The last time we found ourselves transfixed by a parasitic botfly larva, entomologist Piotr Naskrecki was removing it from his arm. This time around, one of the blood-sipping squatters makes its exit from the body of a mouse – and the process is no less gruesomely fascinating to watch (sensitive viewers, you've been warned).

Video courtesy of Evin Gladin and Erica Peyton

Also known as warble flies, botflies are certainly not among the world's best-loved insects. But removing the creature offered up a valuable learning opportunity for Virginia Tech ecologist Dr Marcella Kelly and her assistants during a recent field expedition.

"This was part of a class," she explains. "I teach Wildlife Field Techniques and the class includes a ten-day intensive at Mountain Lake Biological Station in Virginia. Students were learning techniques for capturing small mammals."

While their life cycle is undoubtedly freaky, botflies are fascinating animals with a number of survival tricks in their arsenal. After mating, a female will lay between 1,200 and 4,000 eggs. Using chemical cues, she'll locate a site that's likely to have repeat visits from a warm body – the perfect future host for her progeny. This mouse, for example, probably picked up its larval guest at the opening of its burrow. 

Baby botflies are excellent heat-seekers, and typically enter their hosts through the nose, mouth or any open wound. Once inside, it takes them some three to six days to reach the optimal position, always just below the skin. Interestingly, different species of botfly occupy different parts of the host's body – but contrary to some reports, this particular larva wasn't in the mouse's rectum; it was actually living under the skin of the animal's crotch (we'll leave it up to you to decide if that's any better).

"The experience for me was both really cool and really disgusting," says teaching assistant Erica Peyton, who helped with the removal. "On one hand, [I got to] see what a botfly larva looks like and help the mouse. But on the other hand, it was like popping a really big, gross pimple."

Botfly larvae that parasitise rodents feed on the white blood cells of the host animal. They're fully developed after about a month, achieving a 100,000-fold increase in size. The invaders don't typically kill their hosts – they simply take the nutrients they need to complete their metamorphosis. Judging by this one's dark colouration, it was likely nearing the end of its stay.

"I have had [around] six bot flies myself," Kelly says. "But the size of these relative to the size of the poor mouse is extreme. Imagine having a parasite the size of your thigh – wow!" (While the US is home to several species of botfly, none of them affects human hosts.)

After its eviction, the larva was presented to the mouse, but the whiskered patient showed little interest, so the team simply discarded it in the leaf litter. 

It's hard to say whether the homeless parasite was strong enough to survive, but under normal circumstances, botfly larvae burrow into the ground once they leave their host behind. Depending on environmental factors, the next phase (becoming an adult fly) can take up to 11 months!

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Top header image: Richard Crook, Flickr