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Male Reproductive Tract
| Sperm & Sperm Production
| Spermatophores
Male Reproductive Tract
- Testes:
- Two testes in monarchs (and other animals) produce
sperm. Monarch testes are bright red, and are held together in
a single sac.
- Vas deferens:
- The vas deferens is a tube with a fairly thick
covering that connects the testes to the ejaculatory duct.
- Accessory glands:
- Lepidopteran accessory glands produce secretions
that may facilitate sperm transfer, produce cues to prevent remating
by the female, and provide nutrients for the female.
- Ejaculatory duct:
- The ejaculatory duct is a long tube that carries
sperm from the vas deferens and secretions from the accessory
glands to the aedeagus (penis). The ejaculatory duct is quite
complex in insects in which a complex spermatophore is formed,
like monarchs.
- Aedeagus:
- The insect intromittent organ (penis) is called
the aedeagus. In Lepidoptera, this is held within the last abdominal
segments except during mating. It is a continuation of the ejaculatory
duct, and is inserted into the female during mating. Sperm and
accessory gland materials move through the aedeagus into the bursa
copulatrix of the female.
Sperm begin to mature during the third and fourth larval instars;
the bright red testes are easily visible in dissected male larvae.
Sperm production (spermatogenesis) occurs through a series of
mitotic and meiotic divisions. A single original cell,
called a spermatogonium, is enclosed in a cyst that is formed from
other cells in the testes. This original cell undergoes six mitotic
divisions, to produce 64 spermatocytes. The spermatocytes undergo
two meiotic divisions, resulting in a total of 256 sperm cells.
These divisions take place during the larval and pupal stages. The
last part of sperm development involves transformation into a cell
with a head, which contains the nucleus and DNA, and a filamentous
tail, which propels the sperm forward. The sperm stay in bundles
of 256 until after they have moved out of the testes.
 All
butterflies and moths produce two kinds of sperm; eupyrene sperm
have a nucleus and can fertilize eggs, while apyrene sperm
do not have a nucleus. The function of the apyrene sperm is unknown,
but many researchers think that they may increase the chances that
the eupyrene sperm from a particular male are actually used to fertilize
the female’s eggs. The bundles of apyrene sperm separate before
being transferred to the female, but the eupyrene bundles stay together
and can be observed under a microscope shortly after copulation
ends.
Lepidopteran sperm are transferred within a protein-rich ejaculate
called a spermatophore. This spermatophore can represent a significant
investment by the male; some male monarchs transfer spermatophores
that weigh up to 10% of their own mass! But this isn’t the
lepidopteran record; males in another species (Pieris napi)
can transfer up to 23% of their mass during mating (Forsberg and
Wiklund 1989). The spermatophore is not transferred intact to
the female; most of it forms during mating within an organ in
the female called the bursa copulatrix. The roundish body of the
spermatophore is covered with a tough, white sac, and contains
a granular substance. The stem-like structure is called the collum.
It forms within the male’s aedeagus and
is transferred with the sperm at the very end of copulation. The
collum has an opening that is positioned next to the opening of
a duct in the female that leads to the sperm storage organ. The
sperm are contained in a discrete sac in the pointed end of the
spermatophore. It takes a long time to transfer all of this material
to the female; mating monarchs often remain paired for 16 hours
or longer.
At the University of Minnesota, we have studied
the size, composition, and function of monarch spermatophores.
on to Female Anatomy
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