MY SCHOOLFRIEND, CHARLIE, AND I FINALLY FIGURED OUT a way to while away the crushingly boring hours while we sat idly waiting our turn to bat at Wednesday afternoon cricket games. Since cricket is played in summer, and since our city in those days suffered endless plagues of flies, we were constantly pestered by the winged demons intent upon making life miserable by landing on any sweaty part of the body. You know how fiendishly ticklish a fly crawling on your exposed parts can be. Try to slap them with one hand, and they only laugh at you as they skit off. But we discovered that if you crept up on them slowly from two sides with two hands and then, when your hands were about 12 inches apart, you clapped them smartly together, nine times out of ten you had your revenge. Of course, having a natural flair for fly slapping helps, but I don't think either of us had genes for that. Nevertheless, with much practice we slowly became reasonably skilled even at clapping them in mid air. Little did I realize at the time how helpful this skill would prove some years later…
My family was sitting on the patio having a little picnic. By that time, municipal councils had learned that fly problems could be reduced by covering trash at city dumps, so we had far fewer fly problems than during my teen years. We still had a few, though. While my children were munching on their snacks, a fly flew by (thanks, Dr Seuss). But instead of either diving straight for our plates of goodies or flying around in a perfect circle like any respectable Musca domestica would do, this one seemed partially drunk. It almost aero-waddled while following an erratic path. Strange, I thought. This fly has something wrong with it. Old juices began to flow and before you knew it… one clap and there it was on the patio, legs up, stunned and unhappy. Then came the eureka moment — a couple of tiny “bugs” barely a few millimeters long were staggering stunnedly around within a few inches of where the fly bit the dust. Out came my newly-purchased, university-cast-off, dissecting microscope. Wow! By sheer coincidence I had recently read about pseudoscorpions — miniature, sting-free scorpions — and I immediately realized I was eyeballing one for the first time. The featured fly had been carting them around!
I had been introduced to a very common theme in nature — freeloading hitchhikery. Some “bugs” flag down suitable unsuspecting fellows for the purpose of getting a free ride. Jesus Christ, without whom was not anything made that was made (John 1:3) has unlimited imagination. Try to conjure up a bizarre way of life that He never thought of, and you will fail. Anything you can think of He thought of long ago. He not only came up with the idea of hitchhiking organisms but He also gave them and their carriers the special design features needed to expedite the transaction. The variations on the basic theme of catching a ride are many; undoubtedly, an exhaustive search would find that they would be sufficient to fill a book or two.
Some hitchhikers have devious ends in mind when they hop aboard an unwitting carrier. Twisted-wing insects produce bizarre triungulinid larvae that sit in flowers until visited by an insect, whereupon they literally leap on board. If luck has come their way and they have boarded Mrs. Right, such as a bee or wasp, they will be taken to the carrier's nest. Upon arriving, they disembark and search out any larvae in the nest, which they promptly puncture, invade and parasitize. Blister beetles behave in a very similar manner, except that their larvae must board a bee if they are to survive. When they arrive at the bee's nest, they eat any bee eggs they find and then proceed to live off the honey that had been provided for the bee grubs.
In 2006, Biology Letters reported a bizarre method of hitchhiking among certain species of tiny mosquito-like flies known as midges. Researchers in Spain discovered living larvae of the insect in the gut of migrating waterfowl known as Black-tailed Godwits (When Worms Fly). En route to Africa for the winter, these birds stop off in Spanish salt marshes, where they can be seen gorging by the thousands, to top-up their fuel tanks. Examination of the droppings revealed the first-known example of insects being carried inside birds. (Ever since Darwin discovered snails stuck to a duck's foot, scientists have been aware that invertebrates sometimes spread by hitchhiking on birds, but only recently have they become aware of some stowing away inside. In 2003 the tough eggs of Daphnia, a crustacean, were found to travel unharmed in bird guts.) Not all the midge larvae survive, but enough do; out of 95 larvae examined, 12 showed no signs of the ravages of bird digestive juices. Evidently, this method of dispersal enables these midges to extend their range over great distances.
Many other creatures appear to use living transport for no other purpose than to either find richer pickings or a suitable place for laying their eggs; hitchhiking enables them to do so without any expenditure of precious energy on their own part. When food becomes scarce, they decide it's time to look for greener pastures. This benign form of transport, in which the freeloaders use a carrier for transport alone, is known as “phoresy” to zoologists. Some small flies use dung beetles to pilot them to their dream home. But the medal for the tendency to board living carriers a little bigger than themselves goes to pseudoscorpions (also known as false scorpions) and mites, both of which are frequently observed using an insect or arachnid for transport. Some employ air carriers, mainly flies, while others use a bus service, particularly harvestmen and ants, to find new quarters.
The number of hangers-on found on any given carrier varies considerably, from one to many. Up to eight false scorpions have been observed clinging to the legs of a crane fly. Dozens are sometimes found on the belly of three-inch long giant harlequin beetles that live on rotten fig trees in the American tropics. Somehow or other, these pseudoscorpions are able to tell when the beetle intends to depart for a new tree, and they swarm aboard, making for the beetles' abdomen. There, violent struggles for territory break out and the weaker are thrown off by the stronger. Upon arrival at a new tree, an orgy of mating takes place among the pseudoscorpions. Mites are often found in large numbers upon a suitable carrier.
The method employed by hitchhikers to stay firmly saddled on their steed varies, too. The victorious pseudoscorpions in the melee just described on harlequin beetles quickly spin safety harnesses of silk on the beetle's belly and cling to these in transit ( National Geographic June 1993, Earth Almanac). Many other pseudoscorpions simply clamp down with their pincers on some suitable-sized part of their host, such as a hair. Mites sometimes glue themselves to their taxi, but other methods are used, too. A couple of years ago an attendee at a field naturalists get-together brought a skink that was covered here and there with patches of tightly-clustered “grapes” the size of pinheads. Our most steady-handed member gently dislodged a few with a pair of tweezers which, upon inspection, proved to be mites. Microscopic examination showed that they sported a thread from their rear end that, from memory, was about five times the length of the body and ended with some kind of tiny hook that presumably was wedged between the lizard's scales. Museum experts later told us they were a nymphal stage of a genus of mite that commonly uses reptiles for transport.
Though the triungulinid larvae of twisted-wing insects and blister beetles are undiscriminating generalists and will leap upon the first insect that blunders into their flower, with dire consequences for the unlucky ones, examples abound of considerable choosiness on the part of hitchhikers — they refuse to leap aboard any but “their” special taxi. Particular rider, particular carrier. I suspect that my patio pseudoscorpion fits in the latter category.
A couple of particular cases of hitchhiking that have been studied in some detail show that the simple concepts and general principles so far outlined don't even begin to do justice to what actually goes on. To be successful at their game, hitchhikers often need special skills — finely-honed senses, powers of rapid decision making, unbelievable athletic prowess — that no amount of mutating and natural selecting could possibly give them.
The nasals have it
To our eyes, the red and orange flowers of hummingbird-pollinated plants stand out in the deep shade of the rain forest like rubies on green velvet. To hummingbirds — whose range of color vision is even greater than ours — the contrast must be even more striking. But to hummingbirds, for whom the rich world of odors is closed tight, the numerous chemicals pouring out of these very same flowers mean nothing.
Try entering this world of hummingbirds and flowers on a different scale. Imagine yourself a mite just twice the size of the period at the end of this sentence. You are breakfasting on nectar and pollen with your family and neighbors inside the orange-red, inch-long flower of a Hamelia plant at the edge of a rainforest clearing. You have no eyes, and need none, because your four pairs of legs and your body are equipped with precisely arranged, minute hairs that provide exact information about the world around you. When you meet another mite in your flower, a quick touch with your forelegs tells you its age and sex, and the rate of encounters tells you how many other mites share your flower. Of enormous importance to you are very special hairs on the tips of your forelegs which are sensitive to the many chemicals wafting around you all the time. Your flower has a distinct and very important fragrance for you. It smells like home, and home means safety, your own kind, a mate and… food!
But you have a problem. The flower you have been dining in since midnight — starting with its pollen and followed, in the early hours of the morning, by its nectar — is shutting down its nectar-making factory. By mid-morning all will be lost forever.
Suddenly you sense the vibrant approach of a hummingbird outside the flower. A half second later, a bill 60 times the length of your body is thrust deep into your home and you must dodge the rapidly-flicking tongue that is draining the leftovers in your larder. In the one to five seconds before the hummingbird leaves for the next flower, you must make a decision that will effect the rest of your life. What are the future prospects for food, mating and the welfare of your offspring on this twig of this Hamelia plant, compared with the chance that a risky journey will turn up better prospects elsewhere?
Somehow you figure that you would be better off on a new plant elsewhere in the jungle, so you dash up the bill of the visiting hummingbird and run into its nostril; the odyssey begins. Accommodations inside the bird's nasal cavity are primitive; you have nothing to eat or drink and the place is crawling not only with your own kind but also with other species of hummingbird flower mites all hitching a ride to their own promised land. You know they are not your kind by their smells and by the strange arrangement of their hairs — long where yours are short, straight where yours are curved.
But the ventilation on this feathered freighter is absolutely first-class. About 100 times every minute, a blast of fresh air rushes in providing you with oxygen and chemical information about the ever-changing outside world. You are waiting for that familiar smell — the odor of your own host plant, Hamelia . The hummingbird may visit the flowers of half a dozen other plant species, and the aliens on board may come and go, but you will wait until the scent of Hamelia beckons. You have only one to five seconds to disembark. With luck, if you are a male, you will find females in need of a mate in your new home: if you are a female, you will be delighted to find a nearly empty flower, with a male or two present. An ideal place to start your own dynasty.
All that has been said so far may sound far-fetched, but it is all true! No one knew of the existence of hummingbird flower mites until 1969. In that year, Robert Colwell stumbled upon them while doing research in the Costa Rican highlands. Since then, much study has determined that over 200 species exist, being found nearly everywhere hummingbirds are found. So far, hummingbird flower mites have been found on more than 100 different kinds of plants, and from over 60 different species of hummingbirds.
These freeloading stowaways do nothing of note for either the plants they dine on or for the hummingbirds they ride on. But their survival depends upon the interdependence of birds and flowers. Hummingbirds benefit from the flowers they visit by sucking up the nectar the flowers produce. The flowers benefit because while the birds' bills are probing for nectar, they also pick up and drop off pollen. Thus they provide the crucial service of pollinating the flowers they feed from. The mites depend on flowers for food and nursery arrangements and on hummingbirds for transport to the next flower.
Hummingbird flower mites are very discriminating in regard to their host plants. Most are found on only one species of plant. And on the other side of the coin, most plants act as host to only one of the many different kinds of such mites. A few welcome two different kinds.
Life begins for a hummingbird flower mite as an egg nearly one quarter of the volume of its mother's body. Some species whose host plants have long-lived flowers lay their eggs right in the nectary of a flower. Here they wallow in nectar for a few days until they hatch. Mites that live on plants whose flowers open during the night only to wither and drop later in the same day lay their eggs beneath leaves near the flower buds. How they know the right spot to lay their eggs is one of the billions of little miracles of creation. The eggs hatch into active six-legged larvae; this stage is quickly followed by two molting episodes which produce eight-legged nymphal stages followed by a final molt into adulthood, the whole drama taking about one week.
When Hamelia flowers first open, the mites use a pair of short appendages to move one grain of pollen at a time over their mouthparts, removing nutritious substances that coat the pollen grains. Calibrating for scale, it would be like a human being licking peanut butter and honey off a basketball. A few hours later they switch to a diet of nectar, which contains sugars, fats, proteins and amino acids. When all the flowers in a cluster are exhausted, the mites must move on or die. Hitching a ride on a hummingbird requires timing and agility. Taking the length of the mite as ¼ of a millimeter (two-hundredths of an inch), the bill length of the bird as about 30 mm (1¼ inches), and allowing a generous five-second feeding visit by the bird, the mite must run as fast as a cheetah for its size — 12 body lengths in a second — to make it into the nostril before the bird moves on!
Disembarking presents new difficulties. Before racing down the bill into a flower, the mites must first determine whether or not the flower being probed is of their own host species or starvation may await them. The decision must be made extra quickly; once they have smelled a home flower they must not hesitate, or all will be lost. Records for many thousands of mites show that no more than one in 200 disembarked in the wrong flower!
No one says you must love mites. But only spiritual dullards could fail to recognize the genius of a supremely brilliant mind in the design and execution of these teensy stowaways that are perfectly suited to life on the run!
Take me to your larder
Certain rove beetles provide an extraordinary variation on the theme of hitching a ride to a new source of food. In 1875, a European beetle collector discovered some rove beetles clinging tenaciously to the fur of Peruvian rice rats. He offered his opinion that the beetles were parasitic on the rats. Studies in the 1960s showed that the beetles had mammal blood in their guts, seemingly confirming their parasitic, blood-sucking status. But some investigators were not convinced. Philip Hershkowitz, for one. His studies of captive rats showed that the rats seemed not the least concerned by their passengers, even when the beetles walked over their eyes and whiskers. He further noted no sign of the skin damage that would be expected if the beetles sucked blood. Other features of the beetles just did not match typical parasitic designs.
So in the late 1980s, Robert Timm and James Ashe decided to find out the truth about this intriguing relationship between horse and rider by a combination of field and laboratory observation and experimentation. As scientists discover again and again, detailed studies bring facts to light that should unnerve the most diehard evolutionist. Why won't they open their eyes to see the brilliant planning that has gone into nature and bow before the glory of God? Don't you just sigh with frustration?
They found that host rodents carried an average of about four beetles each. Yet, as Hershkowitz had reported, the rats seemed quite unconcerned. Further, they never found a beetle with its mandibles embedded in the carrier's skin; the beetles always used the mandibles to cling to the host's fur. Further, they found no evidence that the beetles fed on any part of the rats.
The investigators made a critical observation. Previously, all “infected” rats had been collected at night for the simple reason that they sleep in dens during the day. Careful observation of caged rats showed that as day breaks the beetles disembark and spend the entire day in the host's nest. Lights started going on. Timm and Ashe suspected that the beetles ate other lurkers in the dens. When they put some of the beetles in a container with fleas taken from the same host they were treated to a cheering sight — the beetles immediately grabbed a flea by their mandibles and ate it. Close investigation of their cages turned up the remains of eaten fleas. Of course! These beetles were not parasites but predators upon parasites. Herein lay the explanation for the 1960's discovery of blood in the beetles' guts. Like the dog that chased the cat that ate the rat from Jack's house, the blood had passed from rat to flea to beetle.
But why did the beetles lay hold of the rodents' hair as night fell? Why did they want to bounce around the jungle on their bucking rato while it went on its nocturnal rounds? For the same reason that hummingbird mites board hummers — to get to the other side. No, no, no. sorry. To stave off starvation! You see, these rats have a number of different dens dotted all over the countryside, and they sleep away each day in a different one. To receive their daily bread the beetles must, like camp followers of the past, move with their nomadic benefactors. How did they figure out this amazing survival strategy? Isn't it amazing what millions of years of evolution can do?
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