12 A Canadian Arctic Expedition, 1913-18 
shaped spot. Eyes 8 + 8 (fig. 53) subequal; or two inner proximal eyes of 
each group a little smaller than the others. Postantennal organs (fig. 53) near 
the eyes, elliptical, slightly longer than, to twice as long as, the diameter of an 
adjacent eye. Antennae once and one-half to twice as long as the head, with 
segments in relative lengths about as 3:4:5:6. Sense organ of third antennal 
segment with a pair of linear feebly curving sense rods, a thick basal ridge, and 
two guard setae. Very short curving sensory setae occur on all the antennal 
segments as follows: segment 1, 2-5; 2, 3-7; 3, 3-7; 4, 10-15. On the first 
three segments these are on the under side near the distal outer end; on the 
fourth segment they occur on the distal half along the outer side (Agren, '02). 
Mesonotum almost covering the pronotum. Third abdominal segment a little 
longer than the fourth (about as 5:4). Abdominal segments without ankylosis. 
Unguis (fig. 54) stout, curving, with a pair of small lateral teeth, and with inner 
margin untoothed. Unguiculus broadly lanceolate, with inner lamella roundly 
dilated basally, untoothed as a rule, extending a little beyond the middle of the 
unguis. Tenent hairs absent, represented by a single long simple hair. Furcula 
apparently appended to the fifth abdominal segment, and extending to the 
anterior border of the ventral tube. Dentes twice as long as manubrium, 
slender, gradually tapering, crenulate dorsally. Mucro two-thirds as long as 
hind unguis, quadridentate (fig. 55). Apical tooth small, at the base of the 
second tooth; second and third teeth dorsal, large, subequal, subconical, slightly 
hooked, in longitudinal alinement; fourth tooth lateral, oblique, acute, extending 
almost half the length of the mucro. Basal lateral mucronal seta present. 
Rami of tenaculum quadridentate (fig. 56) ; corpus with piany (fifteen or more) 
ventral setae. General clothing of abundant short simple setae (fig. 57). Long 
outstanding feathered sensory setae occur on the last five abdominal segments; 
there being one or two pairs of these on each of the segments, and sometimes 
three pairs on the fourth abdominal segment. Length, 3 mm. 
Packard's six cotypes of his Isotoma tricolor, from Waco, Texas, in the 
Museum of Comparative Zoology, Cambridge, 'Mass., for which MacGillivray 
('96, p. 48) retained the name of tricolor, I found to be pahistris. In the same 
tube with them were five specimens of Isotoma viridis, from Salem, Mass. 
/. aequalis MacG. is also palustris, as I have found from a study of a cotype 
sent to me by MacGillivray. 
Isotoma palustris lives in moist places, and is especially abundant along the 
edges of ponds and streams, frequenting the vegetation in preference to the 
water, though it is at home on the surface of the water, where it leaps vigorously 
and I'epeatedly. The species occurs on the seashore also, under seaweed, drift- 
Avood or stones, and is sometimes found on the snow. 
This is one of the dominant species of its order. It is cosmopolitan in 
distribution, everywhere common, and highly variable in colouration, several 
varieties having received names. The typical form of the species is known 
from all parts of Europe, from Canada and the United States, Azores islands, 
India, and Java; the recorded Arctic distribution being as follows: Siberia, 
Nova Zembla, Sjiitzbergen, Bear island. 
Isotoma palustris var prasina Renter. 
Plate 6, figs. 53-.57. 
Isotoma Stuxbergi var. prasina Reuter, 1891. 
Isotoma palustris var. prasina Schott, 1894. — Dalla Torre, 1895. — Reuter, 
1895.— MacGillivray, 1896.— Schiiffer, 1896, 1898, 1900a, 1900b.— Poppe and 
Schaffer, 1897.— Carl, 1899, 1901.— Carpenter and Evans, 1899.— Wahlgren, 
1899c, 1906b.— Borner, 1901a.— Krausbauer, 1902.— Agren, 1903. 
Isotoma palustris vai'. pallida Schaffer, 1896. — Borner, 1901a. — Krausbauer, 
1902. 
