Horsehair worms parasitize only invertebrates such as insects . To complete their life cycle , the worms must infect large invertebrates that are relatively long lived . Generally , horsehair worms aren ' t considered an effective biological control agent , because they parasitize only a small percentage of a host population . Horsehair worms are harmless to vertebrates , because they can ' t parasitize people , livestock , pets , or birds . They also don ' t infect plants . If humans ingest the worms , they may encounter some mild discomfort of the intestinal tract , but infection never occurs . <| sep |>. And now I know a new , very useful shortcut . <| sep |> There are many really welldesigned ones on thingiverse , seriously people on that site are incredible but I went with a simple , quickly printable one .