Introduction | Male Anatomy | Female
Anatomy | Fertilization |
Male Reproductive Tract
- Two testes in monarchs (and other animals) produce sperm. Monarch
testes are bright red, and are held together in a single sac.
- Vas deferens:
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:
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.
- 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 and Sperm Production
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.
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.
Continue to Female Anatomy