The biota of Plummers Island, Maryland, the research home of the Washington Biologists' Field Club, has been the subject of countless biological investigations over the last 100 years. While the flora and vertebrate fauna are fairly well known, the invertebrate fauna remains poorly documented with the exception of several families of insects. This paper presents a brief description of the site, notes on land-use over the last 100 years, and comments on collecting and research activities focused on invertebrates. It also serves as an introduction for the contributions that constitute this volume—a collection of papers on various aspects of the invertebrate fauna.

Five species of freshwater triclad planarians were found on Plummers Island and the adjacent mainland property, Montgomery County, Maryland. One species (Dugesia [G.] tigrina) occupying the Potomac River and a tributary, is tolerant of degraded habitat. The other four occupy vernal pools (Hymanella retenuova and Phagocata velata) or spring-seeps (Phagocata morgani and Paraplanaria dactyligera) and appear to be indicators of high quality aquatic habitat. These five species represent 36% of the total known Maryland fauna of 14 species.

The nematode fauna of Plummers Island, on the Potomac River near Washington, D.C., is virtually unknown. Only three species have been reported from the Island: Daptonema dubium (Bütschli, 1873) Lorenzen, 1977, Isolaimium papillatum Cobb, 1920, and Tylolaimophorus cylindricum (Cobb, 1920) Goodey, 1963. We surveyed terrestrial and aquatic habitats on Plummers Island to begin to characterize its nematode fauna. Nematodes of 47 genera were found, representing 9 orders and 32 families, including an estimated 59 species. We provide diagnoses of the 3 species reported from the literature, a classified list of genera, their habitats and life stages found, and an estimate of the number of species found in each genus.

We found 19 species of freshwater mollusks (seven bivalves, 12 gastropods) in the Plummers Island area, Maryland, bringing the total known for the Middle Potomac River to 42 species. We found seven species not previously known from the Middle Potomac River, but we failed to find 23 species that had been reported there previously. The new records probably reflect our searching in backwater and woodland pools, habitats that had been poorly searched previously. Species not found but represented by historical records (including 15 bivalves, of which 13 are Unionidae) may represent extirpations or species missed owing to low search effort for mussels. The introduced bivalve Corbicula fluminea, apparently absent from the Middle Potomac in 1978, was the third most numerous species collected in our survey. We found three bivalve species not previously reported in the Middle Potomac: Utterbackia imbecillis (Unionidae), and Pisidium compressum and Pisidium nitidum (both Sphaeriidae).

A survey of the landsnails inhabiting the 20.5-hectare portion of Chesapeake and Ohio Canal National Historic Park formerly owned by the Washington Biologists' Field Club was conducted. Twelve stations, representing all major habitat types present, were intensively studied, and general collecting was done throughout the area. Twenty-five species of snails and slugs representing 12 families were documented. Survey of the literature and pertinent museum collections documented very little change in the composition of the gastropod community over the past 100 years, despite vegetation community succession of the property from largely open field to mature forest. Of nine species known historically from the study area, only one was not found during this survey. Although the mollusk community was diverse, the number of small species was less than expected. This may be the result of the near absence of a leaf litter/humus layer. The absence of those forest floor layers from the study area may be due to the presence of a large population of the introduced Asiatic earthworm, Amynthras agrestis. Regular inundation of flood plain areas along the Potomac River appears to depress or eliminate discrete colonies of land snails.

A survey of the terrestrial isopods of Plummers Island, Maryland and its immediate vicinity revealed the presence of ten species representing nine genera and seven families. All are believed to have been introduced into North America from Europe. Eight of the ten species occurred throughout the study area, while two generally were limited to the edge of the Chesapeake and Ohio Canal towpath and the clearing around the historic lock keeper's house at Lock 11.

Copepod (Crustacea) species diversity was remarkably low on Plummers Island, Maryland, and its immediate environs over a year-long sampling period in 1996 and 1997, compared to that previously and contemporaneously reported nearby within the Potomac River Basin; only two species were found on the Island and two others in the adjacent side channel of the Potomac River. Subsequent sampling in 2004 resulted in records of eight species, seven of which were not found in 1997; six of these were collected on the Island. One explanation for the paucity of species in 1996–1997 is the extreme floods of January and September 1996; the January flood reached a height not recorded since 1972 and destroyed canal locks. These floods scoured much of the Potomac floodplain and may have caused severe local population losses. The scarcity of aquatic microhabitats on the Island undoubtedly also contributed to the low local diversity.

A review of the crayfishes and entocytherid ostracods occurring in the vicinity of Plummers Island, Maryland revealed that four species of crayfishes and four species of ostracods were present. One of the crayfish is exotic and appears to be spreading throughout the Potomac River drainage. Three of the ostracods are common in Maryland, but one is known only from Plummers Island and a single locality in Indiana.

Two species of pseudoscorpions, Tuberocreagris rufula (Banks) and Chthonius virginicus Chamberlin, are reported from Plummers Island, Maryland. Several specimens of C. virginicus were found in empty snail shells, and although this behavior has been reported previously, it is not well documented.

Plummers Island, a small site situated along the northern shore of the Potomac River in Montgomery County, Maryland, has been the research home of the Washington Biologists' Field Club for more than 100 years. Field work conducted by club members from 1901 to about 1925 resulted in the accumulation of thousands of insect specimens of all orders from the Island, most of which are deposited in the collections of the National Museum of Natural History, Smithsonian Institution. Little collecting was conducted from ca. 1930–1950. In the 1960s sampling by Karl Krombein focused on bees and wasps and that by Terry Erwin on carabid beetles. Since 1998 the Lepidoptera fauna, leaf beetles (Chrysomelidae), and darkling beetles (Tenebrionidae) all have been the subject of investigations. In 2005 and 2006 Malaise traps were deployed to sample other orders (e.g., Trichoptera, Diptera, Hyemenoptera). While the four major insect orders (i.e., Coleoptera, Diptera, Lepidoptera, and Hymenoptera) are represented by large numbers of historical specimens, only Lepidoptera have been surveyed thoroughly in recent times; notable exceptions include specific families: carabid beetles, leaf beetles, darkling beetles, sawflies, and bees and wasps. Based on an examination of the insect collection of the National Museum of Natural History and a review of relevant literature, we document 3012 insect species in 253 families, encompassing 18 insect orders: Collembola, Odonata, Dermaptera, Blattodea, Phasmatodea, Orthoptera, Psocoptera, Thysanoptera, Hemiptera, Neuroptera, Megaloptera, Coleoptera, Mecoptera, Trichoptera, Lepidoptera, Diptera, Siphonaptera, and Hymenoptera.

Based on the examination of approximately 8100 specimens of Lepidoptera in the collection of the National Museum of Natural History and a review of relevant literature, we document 836 species in 488 genera and 48 families from Plummers Island, Maryland. Although the Lepidoptera are probably the best studied insect order on Plummers Island, data from the Washington, D.C. area indicate that there likely are many more microlepidoptera and butterflies on the site that are yet to be documented. Most families that were sampled adequately both historically (1901–1920) and in recent years (1998–2005) show a reduction in species richness and considerable species turnover. However, interpretation of these data is difficult owing to differences in sampling techniques and sampling frequency over the last 100 years.

During the summers of 2004 and 2005 host plant associations of larvae of Lepidoptera were studied on Plummers Island, Maryland, with an emphasis on the family Limacodidae (slug caterpillars). A total of 37 species from 12 families was recorded, representing 79 caterpillar-host plant combinations. Eleven species of Limacodidae were collected and reared from six target host plants—box elder (Acer negundo L.), sugar maple (Acer saccharum Marshall), pawpaw [Asimina triloba (L.) Dunal], pignut hickory [Carya glabra (Mill.) Sweet], spicebush [Lindera benzoin (L.) Blume], and red oak (Quercus rubra L.).

A total of 36 species of butterflies was documented from Plummers Island, Maryland, based on four sources: transect survey (2006), diurnal collecting (2005), Malaise trap sampling (2005–2006), and historical records (1904–1997). Each of the four sources contributed species not recorded in the other three, demonstrating the importance of combining methods when compiling inventories. Twenty-five species (69% of the total) were observed during a 10-week transect survey in 2006, with six species not recorded in other sources. Nineteen species (53%) were captured during diurnal collecting in 2005, with five species not recorded in other sources. Eight species (22%) were collected in Malaise traps, with one species not recorded in other sources. Ten species (28%) were represented by historical records in the collection of the National Museum of Natural History, Washington, D.C., with four species not recorded from other sources. The number of species (i.e., 36) is less than what would be expected based on previous inventories of the butterflies of the Washington, D.C. area, and this is most likely the result of the under-sampling.

From 1902–2005, 75 species of Crambidae and 48 species of Pyralidae were collected on Plummers Island, Montgomery County, Maryland. An annotated list of the two families is provided, along with photographs of all recorded species. The Pyraloidea of Plummers Island have wide distributions in eastern United States with some species occurring as far west as Texas and a few others ranging from coast to coast. Hosts recorded in the literature are given, but they are unknown for 36% of the species. The majority of Pyraloidea feed on vascular plants, but hosts are diverse including algae, scale insects, and immatures of wasps, and bagworms.

From 1902–2005 six species of Nolidae, 88 species of Erebidae, and 118 species of Noctuidae have been recorded from Plummers Island, Montgomery County, Maryland. Extensive collecting conducted from 1998–2005 resulted in five species of Nolidae, 70 species of Erebidae, and 88 species of Noctuidae. Only the Nolidae had sufficient specimens collected from 1902–1930 to compare with collections taken from 1998–2005. Five species were present from 1902–1930, and five from 1998–2005 with no change in species richness. A 20% species turnover was noted, with one extinction and one colonization. Owing to probable under-sampling from 1902–1930, the Erebidae and Noctuidae show a 37% and a 32% increase, respectively. This is contrary to other studies at Plummers Island, which showed a decrease in species richness. Species accumulation curves were based only on material collected from 1998–2005. Both abundance and incidence-based estimators were used to predict six species of Nolidae, 79–102 species of Erebidae, and 101–135 species of Noctuidae. Of the total of 212 species for all three families, 75.9% of the species were represented by 10 or fewer specimens. A checklist of the Nolidae, Erebidae, and Noctuidae, and their monthly abundances from 1902–2005 is provided.

During 2004 and 2005 insects were collected on or adjacent to Plummers Island, Montgomery County, Maryland using an ultraviolet light trap (2004 (2005) and two Malaise traps (2005). Forty-seven species of Trichoptera were identified from this material, representing 13 families and 25 genera. Most of the species are common and widely distributed over eastern North America, and none is considered of special concern or sensitive.

The National Insect Collection was searched for specimens of Mecoptera collected on or near Plummers Island, Montgomery County, Maryland, and the records were taken from those found. A special effort was made in 2004 and 2005 to collect insects on or adjacent to Plummers Island, using a number of techniques. In addition to the usual ultraviolet light traps, two Malaise traps were operated during the season in 2005. Eleven species of Mecoptera were identified from this material: nine taken in the 1900s and three in 2004–2005. They belong to four families and four genera. Most are species widely distributed over eastern North America, and none is considered endangered.

During 2004–2006 insects were collected on or adjacent to Plummers Island, Montgomery County, Maryland by a number of techniques. Most neuropteroids were attracted to an ultraviolet light trap, but some were taken in Malaise traps operated during the season in 2005 and early 2006. Sixteen species of neuropteroids were identified from this material; they belong to 6 families and 14 genera. Although the samples included most of the megalopteran genera and species that are known from the region, the Neu-roptera were poorly represented. Five families almost assuredly present on the Island were not found. Most of the documented species are rather widely distributed over North America and none is considered endangered.

Species occurrences of darkling beetles (Coleoptera: Tenebrionidae) are listed for the historically collected locality of Plummers Island, Maryland, on the Potomac River just upstream from Washington, D.C. The list is compared to that of the currently known Maryland species, which includes a number of new state records and range extensions. Notes on some of these occurrences and the absence of certain species are discussed. Maryland records from multiple sources now total 128 species of this family of insects. Plummers Island records, from the beginning of the 20th century to the present, include nearly 50% of the state's fauna, and an additional 25% of the state's tenebrionids are expected there.

The Chrysomelidae fauna of Plummers Island, Maryland consists of 161 species. This represents 43% of the known Maryland fauna. Work conducted in 1997 and 1998 showed a species turnover rate of 72.6% but a decline in species richness of only 2.1%. Nine species have been collected nowhere else in Maryland. Tricholochmaea decora decora (Say) is reported from Maryland for the first time.

Based on historical records in the collection of the National Museum of Natural History, Smithsonian Institution, Washington, D.C., 63 species of Cerambycidae (Coleoptera) are reported from Plummers Island, Maryland. This number represents 24.9% of the Maryland cerambycid fauna. Specimens were collected from March to September, with July yielding the highest total (160 or 38.8%). Only 25 specimens were collected after 1927: three in 1931, three in 1932, one in 1933, one in 1941, 16 in 1968, and one in 2004. Four specimens of the state-listed endangered species Dryobius sexnotatus Linsley were collected prior to 1920.

Based on historical specimens in the collection of the National Museum of Natural History, Smithsonian Institution, Washington, D.C., 25 species of Coccinellidae (Coleoptera) were collected on Plummers Island between 1902 and 1960. This represents approximately 32% of the recorded fauna of Maryland. Neoharmonia venusta venusta (Melsheimer) was the most commonly collected species.

Eighteen species of Hydrophiloidea were collected on Plummers Island, Maryland between 1903 and 1972, representing 17.5% of the known fauna of Maryland. Of these, eight species are aquatic and ten are terrestrial. The most commonly collected species were Cymbiodyta chamberlaini Smetana and Enochrus cinctus (Say).

Based on historical records in the collection of National Museum of Natural History, Smithsonian Institution, Washington, D.C., 18 species of Dytiscidae have been collected on Plummers Island, Maryland. This represents 21.4% of the known Maryland dytiscid fauna. From 1903–1932 to 1960–1970, collection records show a turnover of 50% in the species composition of dytiscid beetles of the site. Agabetes acuductus (Harris), a species of special concern, has been collected twice on the Island.

Based on an examination of the collection of the National Museum of Natural History, Smithsonian Institution, Washington, D.C., seven species of Silphidae (Coleoptera) were collected on Plummers Island, Maryland, from 1905 to 2004. This is 38.8% of the known silphid fauna of Maryland. The most commonly collected species is the habitat- and carrion-generalist Nicrophorus tomentosus Weber.

The published records of the fleas (Siphonaptera) known to occur on Plummers Island, Montgomery County, Maryland are reviewed. Peromyscopsylla scotti is reported for the first time, bringing the total number of species known from the Island to ten.

Ninety-one species of sawflies (Hymenoptera: Symphyta) are recorded for Plummers Island, Maryland. Records are from collections during the periods of 1902–1924, 1958–1972, and 2005. An estimated 97 species currently may occur on the Island. Indications are that species composition has changed through the years. Only 22 of the 51 species (43%) collected during 1902–1924 have been collected in subsequent years, and only 26 of the 48 species collected in 2005 (54%) have been collected previously. Records also are given for 20 species of 11 families of Apocrita: 6 species of Aulacidae, 3 of Evaniidae, 1 of Gasteruptiidae, 1 of Heloridae, 1 of Ibaliidae, 1 of Pelecinidae, 1 of Roprionidae, 2 of Rhopalosomatidae, 1 of Stephanidae, 2 of Trigonalidae, and 1 of Vanhorniidae.

Plummers Island, Maryland has been studied by naturalists for over 100 years. The bees collected on the Island and the immediately adjacent mainland represent six families, 41 genera, and 232 species. About 20% (47 species) are parasitic and do not collect pollen. Most bees are generalist (polylectic) foragers, but there are a few species that appear to visit only a few species or genera of plants (oligolectic foragers). Three exotic species are among the fauna, including the European honey bee (Apis mellifera L.). Based on historical specimens in the collection of the National Museum of Natural History, Smithsonian Institution (USNM), and contemporary survey efforts, the number of bee species on the Island appears to have increased since the 1920s–1960s, and there is no evidence of local species extinction. It is possible that the use of Malaise and pan traps in addition to hand nets have increased collecting efficiency so that the increase in species richness is an artifact of collecting techniques rather than a biological phenomenon. Alternatively, increased species richness may reflect the resiliency of bees and an increase in available nesting sites as heavily shaded forests of the eastern United States have become open through deforestation and urbanization. While the vegetation of the Island has matured through natural succession, the surrounding Washington, D.C. metropolitan area has seen major urban, industrial, and infrastructure development and the resultant opening of forests, increasing bee habitat. Plummers Island is likely a refugium for surrounding bee populations.

Twenty-nine species of shore flies in 17 genera have been collected and identified from Plummers Island. Although relatively diverse, the shore-fly fauna known from the Island represents only 21% of the species known from the surrounding states, indicating that we probably are dealing with sampling error and that many more species are likely to be found there.