A total of 584 adult nematomorphs, Gordius difficilis, was collected from 2 man-made ponds and their overflow stream in southeastern Wisconsin. Ponds were surveyed throughout the year, but all free-living worms were found during July–August of 1996, July–September of 1997, and June–August of 1998. Overall sex ratio was male biased; however, sex ratio was variable during different months. Observations during 1998–2000 indicated that worms mated within 24–48 hr of emergence from their hosts and began laying eggs by mid-August, continuing until mid-October. Eggs with well-developed larvae were recovered during October and November. Encysted larval nematomorphs were recovered from aquatic and semiaquatic invertebrates (gastropods, earthworms, and insects), whereas developing hairworms were found in terrestrial European ground beetles Pterostichus melanarius. It is hypothesized that semiaquatic invertebrates may serve as intermediate/paratenic hosts in this system and are preyed upon by terrestrial carabid beetles, thus completing the life cycle. In addition, scanning electron microscopy observations of G. difficilis add previously unreported observations on intraspecific variation in body length, cuticle morphology, and gametes of this species. This is the first report of G. difficilis from Wisconsin as well as the first report of this species from P. melanarius and aquatic and semiaquatic invertebrates.
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