Seasonal occurrence, morphology, and observations on the life history of Gordius difficilis (Nematomorpha: Gordioidea) from southeastern Wisconsin, United States

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 inverte- brates (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. difficilisadd previously unreported observations on intraspecific variation in body length, cuticle morphology, and gametes of this species. This is the first report of G. difficilisfrom Wisconsin as well as the first report of this species from P. melanarius and aquatic and semiaquatic invertebrates. In 1996, a large population of a rare North American hair- worm, Gordius difficilis (Montgomery, 1898) Smith (1994), was located in southeastern Wisconsin. The natural history and ecology of nematomorphs, also known as gordian or horsehair worms, is poorly known (Chandler, 1985; Poinar, 1991a, 1991b; Schmidt-Rhaesa, 1997a). Larvae are most commonly reported as obligate parasites of orthopteran and coleopteran insects, whereas adults occur in aquatic habitats and do not feed (May, 1919; Schmidt-Rhaesa, 1997a). Adult worms often occur in small numbers; little information exists on their general biology and on variation in morphology among populations. Gordius difficiliswas initially described from a single male from North Carolina as Gordius aquaticus difficilis(Montgom- ery, 1898). Montgomery (1907) later suggested that this taxon was a form of Gordius robustus Leidy 1859. Smith (1994) re- cently redescribed this species using scanning electron micros- copy (SEM) from a collection of 16 male and female specimens from Massachusetts. This species differs from G. robustus in having an iridescent cuticle, white in females and yellowish- brown in males and, in general, is smaller in size and thickness than G. robustus. Males posses a parabolic ridge anterior to the cloaca containing a discrete row of hairlike structures. Both sexes lack a dark ring behind the head as seen on G. robustus and possess distinct areoles. Recently, Hanelt and Janovy (2000) reported G. difficilisfrom a vivid metallic ground beetle Chlaenius prasinus from western Nebraska. No other infor- mation is available on its basic biology; the present study pro- vides some of this information. In this paper, the seasonal oc- currence and observations on the life history of G. difficilis from southeastern Wisconsin are reported. Also provided are previously unreported observations on cuticle morphology, gametes, and hosts of this species.