Silent Sparks; The Wondrous World of Fireflies by Sara Lewis. Princeton University Press, 2016.

As kids we spend evenings, gathering fireflies into jars to make lanterns. Once that wonderment was over, the jars were opened and the fireflies took off.  Depending upon the species, they tended to fly and flash differently, some close to the ground from which they had emerged so recently.

I have never seen    Sara Lewis’s Silent Sparks brings back my childhood memories of summer a firefly in my Florida backyard.

Fireflies are not flies nor bugs. But rather beetles, a very diverse family. Beetles arose some 150 million years ago and today constitute 25% of all species. The Latin names for the two species that Lewis focusses on are Photinus and Photuris. These species were described years ago by James Lloyd an entomologist and Professor Emeritus at the University of Florida.

Lewis describes the complicated metamorphosis of fireflies (sometimes called lightening bugs or glow worms). During their lifetimes, all beetles undergo complete metamorphosis. That requires drastic changes in their physical structures and habits, and in their habitats.

In the northern latitudes they live longest – sometimes one to three years – in their larval state, where they are voracious eaters of snails and earthworms. . They are pupae for a comparatively brief period of two weeks or so, when they rearrange their bodies in metamorphosis to better survive when they emerge in their new, adult habitat – mostly on or near the ground under damp leaf mold in wooded areas or grassy meadows.

The adults exhibit what biologists call sexual dimorphism. Males and females are recognizably different as adults, easy to identify because of the differing shape of their light-producing lanterns. They emerge from metamorphosis with a much distorted male/female ratio of 218/12. That ratio results in considerable female promiscuity.  So a lof of energy must be expended by the male in his search for females of the same species with which to mate.

The males of some species have evolved an ability to synchronize their flashes. The pattern of flashes that they exhibit varies from species to species. Species notification, Sarah Lewis calls it. That would seem to violate evolutionary logic because that synchronization would seemingly be an exhibit of co-operation. Biologists have scurried to put up an explanation for this phenomenon. Lewis favors what is called the “beacon hypothesis.” Together, gathered on a blade of grass or on tree trunks, the males gradually synchronize their flashes and since their flashing patterns vary between species, their beacons better attract the attention of the females of their species, who take up perches nearby.  The females of some species assist the hunt by releasing pheromones.

Entomologists talk about the various firefly species as being reproductively isolated, i.e. they breed only with others of their same species. They are true to their own gene pool. They have species specific signals.

Lewis spends less time in describing their use of “perfumes.” Better is her description of what she calls gift-giving. These nuptial gifts are valuable when nutrients are scarce. Their examination requires a scrutiny of the interior spaces of the male reproductive glands where their sperm is wrapped into packages. Once the “bundle” reaches the female reproductive tract after mating, it is stored in a pouch and slowly digested over the next several days.

How is the male controlling his flash? Chemicals, and it is difficult to explain, the mechanism, but Lewis has it controlled by “light switches.” Here and elsewhere the author lapses into anthropomorphisms and metaphors.

Lightening bugs have numerous predators – spiders and bugs – and they arm themselves against these predators by using poisons, potent toxic steroids. Most do not eat as adults. They do have blood, which is useful for certain medical tests, and therefore they are harvested.

Females of the group, Photuris hunt, catch, and eat male lightening bugs of other groups. The females have thus become femmes fatales.

What seems to be the leading cause of the fireflies’ declining numbers in addition to the harvesting for commercial uses? Light pollution confuses the male’s flashing, so bring back the night. Yard lights: make sure they are turned off when not in use.  Land clearing destroys the habitat for the larvae. Fireflies are not good at dispersing to new habitats and so this is particularly harmful. Pesticides and fertilizer.

In the last section of Silent Sparks, Sara Lewis provides the reader with a field guide for common North American fireflies that might be found in our backyards, including range maps. Helpful, and you might want to take a trip up to the Appalachians to see many of the species.   Or, even better yet to Japan, Korea, and Malaysia to see more of those wondrous worlds.

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