Waterstriders of the Lower Illinois River Basin
The insect suborder Gerromorpha includes the water striders (Gerridae) four other families occurring in Illinois. These animals inhabit the surface film of both lotic and lentic waters, where they are predators and scavengers feeding upon insects trapped in the surface film. The biology, ecology, and distribution of Illinois Gerromorpha were the subject of my recent (1996) dissertation. Currently, additional specimens are being collected to obtain more data on species distributions and habitats in the state. These data will ultimately be published with more comprehensive distribution maps and keys to the species of the state.
Thirty eight species of Gerromopha are known from Illinois, based on field collections in the southernmost 13 counties of Illinois and by examination of all of the major Illinois museum collections. Counties containing ten or more species comprise only 15% of Illinois' 102 counties, and more than one third (35.3%) have no gerromorphans reported from them. The apparent occurrence of few to no species throughout much of the state may reflect intensive agricultural practices and a limited diversity of habitats. However, the higher numbers of species from Coles (18 species) and Champaign (13 species) counties suggest that, instead, a lack of sampling effort accounts for much of the observed effect.
Understanding the distribution and abundance of Illinois' biota is a vital part of long term management of the states natural resources. My ongoing study aims to contribute important information on a poorly known group of insects which inhabit a unique aquatic habitat (the surface film). Developing a fairly complete picture of the distribution of Gerromorpha within Illinois is expected to require several years of field work throughout the state, focusing on areas poorly represented in collections. As a small step towards this larger goal, this proposed project will provide a more complete picture of the taxonomic composition, habitat distribution, and species associations of gerromorphan species in the lower Illinois River basin and Lincoln Hills.
- To conduct a faunal inventory of the Gerromorpha in six counties in the lower Illinois River basin and adjoining parts of the Lincoln Hills.
- To produce updated statewide distribution maps for all gerromorphan species in the study area.
- To produce a faunal list of gerromorphan species in the study area.
- To compile information on microhabitat and species associations for gerromorphan species found in the study area.
Field collections were conducted three multi-day trips to Brown, Calhoun, Greene, Jersey, Pike, and Scott counties (shown on map to right - green dots are historical records): early Spring (16 March 1999), late Spring (7-9 May 1999), and Autumn (21-23 September 1999). One additional collection, not a part of this study, is included here for convenience (Brown County, 16 September 1998). Adults of many species are seasonally present, so collections during different sampling periods should increase the taxonomic diversity in samples. A variety of types of aquatic habitats (ponds, streams, rivers, springs, etc.) were sampled for gerromorphans. Sites were classified into habitat types based on field observations. Collections were made using an aquatic D-net and a small dipnet, supplemented by hand picking. Samples were preserved in 70% ethanol in the field.
In the laboratory, samples were sorted to species, sex, developmental stage, and wing morph. Data collected were used to develop a species list and distribution maps for the gerromorphans found in the study area. Information on habitats and species associations were also recorded. All material has been cataloged and deposited in the Illinois Natural History Survey Insect Collection.
During this study, 103 sites were visited (shaded circles in map below, open circles are historical records). A total of eighteen species (Table 1) were collected, 12 of which had not previously been reported from the study area. Updated state distribution maps for each of the species are presented in Figures 2 and 3.
Table 1. List of species (number of sites at which species was found).
GerridaeAquarius nebularis (12)
Aquarius remigis (41)
Gerris argenticollis (2)
Gerris insperatus (4)
Gerris marginatus (9)
Limnoporus canaliculatus (3)
Rheumatobates palosi (8)
Rheumatobates tenuipes (9)
Metrobates hesperius (2)
Trepobates knighti (4)
Trepobates subnitidus (7)
Merragata hebroides (1)
Hydrometra martini (7)
Mesovelia cryptophila (2)
Mesovelia mulsanti (4)
Microvelia americana (19)
Rhagovelia obesa (1)
Rhagovelia rivale (1)
Each collection site was assigned to one of eight different general habitat types. Lotic habitats were more common and more commonly examined than lentic habitats (ponds and lakes). Many of the temporary streams examined were dry in May and September, and the March collecting trip was generally too early for many of the species. The majority of the faunal diversity was found in permanent streams. Lake and large river habitats generally yielded more taxa than were found in Taylor (1996), probably because a small kayak, available for use in the present study, allowed access to more microhabitats.
Species presence/absence data were tallied for all sites, and these data were used to construct a phenogram of similarity among species (Figure 1). This phenogram is compared to a similar phenogram constructed by Taylor (1996, Figure 14), based on collections made in the 13 southernmost counties of Illinois. Several of the dominant species in collections, Aquarius remigis, Microvelia americana, Aquarius nebularis, Gerris marginatus, and Hydrometra martini, tended to demonstrate similar patterns of species associations as were found by Taylor (1996). In addition, a cluster formed by Rheumatobates palosi, Rheumatobates tenuipes, and Trepobates subnitidus probably represents a natural association. However, many of the other taxa, especially those less frequently collected, show no obviously meaningful species associations.
Figure 1. Phenogram of Normalized Root Mean Square Distance (average linkage cluster analysis) between gerromorphan species collected in Brown, Calhoun, Greene, Jersey, Pike, and Scott counties, Illinois, during the present study (March-September 1999) based on species presence/absence data at each site. Taxa not identified to species level were excluded from the analysis.
Acknowledgements and Support
The Illinois Natural History Survey provided facilities and other support. Mark J. Wetzel provided valuable field assistance and kindly reviewed a draft of this report. Heidi Stuck assisted with sorting and labeling samples in the laboratory. This project was funded through a grant from the Illinois Department of Natural Resources' Wildlife Preservation Fund, Small Project Program.
Taylor, S. J. 1996. Habitat preferences, species assemblages, and resource partitioning by Gerromorpha (Insecta: Heteroptera) in southern Illinois, with a faunal list and keys to species of the state. Ph.D. Dissertation, Zoology Department, Southern Illinois University at Carbondale, Carbondale, Illinois. xviii + 345 pp.
Mesovelia mulsanti, adult female. Photo by Steve Taylor June 2006.
This web page is based on the following report:
Taylor, S. J. 1999. Inventory of the water striders of the lower Illinois River basin.
Technical Report 1999 (25), Illinois Natural History Survey, Center for Biodiversity.
Natural Heritage Division, Illinois Department of Natural Resources. 27 pages.
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