Like us, insects take in and expel air via internal tubes — tracheae. Instead of one, however, insects have many openings along their bodies, often referred to as spiracles. These structures are frequently depicted as simple openings to the outside, and gas exchange is described in terms of insects making pumping movements with their thorax. ‘When the body expands, air is sucked in through the spiracles and into the tracheae’ (from an A-level textbook) is certainly true, but the openings are considerably more impressive than simple holes. Each has a mechanism that gives the insect muscular control over its opening and closing (see Figure 1). Dust is filtered out of the air on its way in towards the valve, but the valve can be closed — sealing the insect from the surrounding atmosphere. Insects can do this if they detect harmful molecules (e.g. insecticides aimed at ridding humans of head lice, such as the one shown here), and also to prevent dehydration. In dry environments, the loss of water vapour that necessarily happens when animals breathe out can be avoided if the spiracles are closed.
The image of the head louse is a scanning electron micrograph. I placed a live head louse in the freezer for a few minutes to immobilise it, covered a stub (which is like the head of a drawing pin) with sticky tape, and placed the insect on the surface. The specimen was coated with a very thin layer of gold and put into a scanning electron microscope, which sends a focused beam of electrons across the specimen. As the beam hits the gold atoms, electrons are emitted and collected to form the image. All this happens in a vacuum. A few minutes after I removed the stub from the microscope, the louse calmly walked off the stub. So if you see someone with gold head lice, you have evidence of the power of insect spiracles!
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