Larval horsehair worms are internal parasites of insects that burst out of the host, a bit like the aliens in "Alien." Why would a very long, thin shape work well for an internal parasite?

 

Horsehair Worms as insects' Parasites

They are also called  Gordian worms are long, slender worms related to nematodes. When they are immature, they are parasites of insects, arthropods and other invertebrate animals. They are harmless to people in all stages of their lives. When humans ingest the worms, they may encounter some mild discomfort of the intestinal tract, but no infection occurs.

They are similar to nematodes but much longer that is 4 inches or longer and very thin (1/80 to 1/10 inch in diameter).

They are found in water or wet areas, such as in or alongside streams or puddles but they can occur in cisterns, livestock watering troughs or most open outdoor container with water. These harmless, curious creatures writhe slowly, contorting their hair-like bodies into intricate knots.

The long larvae have hooks that allow them to penetrate into the body of the insects and other Arthropods they parasitize.

The thin and long body allows the parasites to be released in water in order to mature into adults. This body adaptations allow them to move in water ponds, domestic bath troughs etc with ease

The thin body allow the parasitic horsehair to move from one host to other without disturbances through direct contact or sharing of forage objects by the Arthropods

The thin and very long body is a mechanism that help them evade the host insect defense mechanism because the length would be difficult to handle