Interesting interactions: Part 1: Horsehair worms, insects and a trip
This is a two part series — in each of which I shall take you on a trip(something we are starved of as a result of this pandemic, of course) — to a different place. For the first part, let’s go off to a summer retreat. It’s a warm evening, and you decide to take a swim in the swimming pool of the resort you are staying in. You take a shower, approach the pool and are about to dive in when you notice a large, stout cricket emerge from the bush trying to do the same thing. But you know that crickets don’t swim. Wait! What is happening?This is Bizarre!!!So, you observe it diving in. After sometime, you see long, slender, pencil-like things emerging out.On careful examination, those pencil-like things emerging oddly resemble worms! Now what? This is absolute madness!!!!! A cricket diving into the water to allow worms to come out of it? How’s this possible?
This “worm” — the horsehair worm, is what we shall talk about for the first part of the Interesting Interactions series. The horsehair worm, which belongs to the nematomorph group of animals is found in places with water such as swimming pools, puddles, livestock troughs, cisterns or even in damp soil, albeit in varying depths. The most interesting part of the way a horsehair worm lives its life is its parasitic nature, which I shall elaborate on very shortly.
The life cycle goes as follows: adult horsehair worms mate and lay eggs in the form of long, jelly like strings. These eggs hatch in around three weeks to give rise to larvae, which, within 24 hours of being born; form a protective cover around themselves, called cysts — while lying in wait around vegetation for any potential predators to ingest them along with their food. The cricket you saw on the edge of the pool, happens to be one of those few lucky — well, not so lucky,winners. Some of the other targets, though, can be grasshoppers and cockroaches, depending on the species of hairworm we are talking about.
As soon as the hairworm larvae enter the insect’s gut, the cyst is sloughed off, and the larvae bore into the gut to enter the body cavity of the host(the body cavity is the space between the gut and the body wall). Imagine rolling two chart papers — one tight, and the other loose. Put the tight one inside the loose one. The smaller hollow cylinder is the gut and the inner wall of the larger hollow cylinder is the gut wall, so the part in between is the body cavity). Now, the larvae gradually eat all the fat present in the body cavity of the insect, while they mature. Although there are no obvious symptoms shown by the insect host, a lack of chirping and a possible induction of sterility can be observed in them.
However, the real weapon the hairworm uses to complete its life cycle is a source of amazement. The secret weapon I am talking about is…… MIND CONTROL. Neurotransmitter release is manipulated in these host insects by the hairworm as a result of certain special proteins released by the latter — as found by researchers. This causes the affected insect to behave in certain uncharacteristic ways. The first thing that happens to such infected host insects is that they walk unusually faster than the unaffected insects. This anomalous behavior begins even before the maturation of the larvae inside the host, and is perceived to be absolutely erratic by any observer.
By the time the worms mature the host insects have a tendency to run towards light, an abnormal behaviour for insects like crickets and cockroaches, which prefer dark, cool places to live(that’s why the dense forest/thick bush in the case of urban settings is an ideal habitat for them). When they see light, they walk straight towards it and their pursuit leads them to a river, pond, swimming pool in a resort like yours or even an idle pot of water; after all, water bodies are naturally more luminous than the dense jungle, as they are more open to the atmosphere. Their liking for light becomes so much that they aren’t able to distinguish between land and water and hence they take a dive. This dive they take gives the mature hairworms the opportunity to come out of the host.
They do so in large numbers, each being about 4 to 14 inches long. If the insects end up in water bodies deep enough, they may end up drowning; while if they end up in shallower water bodies, they survive, and lead relatively normal lives; while some scars(both physical and mental) remain.
What makes the kind of parasitism different from parasitism displayed by many other living organisms, is an active long term planning strategy. I say so that unlike some other parasites, which typically work by entering a target host, siphoning off readymade resources from it and live out their entire lives in the body of the host on the host’s resources; horsehair worms actively manipulate these insects to behave in a way they wish them to behave. In other words, they start forcing their hand on the insect source. While the fat from the host helps the larvae to grow by acting as a food source for the larvae, simultaneous manipulation of the insect host plays an important role later on in the life of the hairworms — as an aqueous medium is needed for the adults to lay their eggs. This kind of strategy is highly energy demanding and requires a lot of resources to achieve. However, the need to maintain continuity of its species has led the hairworm to develop this unique strategy, despite the expense.
This is the beauty of biodiversity, i.e. despite the hardships a living species has to face while adapting a particular way of life, it finds a way to fulfill its requirements to survive and even thrive in this wonderful world. Horsehair worms are a beautiful case in point; and have given ecologists the world over a lot of things to ponder upon on their behaviour. Some questions have been answered, some have yet to be answered: but rest assured that an intensive, constant effort is on to understand in depth in horsehair worm parasitism in particular and how the natural world works in general. On that note, I would like to end this article right here. Hope you enjoyed it! See you in part 2…
