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. 

on to Male Anatomy