Cockroaches groom to improve their sense of smell, scientists say.
The insects are known to manipulate their antennae, the protrusions on their head which they use to sense the smells, temperature and physical layout of their environment.
US researchers found that the cockroaches groom to wipe away secretions that would otherwise interfere with their sense of smell.
They also identified the same behaviour in carpenter ants and houseflies.
The findings are published in the journal Proceedings of the National Academy of Sciences.
"The insect antenna is full of sensory structures, called sensilla, used by insects to gather information about their surroundings, which is essential for their survival," said Dr Katalin Boroczky from the North Carolina State University, US, who authored the study.
"Sensory neurons sitting in the sensilla are the receivers of outside signals: humidity, texture of physical surfaces, taste, or smell."
Dr Boroczky observed that when cockroaches were prevented from grooming, their antennae became coated with a shiny substance.
"The main function of insect grooming is thought to be the removal of foreign material from the body surface," she explained.
"In particular, it has been shown that insects can remove powdery substances and pathogens via grooming."
Analysing the substance on the cockroaches' antennae, Dr Boroczky and colleagues discovered it was actually made up of cuticular lipids, which they describe as natural waxy secretions that regulate water loss in the insects.
Wax offTo understand the effects of the grooming behaviour, they compared groomed and ungroomed antennae.
They found that ungroomed antennae accumulated nearly four times the amount of lipids that groomed antennae did.
Ungroomed antennae were also found to be less responsive to aromas and sex pheromones.
"A male American cockroach... has thousands of sensilla on each antenna, some of which house neurons that detect air-borne compounds, such as the female-produced sex pheromone or odour molecules from the environment," she said.
Using an electron microscope to investigate further, Dr Boroczky found that the substance covered the pores on the antennae which connected with these essential smell-receptor cells.
American cockroaches prefer to pull their antennae into their mouth with a foreleg in order to clean them.
Dr Boroczky and the team identified other species that groom their antennae, although they employ different methods.
"The carpenter ant uses a foreleg to groom its antenna followed by cleaning of the foreleg with the mouth, whereas the housefly uses the foreleg only," she told BBC Nature.
"Our study insects represent three different modes of antennal grooming that are common to many insects. It will be exciting to see the extension of this phenomenon to other insect species."
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