The Dog Days of Summer (Article by Ken Sproule)

Most people are probably familiar with the buzzing mating call of the male dog-day cicada (Neotibicen canicularis) on a hot summer or early fall day.

Dog-day cicadas are also known as annual cicadas or harvestflies (these refer to the dog-day cicada in particular but also to the entire Neotibicen genus). The term dog-day refers to their appearance during the hottest days of summer. Annual refers to the appearance of adults every year (a bit of a misnomer as their life cycle is longer than 1 year but broods overlap and adults appear every year) in contrast to the periodic cicadas which have 13 or 17 year life cycles. Harvestfly refers to their appearance during the harvest season.

Cicadas undergo incomplete (or simple) metamorphosis (from egg to nymph to adult) bypassing the pupa stage of complete metamorphosis (from egg to larva to pupa to adult). The adult cicada inserts eggs into tree twigs where they later hatch into nymphs and fall to the ground. The nymph then burrows underground where it spends the majority of its life feeding on the sap from tree roots. After a number of years (depending on whether it is annual or periodic) it emerges. Insect nymphs usually resemble the adult without wings (as opposed to the larvae of insects that undergo complete metamorphosis which look quite different from the adult – a caterpillar becomes a butterfly or moth for example).

The picture below left shows the dog-day cicada nymph (approximately 2.5 cm), newly emerged from underground, searching for a tree, fence post or any vertical surface to climb. In the picture below right, the nymph has found a tree, secured itself and begun its transformation. The wings are starting to show.

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The picture below left shows the dog-day cicada emerging from the nymph stage. The exoskeleton (shed skin) it leaves behind is known as an exuvia. You may have seen many of these exuviae clinging to a tree trunk. In the picture below right the cicada has finished emerging from the nymph stage and is resting on its exuvia while its wings dry.

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The picture below shows the short-lived adult dog-day cicada (approximately 3 cm body). Both the male and female die shortly after mating.

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You may also have heard the swamp cicada (Neotibicen tibicen). It has a lower frequency sound compared to the dog-day as shown by the audio waveforms below. The swamp cicada is a rarer species in Ontario (at least I have only heard it a few times and seen it only once). There are 30 Neotibicen species and over 160 cicada species in North America.

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Incidentally, the phrase “dog days of summer” does not originate from the fact that a dog will find a shady tree to lay under on a hot day. It originates from a belief that the rising of Sirius (the dog star), the brightest star in the sky, contributes to the heat of summer. In North American summers Sirius is above the horizon during the day. Sirius is a star in the constellation Canis Major (Latin for greater dog). The species name canicularis is from canicula and canis (Latin for dog).

Why do periodic cicadas have a life cycle of 13 or 17 years? Hint: they are both prime numbers. For a possible explanation see reference 5.

For an explanation of how the cicada makes sound, see reference 2.

 References:

1) Field Guide to Insects of North America, ISBN 978-0-618-15310-7

2) Insects: Their Natural History and Diversity, ISBN 978-1-55297-900-6

3) BugGuide (www.bugguide.net)

4) Images – Ken Sproule (www.toronto-wildlife.com)

5) Periodical Cicadas – https://en.wikipedia.org/wiki/Periodical_cicadas

6) Dog Days – https://en.wikipedia.org/wiki/Dog_days

 

 

 

 

 

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