Reproduction
The
Photinus marginellus reproduce through means of
reproduction called nuptial gifts. Nuptial gifts are nutritional
contributions delivered from the male to the female during
sexual intercourse (Lewis et al., 2004). The nuptial gifts are
transferred through a spermatophore, which is a capsule of sperm
produced by the male accessory glands, and function to carry the
nuptial gift to the female during intercourse. The flash duration of the male
determines their spermatophore mass. The longer the flashes last,
the less spermatophore mass they have (Cratsley, 2004). As adults
most Photinus do not feed, therefore, reproduction is primarily
based on the resources received through the nuptial gift, and given
to the larva (Lewis et al., 2004). Nuptial gifts are of precise
economic importance within this insect group.
The male reproductive system for Photinus marginellus consists of
four pairs of accessory glands, which are tightly coiled and spiral
in shape. The most important of the four specific glands is the one
containing the spermatophore components, which house the nuptial
gifts (Lewis et al., 2004). The spermatophore, enclose the
pre-spermatophore, which are also spiral in shape, with two rows of
longitudinal pyramidal scales (Lewis et al., 2004). Aside from the
most prominent gland, the male has three additional glands that are
tubular and are each different in length. The long gland extends to
approximately 18-24 mm in length, while the medium gland greatly
declines in size into 5-7 mm long. The short, and last gland of the
three is approximately only 1 mm in length (Lewis et al., 2004).
During the early stages of intercourse, the four accessory glands of
the male secrete different fluids, which then all combine in the
male ejaculator duct with the sperm. The sperm have been pre-stored
in the seminal vesicles of the male organism. When the sperm has
come into contact with the fluid from the accessory glands, the
sperm become packaged into ring-shaped bundles, and are attached at
the end of the spermatophore (Lewis et al., 2004). This entire
spermatophore now has a gelatinous structure due to the fluid and
sperm combination. Within an hour of copulation, only part of the
spiral-coiled, gelatinous spermatophore has been transferred from
the Photinus marginellus male to the Photinus
marginellus female’s spermatheca, the female’s reproductive
storage for the encountered sperm. The remaining end of the
spermatophore enters the Photinus female’s reproductive tract in a
specialized structure called the spermatophore-digesting gland,
where the digesting gland functions to disintegrate the remainder of
the spermatophore within a few days (Lewis et al., 2004).
Figure 1. The Spiral spermatophore of Photinus marginellus,
inside the female reproductive tract. The spiral coils are visible,
and the sperm rings have been
released from the tip of the spermatophore into the female’s
spermatheca. This depicts the early stage of copulation (within
first hour) (Lewis et al., 2004).
The female Photinus marginellus then uses the spermatophore protein (nuptial gifts) to
develop their oocytes (a cell in an ovary, which undergoes mitosis
to form an ovum). The nuptial gifts, therefore, are important
supplements for support and nutrients to the larva in the female.
Nuptial gifts have evolved to be more than just a benefit to the
female, but a necessity as well. The nuptial gifts housed within the
spermatophore of the Photinus marginellus male provide the
Photinus marginellus females with energy reserves,
most importantly for non-feeding Photinus adults (Lewis et al.,
2004). In the non-feeding adults in particular, the energy reserves
diminish gradually in the larva, and need the nutrients in the
nuptial gifts from the Photinus male to supply the energy supplement. Also,
parasites could further reduce the resources needed for female
reproduction, which could be resupplied through the nuptial gifts as
well (Lewis et al., 2004).
Although production of a spermatophore is a beneficial necessity for
the Photinus females for reproduction, and the Photinus
males for sexual selection, it
is costly for the males to produce them. The spermatophore mass
decreases steadily, on average about 75%, between the first and
fourth sexual encounter with a female (Lewis et al., 2004). Also,
the cost of the spermatophore production may confine the male’s
mating success. Even if the Photinus males have access to a female on a
daily basis, the mating success declines, on average, during the
last half of the Photinus male's adult existence (Lewis et al., 2004). Due to
the limitations of the spermatophore production, this eventually
leads to the nuptial gift ability to decrease as the mating season
continues. The diminishing spermatophore mass, along with the
decreasing nuptial gift ability, could potentially lead to weakening
reproductive returns for the Photinus males (Lewis et al., 2004), meaning the
males would be unable to provide the spermatophores, and can no
longer copulate with a female.
It is clear that nuptial gifts influence Photinus marginellus in
very valuable ways. The nuptial gifts provide nutrients and energy
to the larva and to the Photinus female. The cost of the nuptial gifts and
production of spermatophore is very costly, and decreases with every
female the male encounters in sexual intercourse. The less
spermatophore the Photinus male can produce, the less nuptial gift
availability (Lewis et al., 2004), which, therefore, affects sexual
selection of the male (Cratsley, 2004). Early in the mating season,
when the male is the most readily able to generate spermatophores,
the sex ratios are male-biased, and can result in male-male
competition due to the Photinus marginellus females primarily preferring the
Photinus marginellus males that can provide the largest
amount of nuptial gifts. This is again expressed to the female based on the
intensity of the flashing
signals the male provides. However, later in the mating season,
females greatly outnumber the males as the spermatophore frequency
declines. The lower the intensity of flashes from the male’s abdomen, states
low spermatophore mass, which then causes the female to respond
elsewhere for greater production of the beneficial spermatophores
(Cratsley, 2004). To look at more information and pictures of
the difference between male and female Photinus marginellus
look on the website for the
Muesum
of Science, Boston; identifying genders. This page will show
you the difference between the male and female light organelles
and were they are placed. If you would like to gain more knowledge on
nuptial gifts in relation with sexual selection, visit the journal
article:
Nuptial Gifts and Sexual Selection in Photinus Fireflies.
The flashing light signals and how intense the Photinus marginellus male can radiate his bioluminescent glow are essential characteristics for the Photinus marginellus female to consider when choosing a male. These interactions play a giant role in sexual selection for reproduction between the Photinus marginellus male and Photinus marginellus female.