On the lateralised motor behaviour of insects

The late 20th century brought with it substantial evidence showing that many vertebrate species are capable of exhibiting brain and behavioural lateralisation (i.e. functional and/or structural specialisations of the left and right sides of the brain/behaviour), undermining the traditional view that this attribute is uniquely human. Motor lateralisation, a specific form of behavioural lateralisation, in which an organism displays a directional preference whilst making movements, has previously been identified in many vertebrates but has received little attention in invertebrate species, particularly insects. Considering this, I investigated motor lateralisation in the desert locust (Schistocerca gregaria) and the red wood ant (Formica rufa) using a combination of a gap-crossing paradigm and a ‘Y’-maze choice experiment. Using these paradigms, I show that the relatively small nervous systems of insects are capable of producing lateralised motor behaviour, providing evidence that being strongly lateralised can be advantageous and, more generally, adding support to the hypothesis that social organisms are more likely to align their lateralisation with others in their group. The gap-crossing paradigm demonstrates that both desert locusts and red wood ants display a forelimb preference, the first direct evidence that an arthropod possesses a preference in the use of otherwise symmetrical limbs. Studying locusts in this paradigm also produced the first evidence that strong lateralisation confers an advantage to the individual because more strongly biased locusts display fewer reaching errors. In addition, wood ants display inter-colony variation in their preference, the first demonstration of a previously undescribed form of lateralisaton, colony-level lateralisation. By testing wood ants over both the gap-crossing and ‘Y’-maze paradigms I show that within the same insect species, lateralisation can be expressed in different forms of motor behaviour.