Ever wondered how fish lice manage to cling on to the slippery scales of a fish? Or how a chrysalis manages to pupate while dangling underneath a leaf? The answers lie in some intricate natural ‘hooks’.
Many creatures and plants in the natural world have evolved cunning ways to attach themselves to other objects. Studying these natural hooks is enabling scientists to mimic these complex natural designs and find uses for them in other situations.
In a project jointly funded by the EPSRC and QinetiQ (Europe’s largest science and technology solutions company), Dr. Andrew Parker, from the Zoology Department at Oxford University, and his PhD student Abigail Ingram have been studying hooks in nature and their biomimetic applications. By copying caterpillars they have managed to develop a new, extra strong type of Velcro, while observing fish parasites has led to a novel way of tagging fish and other animals.
Being able to cling on to something is a very useful skill in the natural world. Geckos rely on having special sticky feet to enable them to scuttle up walls, while wasps have evolved hooks on their wing tips so that they can clip their wings together during flight and cruise along at high speed. Tapeworms can cling firmly to their victim’s intestine, while leeches get their dinner by gluing themselves to a passing animal.
Parker and Ingram decided to take a detailed look at a couple of natural hooking mechanisms, with a view to taking a design tip from nature and applying the idea to man-made systems. Their first job was to decide which natural hooks would be the most useful ones to study. ‘Eventually we narrowed our study down to looking at a butterfly chrysalis and a fish parasite,’ says Dr. Parker.
Like many butterflies, the Greta oto butterfly from Costa Rica emerges from a chrysalis that dangles underneath a leaf. During the pre-chrysalis stage, the caterpillar spins a three dimensional silk mesh underneath the leaf. Then, as it pupates, the skin of the caterpillar splits and the chrysalis emerges. One end is covered in a bunch of hooks, which are pushed into the silk, enabling it to hang from the silk mesh until the butterfly is ready to fly away.
‘The attachment is incredibly strong,’ says Ingram. ‘It is very difficult to pull it apart.’ To learn how the extra strong bond was created, Ingram spent many hours watching the caterpillars pupate. She set up a digital camera and used time-lapse photography to record their actions. Unfortunately these caterpillars weren’t used to being film stars and they weren’t always very co-operative. ‘Catching the right caterpillar on camera at the precise moment was very tricky. It took several weeks to capture the silk spinning process and then to see exactly how the chrysalis attaches itself to the silk mesh,’ reveals Ingram. Nonetheless, after recording and watching many hours of caterpillars in action, she got the shots she needed to see exactly what was going on.
‘It turned out that the chrysalis creates a hemispherical shape with hooks coming out in all directions,’ says Dr. Parker. When this bundle of hooks is inserted into the silk mesh it creates an incredibly strong attachment. ‘We tested the strength of the connection and used a high speed video camera to film it being pulled apart. It turned out to be forty times stronger than needed to support the weight of the chrysalis,’ says Ingram. The excessive strength of the bond is still a bit of a puzzle, but one idea they have is that this allows the chrysalis to ride out the hurricanes that hit Costa Rica.
Working with Chris Lawrence at QinetiQ, they are now borrowing the design from the Greta oto butterfly to develop an extra strong, three dimensional type of Velcro. ‘One idea is to use this design to make a novel underwater adhesive,’ says Lawrence. ‘The advantage of the caterpillar silk bond is that the caterpillar can swing freely. If we can develop a ‘Velcro’ with similar properties, it could be used to attach objects that need to be able to respond to underwater currents, such as sensors hanging beneath a boat.’
For their other attachment mechanism Parker and Ingram decided to look at a parasite known as Pennella instructa, that lives on the outside of marlin fish. This stringy, worm-like crustacean can be up to 60cm long. One end is firmly embedded in the marlin’s skin. To collect samples Ingram went to one of the largest marlin fishing tournaments in the world, at Port Stephens in Australia. She brought the samples home and investigated the attachment between the parasite and the fish, under the microscope.
‘This parasite grows down into the fish, where it then develops an anchor when it is in place,’ explains Ingram. The advantage of growing this anchor inside the fish is that the parasite causes minimal damage to the fish skin, making it less likely for the anchor to rip out later. Parker and Ingram realised that this idea could come in useful when scientists tag fish to monitor their numbers and follow their movements.
Currently fish tagging is problematic because inserting the tag damages the skin and no-one can be sure that the tag remains in place. ‘If tags are falling out then the data from fish tagging becomes useless,’ says Dr. Parker. Research projects often want to monitor the fish for many months so they need tags that will remain attached to the fish for the duration of the project. In addition, fish tagging can be very expensive, with some of the more sophisticated satellite tags costing as much as £2000 each. At these prices no-one wants their fish tag to drop to the bottom of the seafloor. With the help of Lawrence they are now designing a fish tag that mimics the Pennella.
‘We are designing a tag that will change shape when it enters the fish, ensuring that it stays firmly in place,’ says Lawrence. At the moment they can’t tell us exactly how it is made because they still have to patent the design. However they hope to run trials with their new fish tag early in 2004 and if all goes well it should be available later that year. Once again nature has come up trumps. Over millions of years of evolution, designs have been fine-tuned to perfection. Parker, Ingram and Lawrence have taken advantage of just a couple of these fantastic natural creations, but the possibilities are endless.
Kate Ravilious is a science writer for the EPSRC.