As a caterpillar, Wormy is a light shade of green with black eyes and white spots.
As a butterfly, Wormy is dark orange with black stripes and black on the outermost parts of his wings; this design is that of a monarch butterfly.
At some point, Sandy bought or found him in Texas. Sometime later, she took Wormy to her home in Bikini Bottom.
He later turns into a butterfly overnight.
Role in episode
When SpongeBob and Patrick are pet sitting for Sandy, they and Wormy play games such as hide-and-seek and tag. However, overnight, Wormy metamorphoses into a butterfly and scares everyone in Bikini Bottom because they have never seen a butterfly before.
After Sandy returns from her trip and captures Wormy in a jar, the Bikini Bottomites celebrate, prompting Sandy to believe that she did not know she had be missed so much and should leave town more often.
In The SpongeBob Movie: Sponge Out of Water, a poster of Wormy in his butterfly form can briefly be seen in Sandy's treedome. The image of Wormy on the poster looks very similar to the one seen on the "Beware" fliers made by SpongeBob and Patrick.
The close-up shot of Wormy in his butterfly form is actually footage of a buzzing horsefly.
During the news bulletin about the "monster," the live-action monarch butterfly stock footage depicts a female butterfly (hence the thicker wing veins than the male), suggesting Wormy may have actually been female.
The episode "Wormy" was banned in some countries because of Wormy's terrifying close-up shots.