When I was younger and prior to developing my interest in entomology, cockroaches used to petrify me. When I lived in Sydney, I'd lay awake in bed at night, cockroach spray at the ready on my beside table, waiting for the scuttling giants to emerge from dark crevices. I'm fairly certain the species that tormented me was the large American cockroach (Periplaneta americana). These days I'm no longer scared of this group of insects and have kept Australian native giant burrowing cockroaches (Macropanesthia rhinoceros) as "pets" for the last few years, although my numbers are down from three to now only one lonely specimen.
I think knowledge, experience and understanding are key to overcoming our fear of "creepy crawlies", so I'd like to share these recent findings with you. Researchers have identified neurons in a cockroach's brain that control whether the insect walks slow or fast, turns right or left or downshifts to climb. Electrical stimulation selectively applied to these central-complex cells alters reflexes and consistently causes the insects to replicate movements.
Central-Complex Control of Movement in the Freely Walking Cockroach
To navigate in the world, an animal’s brain must produce commands to move, change direction, and negotiate obstacles. In the insect brain, the central complex integrates multiple forms of sensory information and guides locomotion during behaviours such as foraging, climbing over barriers, and navigating to memorized locations. These roles suggest that the central complex influences motor commands, directing the appropriate movement within the current context. Such commands are ultimately carried out by the limbs and must therefore interact with pattern generators and reflex circuits that coordinate them. Recent studies have described how neurons of the central complex encode sensory information: neurons subdivide the space around the animal, encoding the direction or orientation of stimuli used in navigation. Does a similar central-complex code directing movement exist, and if so, how does it affect changes in the control of limbs? Recording from central-complex neurons in freely walking cockroaches (Blaberus discoidalis), we identified classes of movement-predictive cells selective for slow or fast forward walking, left or right turns, or combinations of forward and turning speeds. Stimulation through recording wires produced consistent trajectories of forward walking or turning in these animals, and those that elicited turns also altered an inter-joint reflex to a pattern resembling spontaneous turning. When an animal transitioned to climbing over an obstacle, the encoding of movement in this new context changed for a subset of cells. These results indicate that encoding of movement in the central complex participates in motor control by a distributed, flexible code targeting limb reflex circuits.