Thank you for your question. The hammerhead worm, is a land planarian. The most common one is Bipalium kewense. They are not native to the United States, however they have been here for over 100 years. This species is native to Indo-China. It was first described in 1878 from a specimen found at the Kew Botanical Gardens in London, England. It has been common in greenhouses here in the U.S. since around 1901.

They have been found in wild habitats in Florida, Louisiana, southern California, Georgia, North and South Carolina and Texas. They have been reported in greenhouses in Alabama, California, Georgia, Illinois, Kentucky, Massachusetts, Mississippi, New Jersey, New York, North Carolina, Ohio, Oklahoma, South Carolina and Tennessee. Once they become established in greenhouses, they can expand to natural habitats. So, you can see they are getting around. Their widespread distribution is attributed to horticulture practices and the shipment of potted plants from greenhouses and nurseries.

Planarians require high temperature and humidity and a food supply consisting of earthworms, slugs and insect larvae. They are also cannibalistic. They can have a significant impact in commercial earthworm farms, but they are not considered a problem for homeowners.

Here's a link to an excellent publication on these guys from the University of Florida's Entomology & Nematology Department:

http://entnemdept.ufl.edu/creatures/misc/land_planarians.htm

I hope this answers your question, and thank you for contacting Ask an Expert.

Jim