The female Monarch will plant all six tarsi (claw-like feet) onto the milkweed plant before she deposits her eggs. To ensure survival of species, she must place the eggs on the right plant; there are sensors located on the tarsi that tell her if this is a milkweed plant. To provide enough food for the hatching caterpillars, the female Monarch will only leave one or two eggs on a single plant. One hungry caterpillar can quickly strip one milkweed plant. Milkweed is the sole host food for the Monarch butterfly larvae.
Following this busy Monarch around the garden while she deposited eggs on the Asclepias incarnata made it easier to later photograph the microscopic eggs. Note how she tucks under the abdomen as she deposits an egg on the leaf of this host plant.
Smaller than a pin-head, the spherical Monarch egg contains a yolk to feed the larvae. The surface of the egg has vertical ridges. The egg breathes through the aeropyles (holes on the surface of the egg). Water and air also enter the indentation at the top, shaped like a funnel, called the micropyle.
The Monarch egg is protected by a tough outer shell called chorion; a waxy substance coats the inside to keep the egg from drying out. There is a slight indentation at the top is where the sperm enters the egg. This pit is called the micropyle which also collects air and water for the egg. This egg is positioned on the underside of the milkweed leaf.
The black head of a first instar Monarch Caterpillar is quite visible and can be seen moving. Soon, the tiny, very hungry caterpillar will chomp its way out of the egg; the exterior shell provides the tiny cat the first nourishment.
Eaten by the caterpillar inside, this is all that remains of the Monarch butterfly egg.
A dime illustrates the size of this first instar Monarch caterpillar, newly emerged only a couple of hours before.