December 2016 Featured Paper

On Sexual Selection in Florida’s Pyractomena borealis by Erin Gentry
Florida Entomologist 86(2), June 2003

Read the Original Paper

In many insects, males mature before females. Once the females mature, the males are ready and waiting to mate with them. This system, called protandry, has a number of advantages.

  • By emerging first, the males have time to produce more sperm before mating with the females.
  • By mating with a freshly emerged female, the males are insured of having virgin females to mate with, ensuring that they will sire the offspring.
  • The largest number of females is available to the males since the female population has not had time to decline from predation.
  • The females are mating with older and, supposedly, more fit males.

Firefly larvae live underground. Once the larvae are fully grown, they enter the pupal stage, then the adult stage. Most fireflies pupate underground, but fireflies in the genus Pyractomena pupate above ground by attaching themselves to vegetation, often on tree trunks.

There are risks involved in pupating above ground that don’t exist below ground. The pupa is exposed to temperature extremes; it is warmer during the day and colder at night. It is much dryer above ground, so the pupa is more exposed to desiccation. Parasites and predators are much more likely to attack an above ground pupa.

So why pupate above ground? It is theorized that by pupating above ground, Pyractomena can avoid flooding, which is important in an insect often found in wet areas. Also, because their rate of development is influenced by temperature, they will pupate in a shorter period of time in the warmer above-ground temperatures.

P. borealis is a protandrous insect. The adult males emerge first and “tend” the female pupa until the females emerge, at which times the males mate with them.

Previous studies have suggested that the timing of pupation in insects is determined by internal physiological changes. This study is the first to show that behavior can also have a major effect on the timing of pupation. This timing is important in a protandrous insect, as the males must emerge before the females. P. borealis males accomplish this by beginning pupation before the females.

They also choose warmer microhabitats for pupation than do females. The males choose larger trees. Large trees retain the warmth from the sun longer than small trees. The males are also found on the southern, sunny side of the trees more often than the females. Since warmer temperatures result in shorter pupation times, these behavioral traits ensure that the males emerged before the females.

In contrast, to P. borealis, P. limbicollis, does not engage in protandry. P. limbicollis pupates on the northeast side of the tree, where the temperatures are less warm but more stable. P. limbicollis does not exhibit behavior that selects for males emerging before females.

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