There's one particular event of every summer in the South that I always await with great anticipation: the emergence of the millions of annual Dog Day cicadas (Tibicen canicularis).
It's not just the event itself that I love. The cicadas are certainly wonders in themselves; but for me, they are more than just insects of the order Homoptera - they are the standard-bearers of my favorite time of year: the "dog days" of summer. It's the time of year when the sun shines the brightest, heat covers the land as lazy dogs curl in cool digs in the shade, and Sirius - the Dog Star and the brightest in the sky - makes its appearance above the Southern horizon.
Spring is nice. Fall is fairly beautiful. Winter could be thrown to the dogs and I wouldn't bat an eye. But Summer? Ahh, summer is the incubator of my soul. When I'm in it, the warmth makes my happiness grow as ideas sprout from the imaginal discs of my imagination.
It is in no small part the fact that cicadas choose late summer to burst newly reformed into the world, leaving their former larval stages behind, that they receive my respect. I like their style.
But they deserve my awe for many other reasons beyond our shared love of summer. Many of us are well aware of the cicadas' prolonged existence as grubs feeding amongst the roots of trees for years, the exact time dependent on the particular species. Many species have synchronized both their development and life-cycles to such a degree that they burst forth from the ground all at once after 13 or 17 years of sucking sap as larvae. They enjoy an incredibly short adulthood, frantically mating for a few weeks, followed by death en masse (much like the death orgies of the market squid).
The advanced life-cycle adaptations of the cicadas and the timing thereof are deserving of their own tribute. However, the focus of this article lies elsewhere in our cicadan wonders. For the cicada contains an organ prevalent among many orders of insects that many of you have likely never even heard of: the "dorsal ocelli".
I took the images above last summer after the poor (or perhaps ecstatically happy) little cicada had already performed its life duties. Shortly after emerging and mating, cicadas slowly become lethargic, then immobile, and finally they simply die. This individual had reached the immobile stage. It was still alive when these pictures were taken, but days later it had died - remaining in the exact same location and position you see it in now.
Now, look more closely. You may notice its head is bejeweled with three orange organs. These are its dorsal ocelli (singular: ocellus).
The ocellus is a strange and still quite mysterious organ. It is present throughout the insect world, but only erratically. Despite their ongoing mystery, the organs have been studied fairly extensively since the 1920s and 30s. The following distribution of ocelli among the insects (for you entomologists) is from The Function of the Insect Ocellus1, by D. A. Parry in 1947:
ORTHOPTERA : always present in Acriidae and Gryllidae; sometimes present in Blattidae, Mantidae, Tettigoniidae; not present in Grylloblattidae. DERMAPTERA : absent. PLECOPTERA : two or three present. ISOPTERA: present. EMBIOPTERA: absent. PSOCOPTERA: sometimes present. ANOPLURA: absent. EPHEMEROPTERA: present. ODONATA: usually present. THYSANOPTERA: present. HEMIPTERA: great variation. Some families separated on the presence or absence of ocelli. Several families in which some genera possess ocelli and some do not. NEUROPTERA: conspicuous in some families, absent in others. MECOPTERA: some genera with ocelli, others without. TRICHOPTERA: some families with ocelli, others without. One family including six genera with ocelli and two without. LEPIDOPTERA: sometimes present. COLEOPTERA : absent except in a few species not all in the same family. STREPSIPTERA : absent. HYMENOPTERA: usually present, but sometimes absent in the Vespoidea. DIPTERA: sometimes present. APHANIPTERA: uncertain.
Many species it seems have found great use in the ocellus, as evidenced by its retention throughout much of the Insecta class, while others have completely disposed of it.
But what is it?
Essentially, the dorsal ocellus is an eye. But dorsal ocelli are not like the large compound eyes always present nearby. Nor are they like our own.
Early studies measuring the focal depth of various ocelli lenses all came to the conclusion that ocelli cannot focus forms on their simple retinas. It has since been shown that this is mostly true, except with some dragonflies which apparently may be able to form images with their ocelli.
What dorsal ocelli can do quite well is sense light. In fact they are much more sensitive to light intensity than the main compound eyes.
Studies in the 40s showed that ocelli nerve impulses were inhibited by light. When the ocellus was occluded, signals would then propogate down the large nerves to ganglia. Essentially, if a shadow passed over the ocellus, signals fired. And because the nerves are very large in diameter (often the largest nerve fibers), they are very fast.
It was additionally shown that light perception in the ocelli alone could not lead to reflexive movement. Thus it was suggested, and some still hold, that light perception (or shadow perception) acts to set the excitatory potential of the nervous system. Thus, if a shadow passes overhead, the nervous system would be primed to react to visual stimuli from the compound eyes.
More recent studies have shown that ocelli are intricately involved in orientiation to light (including UV), particularly to the horizon, and so are integral parts of the flight stabilization machinery, which makes sense when considering that most flying insects have ocelli.
Again, research in dragonflies indicate that the ocelli can form images with very wide fields, and can sense motion. There are other indications that ocelli may play a role in circadian entraining.
To my knowledge, no physiological research has been conducted on the cicada ocelli. Regardless, it appears that whatever the function of the ocellus, it is intricately and physically intertwined with the circuitry of vision from the compound eyes.
The ocellus represents just one more example - among myriads - of a sense that we as humans can hardly fathom. It is hard enough to imagine perceiving the world through thousands of individual ommatidia (the many eyes within a single insect compound eye). Add to that a complex system of light perception wired to the eye circuitry to aid in orientation, flight stability, or to prime the brain for visual stimuli. Such perception is impossible to even imagine.
It's clear from my limited research that science has yet to fully explain the purpose of these beautiful adaptations, despite the prevalence of their existence. It just goes to show that we have not come close to deciphering all the mysteries of life - even mysteries that have stared us in the face for a century.
So this summer, as the cicadas raise their eyes and dorsal ocelli to the summer sun for the first and last time, take a second to give them a closer look. You may just find yourself in awe of these photosensitive jewels.
- Parry D.A. (1947) The Function of the Insect Ocellus. Journal of Experimental Biology. Vol. 24. Nos. 3 & 4. pp. 211-219 (pdf)
- Beament J.W. L. (1966) Treherne J.E. Advances in Insect Physiology. Academic Press. (book)
- Berry R.P., Stange G., Warrant E.J. Form vision in the insect dorsal ocelli: An anatomical and optical analysis of the dragonfly median ocellus. Vision Research. Volume 47, Issue 10, May 2007, pp. 1394-1409.
- Simple eyes in Arthropods. Wikipedia.org