Review Article |
Corresponding author: Jeffrey W. Shultz ( jshultz@umd.edu ) Academic editor: Katalin Szlavecz
© 2018 Jeffrey W. Shultz.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Shultz JW (2018) A guide to the identification of the terrestrial Isopoda of Maryland, U.S.A. (Crustacea). In: Hornung E, Taiti S, Szlavecz K (Eds) Isopods in a Changing World. ZooKeys 801: 207-228. https://doi.org/10.3897/zookeys.801.24146
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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.
Oniscidea , taxonomy, taxonomic key
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
Most terrestrial isopod species in Maryland are widely distributed and common European adventives (e.g.,
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 (
The isopod body has three main regions or tagmata: head (cephalothorax), pereon (thorax), and pleon (abdomen) (Figure
A Cylisticus convexus, head, dorsoanterior view, antennae removed (based on
Each eye consists of either a compound eye (ce, Figure
Trichoniscidae. A Trichoniscus pusillus B Haplophthalmus danicus, highlighting cuticular sculpture of pereon CMiktoniscus cf. medcofi, highlighting cuticular sculpture of pereon D–E Miktoniscus spinosus, male D right pleopod I, ventral view E right pleopod II, ventral view (based on
The pereon consists of seven broad segments (Figure
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
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
The last apparent segment of the pleon is the pleotelson, a combination of a terminal segment and the telson (pt, Figs
1 | Cuticle translucent to white. Eyes absent. Antenna with antennomere V much wider than the other antennomeres (Figure |
Platyarthrus hoffmannseggii Brandt, 1833 |
– | Cuticle color variable. Eyes present, each usually compound or with one or three ocelli. Antenna variable, but antennomere V not significantly wider than the others. Not associated with ants or associated with ants only incidentally | 2 |
2 | Antennal flagellum comprising two to many distinct articles (Figs |
3 |
– | Antennal flagellum superficially appearing to be one article, either robust (fl, Figure |
4 |
3 | Antenna with two distinct flagellar articles (Figs |
9 |
– | Antenna with three distinct flagellar articles (Figure |
16 |
– | Antenna with four distinct flagellar articles (Figure |
Armadilloniscus ellipticus (Harger, 1878) |
– | Antenna with flagellum long, comprising 10 or more flagellar articles (Figure |
19 |
4 | Compound eye present. Antennal flagellum approx. the same width basally as antennomere V and comprising four articles that decrease in size distally (fl, Figure |
Scyphacella arenicola Smith, 1873 |
– | Compound eye absent, but with one or three ocelli. Antennal flagellum abruptly narrower than preceding article, a narrow tapering cone (Figure |
5 |
5 | Surface of pereon smooth; without tubercles, denticles or other sculpture (Figure |
6 |
– | Surface of pereon sculptured, with tubercles and/or denticles (Figure |
7 |
6 | Each eye with three ocelli. Antenna with four or five flagellar articles visible with high magnification. Body length up to 5 mm (Figure |
Trichoniscus pusillus Brandt, 1833 |
– | Each eye with one ocellus. Antenna with six flagellar articles visible with high magnification. Body similar to Figure |
Hyloniscus riparius (C. L. Koch, 1838) |
7 | Pereon with tubercles or denticles on longitudinal ridges (Figure |
Haplophthalmus danicus Budde-Lund, 1880 |
– | Pereon with tubercles in transverse or roughly transverse rows (Figure |
8 |
8 | Male pleopod I (Figure |
Miktoniscus spinosus (Say, 1818) |
– | Male pleopod I (Figure |
Miktoniscus medcofi (Van Name, 1940) |
9 | Exopodite of uropod not extending beyond elliptical perimeter of body in dorsal view; broad, dorsoventrally flattened; protopodite and endopodite largely hidden in dorsal view (Figure |
10 |
– | Exopodite of uropod extending beyond elliptical perimeter of body in dorsal view; protopodite visible in dorsal view; exopodite prominent, attaching at terminus of protopodite, broad basally, tapering distally; endopodite thin, attaching at medial base of protopodite (Figure |
11 |
10 | Head with median frontal projection extending dorsally, with dorsal margin overlapping anterior edge of cephalic dorsum (fp, Figure |
Armadillidium vulgare (Latreille, 1804) |
– | Head with prominent, median frontal projection extending dorsoanteriorly and not overlapping anterior edge of cephalic dorsum (fp, Figure |
Armadillidium nasatum Budde-Lund, 1885 |
11 | Pleon abruptly narrower than pereon (Figure |
Porcellionides pruinosus (Brandt, 1833) |
– | Pleon not abruptly narrower than pereon (Figure |
12 |
12 | Dorsal surface of pereon essentially smooth | 13 |
– | Dorsal surface of pereon with numerous bumps or tubercles | 14 |
13 | Posterolateral margin of first pereonal tergite produced posteriorly into pointed angle (arrow, Figure |
Cylisticus convexus (De Geer, 1778) |
– | Posterolateral angle of first pereonal tergite bluntly rounded, not produced posteriorly into a pointed angle (arrow, Figure |
Porcellio laevis Latreille, 1804 |
14 | Anterolateral lobes very prominent and broad, width of each approx. one-third width of head; in dorsal view, junction between median projection and anterolateral lobes V-shaped or nearly so (Figure |
Porcellio spinicornis Say, 1818 |
– | Anterolateral lobes not so wide, joining convex median projection via curved margin, not V-shaped notches (Figure |
15 |
15 | 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 |
Trachelipus rathkii (Brandt, 1833) |
– | Lungs restricted to pleopods I and II (as in Figure |
Porcellio scaber Latreille, 1804 |
16 | Head with prominent anterolateral lobes. Pleon not abruptly narrower than pereon, body broad and distinctly elliptical in dorsal view (Figure |
Oniscus asellus Linnaeus, 1758 |
– | Head without anterolateral lobes. Pleon abruptly narrower than pereon, body more elongate, oblong in dorsal view (Figure |
17 |
17 | 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 |
Littorophiloscia vittata (Say, 1818) |
– | Found in terrestrial environments. Pleotelson with lateral margins essentially straight (pt, Figure |
18 |
18 | Pleotelson triangular, with lateral margins straight or nearly so in dorsal view, posterior apex blunt (pt, Figure |
Chaetophiloscia sicula Verhoeff, 1908 |
– | Pleotelson with lateral margins distinctly concave, posterior apex pointed (pt, Figure |
Philoscia muscorum (Scopoli, 1763) |
19 | Uropod with protopodite very long, surpassing posterior terminus of pleotelson by more than the length of pleotelson; endopodite and exopodite long and thin, about equal in length, both arising from tip of protopodite (Figure |
Ligia exotica Roux, 1828 |
– | Uropod with protopodite exceeding posterior tip of pleotelson by about one length of the pleotelson or less; endopodite about 1.5 times the length of exopodite, endopodite arising from protopodite proximal to exopodite (Figure |
Ligidium elrodii (Packard, 1873) |
Length up to 15 mm. Compound eyes present. Frontal lamina with broad projection [scutellum] (fp, Figure
Armadillidiidae. A–B Armadillidium vulgare A head, dorsal view B posterior end of pleon, dorsal view C–E Armadillidium nasatum C head, dorsal view D posterior end of pleon, dorsal view E Dorsal view. Abbreviations: ex exopodite of uropod; fp frontal projection; pl5 pleonal tergite V; pt pleotelson. Scale bar: 5 mm.
Armadillidium vulgare (Latreille, 1804) (Figs
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
Armadillidium nasatum Budde-Lund, 1885 (Figure
With features of the family and genus, also the following: Projection of frontal lamina extending dorsoanteriorly, not overlapping anterior margin of cephalic dorsum (Figure
Cylisticus convexus (De Geer, 1778) (Figs
Body length up to 15 mm. Compound eyes present. Antenna with two distinct flagellar articles. Frontal lamina divided by median vertical ridge (carina) that terminates dorsally as a small, triangular median projection at frontal margin (ca, Figure
Porcellionidae, Trachelipodidae, Cylisticidae. A Porcellio scaber B Porcellionides pruinosus C Porcellio spinicornis, head, dorsal view D Trachelipus rathkii, head, dorsal view E Porcellio laevis, head and first pereonal tergite, dorsal view F Cylisticus convexus, head and first pereonal tergite, dorsal view.
Armadilloniscus ellipticus (Harger, 1878) (Figure
Body length up to 4 mm. Compound eye present, each with five to ten facets. Antenna with four distinct flagellar articles (fl, Figure
Littorophiloscia vittata (Say, 1818) (Figure
Body length up to 5 mm. Compound eyes present. Head without anterolateral lobes. Antenna with three distinct flagellar articles (as in Figure
Oniscidae, Philosciidae, Halophilosciidae, Ligiidae. AOniscidae: Oniscus asellus B–EPhilosciidaeB–C Philoscia muscorum B dorsal view C pleotelson D Chaetophiloscia sicula: dorsal view of pleon (after
Compound eyes present, very large with many facets. Antenna with many (> 10) flagellar articles. Uropod with protopodite projecting posteriorly beyond tip of pleotelson; endopodite and exopodite long and thin. Lungs absent. Rapid runners.
Ligia exotica Roux, 1828 (Figure
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:
Ligidium elrodii (Packard, 1873) (Figure
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:
Oniscus asellus Linnaeus, 1758 (Figure
Body length up to 16 mm. Perimeter a wide ellipse in dorsal view. Compound eyes present. Antenna with three distinct flagellar articles. Head with prominent anterolateral lobes. Pleon not abruptly narrower than pereon. Lungs absent. Pleotelson elongate, pointed. Dorsoventrally flattened, not capable of rolling into a ball. Sources:
Compound eyes present. Antenna with three distinct flagellar articles (Figure
Chaetophiloscia sicula Verhoeff, 1908 (Figure
With features of the family, also the following: Body length up to 7.5 mm. Body outline similar to Figure
Philoscia muscorum (Scopoli, 1763) (Figure
With features of the family, also the following: Body length up to 12 mm. Pleon compact with epimera forming serrated lateral margin (Figure
Platyarthrus hoffmannseggii Brandt, 1833 (Figure
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:
Compound eyes present. Antennae with two distinct flagellar articles (Figure
Porcellio laevis Latreille, 1804 (Figure
With features of the family, also the following: Body length up to 18 mm. Large, broad species, body outline similar to Figure
Porcellio scaber Latreille, 1804 (Figure
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 (
Porcellio spinicornis Say, 1818 (Figure
With features of the family, also the following: Body length up to 12 mm. Body outline similar to Figure
Porcellionides pruinosus (Brandt, 1833) (Figure
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 (
Scyphacella arenicola Smith, 1873 (Figure
Body length up to 5 mm. Compound eyes present. Antennae with four flagellar articles that decrease in length distad, base of flagellum only slightly narrower than preceding article. Head without frontal margin, no clear border between cephalic dorsum and frontal lamina. Pereon with numerous scales; cuticle tuberculate, each tubercle tipped with small spine. Source:
Trachelipus rathkii (Brandt, 1833) (Figure
Body length up to 15 mm. Body outline similar to Figure
Body length < 8 mm. Each eye with one ocellus (Haplophthalmus, Hyloniscus, Miktoniscus) or three ocelli (Trichoniscus). Antenna ending in a narrow, tapering, pointed flagellum (Figure
Haplophthalmus danicus Budde-Lund, 1880 (Figure
With features of the family, also the following: Body length up to 4 mm. Each eye with one ocellus. Antennae with three flagellar articles observable with high magnification. Cuticle lacking dark pigments; translucent, white or cream. Head densely tuberculate dorsally, tubercles conical. Tergites of pereon armed with low longitudinal crests with roughened and/or tuberculate dorsal surfaces. Pleon segments III–V with prominent epimera; outline of pleon not abruptly narrower than pereon. Sources:
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
Miktoniscus spinosus (Say, 1818) (= M. halophilus Blake, 1931) (Figure
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
Miktoniscus medcofi (Van Name, 1940) (Figure
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
Trichoniscus pusillus Brandt, 1833 (Figure
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:
An earlier version of this key was developed at the University of Maryland for use in a graduate course on terrestrial arthropods. I thank Katalin Szlávecz for suggesting that I develop the key further for inclusion in this symposium. Reviewers Sandor Farkas and Stefano Taiti provided many helpful and constructive comments. Specimens of Ligidium elrodii were found by the author in pitfall samples provided by the Maryland Department of Natural Resources. I thank Jennifer Selfridge and Gwen Brewer for access to these samples. The author was supported by the Maryland Agricultural Experiment Station.