Reproductive Biology
Introduction | Male Anatomy | Female
Anatomy | Fertilization |
Research Projects
Background
Much of our work on reproduction in monarch butterflies has focused on the factors
that affect the number of offspring that individual monarchs produce. In order to
study this we needed to know basics of reproduction: How do males and females produce
sperm and eggs, and how and when are eggs fertilized? We answered these basic questions
by studying work done by other scientists on insect reproduction, and by observing
these processes ourselves. We were constantly surprised at the amount of basic background
information that was unknown! In this section, we’ll explore the amazing process
that ends in egg laying by female monarch butterflies.
We say that animals are sexually mature when they can produce offspring, and most
animals require time to develop before they reach this stage. In humans, the onset
of sexual maturity is a gradual process caused by hormones released by the hypothalamus
and the pituitary (parts of the brain). A similar process occurs in monarchs. A
pair of glands called the corpora allata release a hormone called juvenile hormone.
High levels of this hormone circulating in adult butterflies cause eggs to mature
in females and cause the male reproductive tract to develop. The reproductive organs
in most animals, including humans and monarchs, begin to develop a long time before
sexual maturity. Monarch reproductive organs actually begin to develop during the
larval stage, but it isn’t until they receive hormonal messages that they complete
development.
In most insects, sexual maturity coincides with the final molt to the adult stage.
However, it may be delayed for several days after the beginning of the adult stage,
and in species with an adult diapause, maturity may
even be delayed for several months. In monarchs, breeding season individuals are
sexually mature four to five days after they emerge as adults, and the generation
that migrates is not sexually mature until after the overwintering period. Research
by Dr. Bill Herman (1975) at the University of Minnesota has shown that diapause
monarchs have low levels of juvenile hormone circulating in their blood.
Continue to Male Anatomy