Study the following and identify the correct statements.
• In cockroach, sub-oesophageal ganglion is the principal sensory centre.
• Ventral nerve cord of cockroach has only nine ganglia along its length.
• The images formed by ommatidia in nocturnal insects are called superposition images.
• During expiration, thoracic spiracles are kept open and abdominal spiracles are kept closed in cockroach.