Cicadas are back in town!

The distinctive deafening sound of cicadas accompanied my mid-morning stroll in Kumeu a few weeks ago.

They followed me home.

The impatient clicking sounds from this insect were hard to ignore.

Wings fit snugly along the fuselage while the insect decides if I am a threat.

It eventually tired of me sticking a camera lens up close and flew off, landing beside my ear in full click mode. It took some wild arm waving and head bobbing to get rid of the pest.

Amphipsalta Zelandica, a sleek machine.

Cicadas are beautifully engineered flying machines with two enormous compound eyes. Their fields of vision overlap slightly, providing some stereoscopic perspective to their environment as they buzz around.

Those big dark eyes.

The Australian ‘redeye’ cicada has about 7,500 individual visual units (ommatidia) in each compound eye.

The three red dots are ocelli, additional “simple eyes” that are optically complex and probably assist stability in flight. More about that in another post.

Compound Eyes, how do they work?

The individual units of a compound eye are Ommatidia. Fused in a dome-like structure, they provide a wide range of views. With many different views to contend with simultaneously, insect vision is probably blurred. Movement is easily spotted as objects cross the field of view of nearby ommatidia. So how do they work?

• Light rays entering the compound eye pass through the lens of each ommatidium before travelling through a transparent cone. This cone converges light onto the tip of the rhabdom.

• Once inside the rhabdom, several photoreceptor cells containing rhodopsin molecules absorb light of varying wavelengths. Cicadas, like us, can detect a range of colours and intensities using the same chemistry.

• Rhodopsin is a complex folded protein found in cicadas and other animals. It has tiny compartments perfectly engineered to lock up a Vitamin A molecule. In humans, the vitamin A in these compartments absorbs light of differing wavelengths loosely corresponding to red, green and blue regions of the visible spectrum. When vitamin A absorbs light of the right wavelength, it changes shape, triggering a cascade of electrical signals. These signals travel to the brain, where they are processed, allowing the insect to navigate onto my head and off again while avoiding flailing arms.