What do lice do when their host dies?

It’s weird and sometimes involves hitch hiking.

The world of parasites is stranger than most can even imagine, especially given how we tend to avoid thinking about them wherever possible. One question most people don’t think about is what happens when the host of a parasite dies, a matter that will not elicit the sympathy of many. One might think death of the host condemns the parasite to death, but some organisms have evolved clever ways to deal with an event as devastating as the death of the host.

The Mallophaga, also known as chewing lice, are well known for their ability to chew through feathers and fur, but less known is their potential for phoresy (essentially using another organism as transit) on hippoboscids, a large group of parasitic flies. The reasons behind this are poorly understood but makes sense given the relative lack of ability lice have to move from one host to another without direct physical contact. This relationship is likely an underestimated part of the louse life cycle due to the way it helps the lice escape a dying host.

For an ectoparasite the death of a host can mean certain death and the means of escape are often less than effective, especially for something as tiny and specialized as a louse, and a hippoboscid fly offers a means to escape to a new host. Hippoboscid flies supposedly attracts lice because they often have a higher temperature than the dead host. This heat is probably what led to the evolution of this phoretic attachment in the first place, since it’s pretty easy to envision a scenario in which lice unknowingly attached to passing flies that made the mistake of landing on their dead/dying host.

The evolutionary benefits of escaping a dead host are very clear in the group of lice called the ischnocera. Risks are still high, as riding the hippoboscid can easily lead to death on an improper host and one would imagine that a louse that could not attach properly would quickly fall off, making the benefit of a free ride moot. Despite all these issues, ischnoceran lice in particular have some strong incentives to ride because they tend to cling to the feathers of their hosts even…

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