Why do leeches feed on blood? Why do they target potential sources of blood? Why, of all things, blood? How do they ‘see’ their targets? What characteristics do they have in connection with feeding on blood? These are the barrage of questions asked by the most curious among people.
Why blood? All leeches whether freshwater, marine, or land dwelling have had a long history on our planet, estimated from 150 to 450 million years ago, especially during the time of the dinosaurs. Perhaps to avoid intense competition with their relatives or ancestors who mostly belonged to the decomposer guild, they began to try and explore other food sources, possibly including freshly killed animals, until they have perfected what we may call now as efficient free-living parasites. Note that the peak of the age of the dinosaurs also saw the origin of the first mammals as well as the ancestors of today’s birds. Blood is a complete meal, protein-rich, warm (if from mammals or birds), contains glucose (as blood sugar), rich in oxygen and minerals that may be essential for many animals like iron etc. What more could a parasite ask for?
The anterior end or what most people interpret as the head part of many freshwater leeches has some sort of a retractable drinking straw called a proboscis. The proboscis of a leech is a actually a part of the pharynx, which can be extended or produced outward. Not all leeches have such proboscis. Land leeches and the medicinal leeches (like the European medicinal leech ,Hirudo medicinalis L., and 5-6 other medicinal leech species), have three jaws. These jaws are arranged at more or less equal angles. Each jaw has numerous tiny sharp teeth. Hence, if you’d look at a leech bite through a magnifying glass, you’d a Y-shaped wound. From this incision, it sucks out the cherished blood from their accidental “donor” while injecting anticoagulants contained in their saliva.
Leeches have no true eyes. They just have 2-10 light sensitive spots, called eyespots on the anterior or “head” end of the body. The eyespots detect light and dark conditions. This means that in searching for their blood donors, they have to have other means and hence, they use a combination of mechanical, thermal, and chemical sensors. They are alerted by vibrations or even the movement of leaves and twigs or branches, even mild tremors or shaking of the ground, sudden warm gusts of air when animals pass by and ever the scent of humans and other animals, probably also CO2 and sweat.
Most of the time, leeches can be seen just “standing” on their “butt” ends while they’re securely attached to leaves, branches, or rocks by their hind or posterior suckers or suction pads. Once a leech detects a passer-by, it bends or coils and contracts and then spring jumps at the next sign of movement or sensed warm body. The coiling and contraction effects a spring action, sort of jumping without legs, and it readily attaches to the animal passing by using its front suckers.
All these detections and movements happen noiselessly but for the frightened screams of sensitive hikers. Most of the time, just like me, hikers, especially those extremely enjoying their outdoor activities or are busy with observing or documenting nature’s treasures, become aware they have donated blood to leeches only when they remove their shoes back at base camp.
Because of the painless bloodletting, not a few have claimed that leeches inject or apply some anesthetic substance through their saliva. However, hitherto there is no evidence to back up those claims. What is known is that leech saliva contains hirudin, an anticoagulant that prevents blood clotting, enabling the leech to feed until it is full. Easily our perfect free-living parasite and its means of detecting blood sources, painless incisions, etc. are thought of as perfect designs of a super inventor or engineer. Yet one smart kid can easily counter, why would a super engineer design a perfect vampire of a machine that would cause harm those who were made to imitate him? Again, my answers dwell on things biology. The ecologist knows that nature or the environment is quite demanding. It selects or chooses from a wide range of choices or variants. Those that are chosen or selected or favored, survive. Those that survive are able to pass on their hereditary traits to their offspring.
And that is the essence of natural selection. – Jun Lit