath with soap and a bowl of water.  Sadly, none of these are options for ants.

An ant in a shower would be pummeled by huge drops that have a great deal more mass and momentum than the ant, sending the ant skittering into the distance at a high velocity. Well, you say, how about if we make the droplets more ant-sized, say in the same mass ratio to the ant as the shower drops were to you (very approximately 50 mg per drop, or less than a millionth of your weight).  So, our 0.04 mg ant would be “showering” with droplets weighing less than a hundred millionth of a mg.  While the mass ratio of such tiny droplets might not batter the ant mercilessly, they would also be literally “too small to fall” because their tiny inertial mass would be very small relative to the viscous friction of air, and they would drift like dust, subject to Brownian motion.  They would, in fact, be much smaller than typical fog droplets, and their huge surface to volume ratio would ensure that, under any humidity less than 100%, they would evaporate in an instant.  The poor ant would only be treated to a stream of humid air, air that would unmoor it and send it flying anyway.

Perhaps, you may say, taking a bath in a drop of water a couple of times larger than the ant would seem a better option, akin to you in a tub. Whereas you would have no problem breaking through the surface tension of such a bath at your scale, the surface tension of this small drop would be very large relative to the size of the ant, and she would find it very difficult to break through the surface to get into her “bath.”  In part, this is because the ant’s surface is coated with a water repellent wax that keeps the ant from drying out fast, but this makes getting through the water’s surface that much harder.  Even if the ant broke through the droplet’s surface tension, she would have an equally difficult time getting back out (sigh).  In this situation, where would be the incentive to stay clean?

OK, the ant could reduce the drop’s surface tension with soap or a detergent like we do, but this would be dangerous because such wetting agents facilitate the entry of water into the ant’s respiratory system and would suffocate it (indeed, soap is a good insecticide).  Or let’s imagine that the ant could actually take the bath, but would enjoy it hot. But a droplet that size would cool in an instant because its surface area would be so huge relative to its volume.

So how is our poor little ant to stay clean?  Clearly, not by using water in any of its forms.  Indeed, water would be of questionable effectiveness anyway because most of the dirt on the ant’s body is hydrophobic (i.e. water repellent. This is why we use soap to wash).  In addition, molecular adhesion of the tiny particles that contaminate the ants’ surface would be so strong as to make them resist dislodging (try blowing a dusty surface to convince yourself of this). 

In the face of all these challenges to using water to clean up, the ants have turned to a purely physical method--- scraping and “licking.”  The ants were doing this long before it became vogue among the ancient Greeks and Romans to use a strigil to scrape the dirt and sweat off their bodies after exercising (works nicely, but don’t try this in a modern gym).

Still, scraping a surface randomly would seem only to move stuff around, so ants (and other insects) have evolved standard, repetitive behavior patterns, called “grooming behaviors” that move the dirt to locations from which it can be separated from the body.  Thus, most insects rub their legs together repetitively in such a way that any dirt on them is moved to the far end of a leg from which it can be either ejected or collected into a pocket below the mouth (the infrabuccal pocket) from which it can later be ejected as a pellet or ingested.  In some cases, the pellet may be fed to larvae, giving the kids a role in garbage recycling.

Ants also have a spur on the tibia of their forelegs.  This spur is particularly useful for cleaning the antennae and legs by drawing them through the angle of the spur with the leg.  This leg- and antennae-rubbing is very frequent in ants, but even more frequent is “licking.”  But this “licking” does not involve watery saliva as it would if we were to lick gravy from a plate or lick a frozen flagpole (huh?).  Rather, it is essentially scraping a surface with protrusile mouthparts formed from separate mouth appendages that function like a (dry) tongue. 

You can see this “tongue” at work in the video below.  Unlike most other insects, ants not only groom themselves, but also their nestmates. In fact, such social grooming is a central process in social cohesion and social behavior among ants. All colony members are part of this social grooming system, but of course, larvae and pupae, being mostly immobile, are recipients of grooming rather than active agents.  In the video below, the licking, rubbing, dabbing action of the worker “tongue” can be easily seen.

The function of this social grooming goes far beyond helping sisters stay clean.  The endless and repetitive licking moves some of the surface chemicals (such as waxes and other non-polar compounds) into the post-pharyngeal gland, a multi-lobed structure in the head.  It is not certain that this structure is actually a gland in the sense that it secretes chemicals, but in any case, it serves as a sort of mixing bowl for the body odors of all colony members, a mixture that is shared and re-shared, and functions as a colony-identifying odor.  The chemistry of this odor varies enough so that each colony has a unique odor that depends on diet, genetic factors, and environment, but allows each colony to distinguish its colony members from non-members. In the (almost) words of Bob Dylan, “everybody’s gotta get groomed.”

But every communication system can be hacked. In many ant species, the colony odor is not present on larvae, pupae, or newly emerged adults but is acquired over the first few days of an adult worker’s life.  It is this fact that has allowed the evolution of dulosis (aka “slavery”) among ants.  The slavemaker ants raid the colonies of the slave species, steal their pupae and larvae and return them to their own colony. When these pupae emerge in the slavemakers’ nests, they acquire and learn the odor of the host nest and function as though they were members of the slavemakers’ colony, doing all the work in that colony. Significantly, dulosis often evolves between species that are closely related.   In the extreme, the slavemakers cannot exist without the slaves, for they have lost the capacity (and even the morphology) to do the necessary work. Although much has been made politically and historically of this ant slavery, it is not the equivalent of human slavery.  Whereas the “slavery” of ants is between different species, that of humans is within a single species, Homo sapiens. However, it seems likely that true slavery (i.e. within a species) exists among ants, but it is hard to detect because slave and slavemaker look alike. Indeed, I have found such true slavery during early colony establishment of the fire ant, Solenopsis invicta, but perhaps that is a story for another essay.