2. THE SPECIES OF KRILL
31
Ib. Lappet on first segment of antennular peduncle large, conspicuously
longer than broad, not triangular, with the short, acute tip turned
mainly or totally outwards
. . .. . . .. . . simplex
Genus Thysanoessa
There is considerable variation in several characters of the species
in this genus and identification from this key can be confirmed through
reference t o Einarsson (1945), Sheard (1953), and Boden et al. (1955).
la. Median keel on dorsal posterior surface of some abdominal segments.
2a. Large process on dorsal keel of fourth abdominal segment ; smaller
process on dorsal keel of fifth abdominal segment ; no process on keels
of abdominal segments 1-3. Second thoracic leg slightly elongate.
No lateral denticle on carapace. Eye subovoid.. . . . spinifera
2b. No process on dorsal keel of abdominal segments, or long process on
third and very short process on fourth and fifth segments. First and
second abdominal segments not keeled. Second thoracic leg greatly
elongate. Eye divided into upper and lower parts.
3a. Lateral denticle on carapace in posterior position . . inspinata
3b. Lateral denticle on carapace in median position . . longipes
lb. No median keel on dorsal surface of abdominal segments.
4a. No lateral denticles on inferior margins of carapace.
5a. Sixth abdominal segment much shorter than the sum of the two
preceding segments, with a strong spine a t its upper distal end.
Second thoracic limbs either elongate (neglecta series) or normal
(inermis series) . . .. .. .. .. . . inermis
5b. Sixth abdominal segment almost equal to, or equal to, sum of two
preceding segments, usually without a strong spine on its upper
distal end. Second thoracic limb elongate . .
longicuudata
4b. Lateral denticles on inferior margins of carapace.
6a. Lateral denticle posterior to mid-point of inferior margin of
carapace. Second thoracic leg greatly elongate.
7a. Sixth abdominal segment somewhat or considerably shorter
than sum of the two preceding segments.
8a. Setae on fifth and sixth (carpal and propodal) joints of
first to third thoracic legs short compared with terminal
setae on seventh joint
.. .. ..
parua
8b. Setae on fifth and sixth joints of first to third thoracic
legs as long as, or longer than, terminal setae on seventh
joint . . .. .. .. .. . . gregaria
2.
31
Ib. Lappet on first segment of antennular peduncle large, conspicuously
longer than broad, not triangular, with the short, acute tip turned
mainly or totally outwards
. . .. . . .. . . simplex
Genus Thysanoessa
There is considerable variation in several characters of the species
in this genus and identification from this key can be confirmed through
reference t o Einarsson (1945), Sheard (1953), and Boden et al. (1955).
la. Median keel on dorsal posterior surface of some abdominal segments.
2a. Large process on dorsal keel of fourth abdominal segment ; smaller
process on dorsal keel of fifth abdominal segment ; no process on keels
of abdominal segments 1-3. Second thoracic leg slightly elongate.
No lateral denticle on carapace. Eye subovoid.. . . . spinifera
2b. No process on dorsal keel of abdominal segments, or long process on
third and very short process on fourth and fifth segments. First and
second abdominal segments not keeled. Second thoracic leg greatly
elongate. Eye divided into upper and lower parts.
3a. Lateral denticle on carapace in posterior position . . inspinata
3b. Lateral denticle on carapace in median position . . longipes
lb. No median keel on dorsal surface of abdominal segments.
4a. No lateral denticles on inferior margins of carapace.
5a. Sixth abdominal segment much shorter than the sum of the two
preceding segments, with a strong spine a t its upper distal end.
Second thoracic limbs either elongate (neglecta series) or normal
(inermis series) . . .. .. .. .. . . inermis
5b. Sixth abdominal segment almost equal to, or equal to, sum of two
preceding segments, usually without a strong spine on its upper
distal end. Second thoracic limb elongate . .
longicuudata
4b. Lateral denticles on inferior margins of carapace.
6a. Lateral denticle posterior to mid-point of inferior margin of
carapace. Second thoracic leg greatly elongate.
7a. Sixth abdominal segment somewhat or considerably shorter
than sum of the two preceding segments.
8a. Setae on fifth and sixth (carpal and propodal) joints of
first to third thoracic legs short compared with terminal
setae on seventh joint
.. .. ..
parua
8b. Setae on fifth and sixth joints of first to third thoracic
legs as long as, or longer than, terminal setae on seventh
joint . . .. .. .. .. . . gregaria
2.