A guide to the identification of the terrestrial Isopoda of Maryland, U.S.A. (Crustacea)

Abstract The terrestrial isopod fauna of Maryland is inferred using the taxonomic literature, internet-based citizen science projects, and original collecting. Twenty-two species are either known or are likely to occur in the state. This includes 17 mostly-European adventive species that comprise the vast majority of records. Of the five expected native species, three occur in or near marine or estuarine littoral habitats and each has yet to be recorded or recorded from only a single locality. This situation likely reflects the long history of systematic work on the European fauna and the availability of keys for the identification of these taxa, which contrasts with the limited taxonomic work on native species. A taxonomic key, illustrations, and brief descriptions of species known or expected to occur in Maryland are provided.


Introduction
There are no up-to-date, comprehensive taxonomic keys to the terrestrial isopod species of the eastern United States, the most recent being that of Muchmore (1990). A standard solution to the identification problem has been to send specimens to taxonomic experts, but such expertise is increasingly rare (Sfenthourakis and Taiti 2015). Alternatively, the internet offers several citizen-science projects: BugGuide (https://bugguide.net/), iNaturalist (https://www.inaturalist.org/), and Maryland Biodiversity Project (https://www.marylandbiodiversity.com/) where photos of specimens can be posted for identification by the internet community. Unfortunately, the photos often do not show key diagnostic features and the specimens thus remain unidentified or even misidentified. Given such problems, it has become common in ecological studies to circumvent the issue by grouping unidentified specimens into look-alike morphospecies, a practice known as "taxonomic minimalism" (Beattle and Oliver 1994), which limits comparisons across studies and may inadvertently inflate or reduce the actual number of species in a study (Krell 2004). This "solution" hampers the ability to associate taxonomic species with particular localities or habitats, to understand the dynamics of communities, and to recognize new native, adventive, or invasive species. The goal of this contribution is to provide a taxonomic key to the 22 species of terrestrial isopods that are known or expected to occur in Maryland as a step toward reducing the need for taxonomic minimalism in ecological studies in the state and adjacent areas.
Most terrestrial isopod species in Maryland are widely distributed and common European adventives (e.g., Hornung and Szlávecz 2003, Norden 2008. Consequently, identification tools designed for use in Europe (e.g., Hopkin 2014) may be useful in the eastern United States and can potentially identify native forms to genus (e.g., Ligidium, Miktoniscus). Still, because such keys deal with a more diverse fauna than exists in Maryland and may omit some European species, such as Chaetophiloscia sicula (Hornung and Szlávecz 2003), the potential for misidentification is increased.
Maryland's native terrestrial isopod fauna is small and probably consists of five species. Three marine or estuarine littoral species are either known from the state, Mitkoniscus spinosus (Say, 1818) (Trichoniscidae) and Scyphacella arenicola Smith, 1873 (Scyphacidae), or are expected to occur based on established distributions, Littorophiloscia vittata (Say, 1818) (Halophilosciidae). Two inland terrestrial species are also expected, Miktoniscus medcofi (Van Name, 1940) (Trichoniscidae) and Ligidium elrodii (Packard, 1873) (Ligiidae), with the first state record of L. elrodii reported here. In addition, M. spinosus has been found in moist inland habitats in North Carolina (Schultz 1976) and this could be the case in Maryland. In fact, a recent photo taken at a suburban park in the District of Columbia, which is essentially encompassed geographically by Maryland, appears to show a specimen of Miktoniscus sp. (BugGuide: https://bugguide.net/ node/view/1469429). The paucity of state records for native terrestrial isopods likely reflects several factors, including the lack of taxonomic work on the group, substantial reduction in native habitat, and the limited accessibility of taxonomic keys and other methods of identification.

Basic external anatomy of terrestrial isopods
The isopod body has three main regions or tagmata: head (cephalothorax), pereon (thorax), and pleon (abdomen) ( Figure 1C). The anterior part of the head is typically divided transversely by a raised frontal margin (fm, Figure 1A) that begins on the lateral surface, passes anteriorly ventral to the eye region and dorsal to the base of the antennae, and traverses the anterior surface of the head. It separates the cephalic dorsum (or vertex) (cd, Figure 1A) from the frontal lamina (frl, Figure 1A), a sclerotized region bordered laterally by the bases of the two large antennae. In some cases the frontal margin is interrupted medially or is absent, with the latter obscuring the distinction between the cephalic dorsum and frontal lamina. The frontal margin is often produced into a pair of anterolateral lobes (al, Figure 1A) between the eye region and base of the antenna. The frontal margin may also be produced medially in various ways (e.g., Figs 1A, 4A, C-F).
Each eye consists of either a compound eye (ce, Figure 1A) or one to three ocelli and is absent in some species (e.g., Figure 1C). Antennules (= antennae I) are very small and positioned medial to the base of the large antennae (antenna II). The five proximal articles (antennomeres) of the antennae constitute the peduncle (pe, Figure 1D), with the distal-most article bearing a terminal flagellum (fl, Figure 1D). Most species have only two or three distinct flagellar articles (Figs 3E, 4A, B, 5A, B) but some have 10 or more ( Figure 5F, G). Species in the family Trichoniscidae have what can appear to be a thin, tapering undivided flagellum (Figure 2A-C), but inspection with high magnification will reveal multiple articles, the number of which can be useful for species identification.
The pereon consists of seven broad segments ( Figure 1C), each bearing a pair of legs (pereopods) ventrally. The dorsal surface of each segment, the tergite (tg, Figure 1D), may be smooth or armed with tubercles or other sculpture (e.g., Figure 2B, C). The tergites bear ventrolateral extensions (epimera) (ep, Figure 1D) that collectively form the protected ventral space within which the legs operate.
The pleon has five free segments. The lateral margins of the first two lack epimera. The remaining three may be abruptly narrower than the last pereonal tergite (b, Figure 1E), often due to the absence or reduction of epimera. However, when large epimera are present, the pleonal margin may appear as a smooth continuation of the pereonal margin (a, Figure 1D).
Each free pleonal segment has a pair of ventral appendages, the pleopods, with a large plate-like part, the exopodite, and a medial part, the endopodite ( Figure  1B). The endopodites of the first two pairs are enlarged and elongated in males (en, Figure 1B) and bear species-specific modifications (e.g., Figure 2D-G). The exopodites may have air-filled respiratory tubules called pseudotracheae that function as a lung. The lungs are visible in live specimens as thickened white patches on the lateral part of the exopodites. The white color typically disappears through loss of air when the animal is preserved. In species found in Maryland, the lungs may be absent, present in the first two pairs of pleopods (lg, Figure 1B) or present in all pleopods.
The last apparent segment of the pleon is the pleotelson, a combination of a terminal segment and the telson (pt,Figs 1B,D,3B,D,. The pleotelson has a pair of uropods, each comprising a basal protopodite that articulates distally with a medial endopodite and lateral exopodite ( Figure 1B-D). The relative sizes, lengths, and shapes of these elements and their position with respect to the pleotelson are often useful in identification.

1
Cuticle translucent to white. Eyes absent. Antenna with antennomere V much wider than the other antennomeres ( Figure 1C). Associated with ants. Antennal flagellum superficially appearing to be one article, either robust (fl, Figure 1E) or thin, tapering and elongate ( Compound eye present. Antennal flagellum approx. the same width basally as antennomere V and comprising four articles that decrease in size distally (fl, Figure 1E). Sandy marine beaches. Surface of pereon sculptured, with tubercles and/or denticles ( Figure 2B, C) .... 7 6 Each eye with three ocelli. Antenna with four or five flagellar articles visible with high magnification. Body length up to 5 mm ( Head with median frontal projection extending dorsally, with dorsal margin overlapping anterior edge of cephalic dorsum (fp, Figure 3A). Pleotelson subtrapezoidal, with posterior margin nearly straight (pt, Figure 3B Head with prominent, median frontal projection extending dorsoanteriorly and not overlapping anterior edge of cephalic dorsum (fp, Figure 3C). Pleotelson subtriangular with posterior apex variably produced, but typically with rounded terminus (pt, Figure 3D) ...........Armadillidium nasatum Budde-Lund, 1885

11
Pleon abruptly narrower than pereon ( Figure 4B). Anterolateral lobes very small. Cuticle often with frosted-gray or dusty appearance, but this may be absent. Antennomeres IV and V usually with white terminal band. Frontal margin without evident median projection. ( Posterolateral margin of first pereonal tergite produced posteriorly into pointed angle (arrow, Figure 4F). Five pairs of lungs present. Frontal margin of head with small median triangular projection ( Figure 4F) corresponding to vertical median carina of frontal lamina (ca, Figure 1A). In life, capable of rolling into a ball with antennae exposed. Posterolateral angle of first pereonal tergite bluntly rounded, not produced posteriorly into a pointed angle (arrow, Figure 4E). Lungs restricted to pleopods I and II. Frontal margin with broad, convex median projection ( Figure 4E). Anterolateral lobes very prominent and broad, width of each approx. onethird width of head; in dorsal view, junction between median projection and anterolateral lobes V-shaped or nearly so ( Figure 4C). Head dark brown to black, contrasting with base color of pereon. Pereon with dark mid-dorsal stripe (sometimes broken), usually bordered laterally by bright yellow markings that may be lost in preservative. Five pairs of lungs. Dorsum of pereon with low, irregular bumps and tubercles; surface usually with pattern of dark brown, reddish brown and tan; lateral surface at base of epimera with tan to nearly-white patches creating a pair of broken lines. Frontal margin with broad convex median projection ( Figure 4D). (Trachelipodidae) ............ Trachelipus rathkii (Brandt, 1833) -Lungs restricted to pleopods I and II (as in Figure 1B). Dorsum of pereon tuberculate; color variable, ranging from solid brown or gray to various patterns, sometimes similar to Trachelipus but usually without a pair of broken light lines at base of epimera. Frontal margin with prominent triangular to subtrangular median projection with rounded apex ( Figure 4A) Found in vegetation or under objects near marine or brackish water. Pleotelson with lateral margins weakly concave, posterior apex bluntly rounded to truncate (pt, Figure 5E). Head with color similar to that of pereon; pereon often with dark mid-dorsal line bordered laterally by bright yellow splotches.  Figure 5D) or distinctly concave (pt, Figure 5C). Coloration differing from above.  Figure 3A, C). Frontal margin interrupted medially ( Figure 3C) but this is hidden by scutellum in A. vulgare ( Figure 3A). Anterolateral lobes prominent. An-tenna with two distinct flagellar articles ( Figure 3E). Pleon not abruptly narrower than pereon ( Figure 3E). Lungs limited to first two pairs of pleopods (lg, Figure 1B). Uropod with exopodite broad, dorsoventrally flattened, completing rounded posterior outline of body in dorsal view (ex, Figure 3B, D, E); protopodite and endopodite, largely hidden in dorsal view (Figs 1B, 3B, D). In life, capable of rolling into a ball with antennae hidden. With features of the family and genus, also the following: Projection of frontal lamina triangular, with broad, transverse dorsal margin overlapping frontal margin (fp, Figure 3A). Pleotelson subtrapezoidal, broadly truncated posteriorly (pt, Figure 3B). Sources: Richardson (1905), Schultz (1982) and Hopkin (2014). U.S. Distribution: Introduced. Throughout contiguous 48 states Klausmeier 2000, 2001). Recorded in Maryland by Richardson (1905), Hornung and Szlávecz (2003), Norden (2008), BugGuide, iNaturalist, and Maryland Biodiversity Project. Habitat: Synanthropic. Litter and under stones or other objects, occurs in somewhat drier conditions than most terrestrial isopods.

Family Halophilosciidae (Littorophiloscia)
Littorophiloscia vittata (Say, 1818) ( Figure 5E) Body length up to 5 mm. Compound eyes present. Head without anterolateral lobes. Antenna with three distinct flagellar articles (as in Figure 5B). Pleon abruptly narrower than pereon (as in Figure 5B). Lungs absent. Unable to roll into a ball. Head color usually similar to that of pereon. Pereon and pleon usually with dark median stripe bordered by bright yellow patches, also a series of lateral submarginal yellow markings. Pleotelson with lateral margins weakly concave, posterior end rounded to almost truncate (pt, Figure 5E). Sources: Schultz (1963Schultz ( , 1974. U.S. Distribution: Native. Coasts of Atlantic Ocean and Gulf of Mexico, New York to Texas Klausmeier 2000, 2001), not yet recorded from Maryland. Habitat: Marine and brackish shores between water and drift line, but also somewhat farther away from shore under objects, in marsh grass, debris, etc. (Schultz 1974).
Ligia exotica Roux, 1828 ( Figure 5F) Only marine littoral species known in Maryland with features of the family, also the following: Body length (excluding uropods) up to 30 mm. Antenna, especially flagellum, very long with more than 20 flagellar articles. Pleon not abruptly narrower than pereon, outline of body fusiform, tapering posteriorly. Uropod with protopodite very long, cylindrical; endopodite and exopodite nearly equal in length, attaching to protopodite at its terminus, total length of uropod up to 18 mm. Sources: Richardson (1905) and Schultz   Klausmeier 2000, 2001). Recorded from Maryland (Chesapeake Bay) by BugGuide, iNaturalist, and Maryland Biodiversity Project. Habitat: Marine to brackish shoreline, especially adjacent surfaces (pilings, rocks, etc.) Ligidium elrodii (Packard, 1873) ( Figure 5G) Only terrestrial species to occur in Maryland with features of the family, also the following: Body length up to 10 mm. Antenna with approx. 11 flagellar articles. Uropod with protopodite extending distad to pleotelson; endopodite articulating with protopodite at a point distad to articulation with exopodite, endopodite almost 1.5 times longer than exopodite. Sources: Richardson (1905) and Schultz (1970Schultz ( , 1982. U.S. Distribution: Native. Known from localities throughout the eastern states, also California [?] Klausmeier 2000, 2001). Recently recorded from forests in Garrett County (J. Shultz, original observation). Habitat: Moist to wet litter and moss especially near swamps, marshes, springs, ponds, streams, also caves.
Chaetophiloscia sicula Verhoeff, 1908 (Figure 5D) With features of the family, also the following: Body length up to 7.5 mm. Body outline similar to Figure 5B, but pleon more elongate and lateral margins smooth, not serrated ( Figure 5D). Pleotelson broad, triangular (width about double the length) with lateral margins straight or nearly so, posterior apex bluntly rounded (pt, Figure  5D). Dorsum with purplish-brown pigment interrupted by median series of small light markings and over much of surface by light longitudinal markings (lineoles); epimera pigmented except for distal light marginal line and proximal light mark that widens anteriorly; head with markings similar to pereon. Source: Vandel (1962). U.S. Distribution: Introduced. Known thus far only from Baltimore, Maryland (Hornung and Szlávecz 2003). Habitat: In Baltimore, litter in mature temperate deciduous forest.

Family Platyarthridae (Platyarthrus)
Platyarthrus hoffmannseggii Brandt, 1833 (Figure 1C) Body length up to 5 mm. Cuticle white to translucent. Eyes absent. Antenna with pre-flagellar antennomere very robust, wider than other peduncular antennomeres; flagellum superficially appearing to be undivided but actually with two articles, basal article very short. Anterolateral lobes prominent. Pleon not abruptly narrower than pereon. Lungs absent. Source: Hopkin (2014). U.S. Distribution: Introduced. Recorded from New York, Connecticut Klausmeier 2000, 2001) and Philadelphia, Pennsylvania (BugGuide). Not yet reported from Maryland, but the record from Philadelphia indicates that it may occur in the state. Habitat: Soil, associated with ants.

Family Porcellionidae (Porcellio, Porcellionides) (Figure 4A-C, E)
Compound eyes present. Antennae with two distinct flagellar articles ( Figure 4A, B). Lungs restricted to pleopods I and II. Uropods extending posteriad beyond end of pleotelson ( Figure 4A, B). Latreille, 1804 (Figure 4E) With features of the family, also the following: Body length up to 18 mm. Large, broad species, body outline similar to Figure 4A. Medial frontal margin projecting anteriorly as a broad weakly convex edge. Anterolateral lobes prominent, each less than third the width of head. Pereon smooth, without tubercles or other significant sculpture.

Porcellio laevis
Pleon not abruptly narrower than pereon (as in Figure 4A). Posterolateral angle of first tergite bluntly rounded, not produced posteriorly (arrow, Figure 4E). Source: Hopkin (2014). U.S. Distribution: Introduced. Widely distributed in the contiguous 48 states Klausmeier 2000, 2001). First records from Maryland are vacant lots in Baltimore City (K. Szlávecz, unpublished observation) as well as agricultural fields in Anne Arundel and Prince Georges Counties and forests in Garrett County (J. Shultz, original observation). Habitat: Diverse environments, especially synanthropic habitats.
Porcellio scaber Latreille, 1804 ( Figure 4A) With features of the family, also the following: Body length up to 17 mm. Head with frontal margin produced medially to form a prominent triangular to subtriangular projection. Dorsal surface of head and pereon with numerous tubercles. Pleon not abruptly narrower than pereon. U.S. Distribution: Introduced. Widely distributed in the northeastern, north-central, and western states, also Florida Klausmeier 2000, 2001). Recorded from Maryland by Richardson (1905), Hornung and Szlávecz (2003), Norden (2008) and Maryland Biodiversity Project. Habitat: Diverse mesic terrestrial habitats.
Porcellio spinicornis Say, 1818 ( Figure 4C) With features of the family, also the following: Body length up to 12 mm. Body outline similar to Figure 4A. Head with frontal margin produced medially into broad convex to rectangular projection that extends laterally almost to the medial edge of large anterolateral lobes; each anterolateral lobe about a third the width of head; in dorsal view, the median and anterolateral lobes join at V-shaped notch. Head dark, pereon generally lighter than head but with dark median stripe; in life, dark stripe bordered laterally by bright yellow markings. Source: Hopkin (2014). U.S. Distribution: Introduced. Northeastern and northern Great Lake states, Nebraska, Tennessee, Arkansas Klausmeier 2000, 2001); also North Dakota, Iowa (BugGuide), and Ohio (iNaturalist). Not yet recorded from Maryland. Habitat: Moist substrates and associated vertical surfaces.
Porcellionides pruinosus (Brandt, 1833) ( Figure 4B) With features of the family, also the following: Body length up to 12 mm. Body often with frosted gray-white appearance due to layer of microscopic beads, structural color that varies with lighting and timing of molt cycle (Hadley and Hendricks 1985). When coating absent, color varying but often red-brown, sometimes with light wavy flagellum (Figure 2A-C) comprising up to six articles when observed using high magnification. Lungs absent. Pleotelson with terminus truncate or with median notch or concavity (emarginate), not pointed (Figure 2A-C).
Hyloniscus riparius (C. L. Koch, 1838) With features of the family, also the following: Body length up to 7 mm. Body outline similar to Figure 2A. Each eye with one ocellus. Antennae with six flagellar articles observable with high magnification. Dorsum smooth, without tubercles or other sculpture. Pleon abruptly narrower than pereon. Male with hook on third segment (merus) of pereopod 7. U.S. Source: Richardson (1905) and Schultz (1965). U.S. Distribution: Introduced. Recorded from Maryland by Hornung and Szlávecz (2003), Norden (2008) and Hornung et al. (2015). Habitat: Moist to wet soil or litter, flood plains.
Miktoniscus spinosus (Say, 1818)  With features of the family, also the following: Body length up to 5 mm. Each eye with one ocellus. Antennae with four flagellar articles observable with high magnification. Dorsum of pereon with transverse to roughly transverse rows of tubercles. Pleon abruptly narrower than pereon. In male: Pleopod I with exopodite an elongate plate narrowing distally to a terminal point, exopodite only slightly shorter than endopodite; endopodite cylindrical, terminating with a distinct medial bend ( Figure 2D). Pleopod II with plate-like endopodite short, with small distomedial lobe; terminus of endopodite spatulate with square apical border ( Figure 2E). Source: Schultz (1976). U.S. Distribution: Native. Coastal reed marshes from Massachusetts south to Georgia (Schultz 1976(Schultz , 2001; unidentified Miktoniscus in DC (BugGuide). Recorded from "edge of an estuary" in Calvert County, Maryland by Schultz (1976) as M. halophilus. Habitat: Brackish and estuarine marshes along the Atlantic Coast, also moist leaf litter of inland forests in river bottoms and near streams in piedmont of North Carolina (Schultz 1976).
Miktoniscus medcofi (Van Name, 1940) (Figure 2C, F, G) With features of the family, also the following: Body length up to 5 mm. Each eye with one ocellus. Antennae with four flagellar articles visible with high magnification. Dorsum of pereon with transverse to roughly transverse rows of tubercles. Pleon abruptly narrower than pereon. In male: Pleopod I with plate-like exopodite tapering distally to broadly rounded apex; endopodite tapering and flattened distally but resulting lamella with central longitudinal "mid-rib", terminus with file-like striations under high magnification ( Figure 2F). Pleopod II with plate-like exopodite comparatively long with long distomedial lobe; terminus of endopodite not spatulate ( Figure 2G). Sources: Van Name (1940) and Schultz (1976). U.S. Distribution: Native. New York south to Florida west to central states Klausmeier 2000, 2001), including Texas (Hutchins and Drukker 2016); unidentified Miktoniscus in DC (BugGuide). Not yet recorded from Maryland, but its presence in adjacent areas suggests that it occurs in the state. Habitat: Moist soil, under logs, caves.
Trichoniscus pusillus Brandt, 1833 ( Figure 2A) With features of the family, also the following: Body length up to 5 mm. Each eye with three ocelli. Antenna with four or five flagellar articles visible using high magnification. Dorsal cuticle smooth, with reddish to purplish pigments. Pleon abruptly narrower than pereon. Posterior margin of pleotelson with median concavity (emarginate). Sources: Richardson (1905), Schultz (1982) and Hopkin (2014). U.S. Distribution: Introduced. Northeastern states from Maine south to North Carolina and west to Wisconsin and Arkansas; also Washington State (Jass and Klausmeier 2001). Recorded in Maryland by Hornung and Szlávecz (2003) and Norden (2008). Habitat: Moist soil and litter.