Monarch butterfly chrysalis

Once the butterfly has emerged the empty chrysalis is called an exuvia¹ (Figures 2 & 3).

There are holes through the cuticle called spiracles (Figures 1 & 2). The spiracles connect to fine tubes (trachea) within the developing butterfly (Figure 3). The trachea enable gas exchange¹. Some of the trachea are visible as fine threads when a butterfly emerges (Figure 6).

The caterpillar attaches itself to a silk button with its anal prolegs. But these are soon shed, along with the rest of the skin of the caterpillar. So the chrysalis replaces the proleg attachment with a peg-like part called a cremaster. The cremaster has many tiny hooklets to secure it to the silk².

The location of parts of the future butterfly are visible on the outside of the chrysalis^4,5 (Figures 1-4). The parts are easiest to see soon after the chrysalis forms and then when the butterfly is about to emerge.