In praise of fleas

Fleas collected from our neighbour's cat.

We don’t own a cat, but a neighbour’s has decided to adopt us. Sleek and affectionate, it joins us for breakfast, curls up on the chair in our conservatory, and lounges around on the top of our rabbit’s hutch. The only trouble is that it isn’t alone. It’s brought lots of tiny friends along with it: cat fleas (Ctenocephalides felis). Still, we’re not the only ones with this problem: the number of reports of fleas infesting houses across the UK is on the up (see: Who, What, Why: Have flea numbers jumped?). This might be because milder winters don’t reduce the numbers of these pests, or perhaps because people aren’t treating their pets as often. Whatever the cause, pet owners are becoming more and more aware of these tiny insects and their antisocial ways.  We didn’t realise that our feline house-guest had parasite problems until a friend spotted a large flea, swollen with eggs, calmly making its way across the cat’s nose. We then decided to comb his fur and found dozens more. A few days later, we started getting bitten around the ankles; they were in the house.

Despite the irritation they can cause, fleas are a wonderful example of natural selection. Not for nothing have they featured in numerous evolutionary studies and in classic biological texts. Their life history is extraordinary. Female fleas can lay hundreds of eggs in a lifetime. Each tiny oval sphere takes only a day or so to hatch into a grub-like larva, which then feeds on a variety of organic matter (usually including the faeces of adult fleas) and grows quickly. At this stage in their lifecycle the fleas don’t bite animals. Once they are large enough, they form a silken cocoon and pupate inside (like a butterfly, but arguably with a less attractive outcome). After a week or so the adults emerge – often triggered by vibrations, heat or carbon dioxide that tells them a potential food-supply is nearby. In our house, all we had to do was stamp our feet on the floor boards and several of the tiny adults would come hopping in search of a meal!

The toughness and speed of fleas is startling. With their flattened bodies and hooked feet, they can move rapidly through the fur of their hosts, making them extremely difficult to catch. Once you’ve managed to get your hands on one, they aren’t easy to kill. Squeezing them between your fingers isn’t enough – open your hand and they just hop away, unhurt. Only by crushing them between your finger nails (hopefully until they pop) can you finally destroy them.

Perhaps the most amazing thing about fleas is their jump. This remarkable acrobatic ability has fascinated biologists for years, and proved quite a problem to understand. Fleas don’t just contract their muscles to jump, like we do. Rather, a clever arrangement of muscles push against special pads of rubber (called resilin), found in their legs, effectively storing up energy as the rubber distorts. By releasing a lock, the fleas allow the resilin to reform into its original shape, rapidly extending their hind-legs and propelling the insect into the air. Although the role of resilin in flea jumps had been known since the 1960s, it was only last year that some of the technicalities of how fleas jump was finally explained. A controversy had always existed as to the exact arrangement and control of the jump and whether fleas left the ground by pushing off from their trochanter (an upper bit of their legs) or their tibia and tarsi (effectively their feet).  Researchers Greg Sutton and Malcolm Burrows, based here in Cambridge, used high speed photography and cunning mathematical models to finally answer the question: fleas jump from their feet! For some beautiful footage of fleas jumping, that may make even the most ardent flea-hater think again, see DiscoveryNews.

Despite several rounds of spraying with insecticide and much vacuuming, we still find the occasional flea at home. Even though it’s irritating to have them, it’s hard not to feel a grudging respect for creatures that are so well-adapted to their environment.

If you would like to find out more about how natural selection works, why not sign up to some of our upcoming courses on evolution at ICE:

Life as we know it, with Dr Matthew Wilkinson

Evolution of you and me, with Dr Neil Shailer

Evolution, a new online course.

or have a look at an ICE presentation about evolution on youtube


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