Lepeophtheirus dissimulatus — Lewis 
207 
The abdomen (Fig. 12 e) is slightly more 
than one-half the length of the genital segment; 
its greatest width is approximately twice the 
greatest length. The lateral margins are flatly 
convex or straight; the lateral posterior margins 
are slightly concave and taper to the median, 
bilobed anal region. 
The bifurcate third part of the frontal organ 
(Fig. 13*0 now extends to the posterior end of 
the smaller semirectangular fourth part. Both 
parts of the bifurcation are club-shaped and do 
not extend laterally as in the third chalimus. 
The alimentary tract is the same as that of the 
preceding stage although there is not as much 
yolk material. 
Three specimens of the fourth chalimus lasted 
for about 36 hr under laboratory conditions 
(temperature approximately 23 C) before they 
moulted into the fifth chalimus. 
The fifth chalimus (Fig. 1 0a, b ) is found both 
attached and free on the host although more fre- 
quently attached. With the exception of a few 
structures such as the genital segment and some 
of the appendages, the somewhat flabby condi- 
tion of the body and appendages characteristic of 
earlier chalimus larvae is absent and the stage is 
quite similar to the adult. The greatest length of 
the cephalothorax, including the frontal region, 
is approximately 1.3 times its greatest width, ex- 
cluding the now present marginal flanges. The 
frontal plates form most of the frontal region 
and a narrow, membranous flange projects from 
its broadly curved anterior margin. The lateral 
frontal region surfaces project slightly, the pos- 
terior lateral margins connecting the lobate pro- 
jecting portions to the lateral margins of the 
cephalothorax. The division between the cepha- 
lothorax and the frontal region is a distinct 
three-lobed groove, the median lobe of which 
is approximately twice the length of the lateral 
lobes. The lateral margins of the cephalothorax 
are broadly curved, with distinct marginal 
flanges extending from the junction of the 
frontal region posteriorly around the posterior 
extensions of the lateral regions to the begin- 
ning of the posterior sinuses. The lateral pos- 
terior margins, outside the posterior sinuses, 
are bilobed and consist of the rounded posterior 
extensions of the lateral regions and the sharply 
rounded lateral surface of the median thoracic 
area. The posterior sinuses are distinct and U- 
shaped. The median thoracic area extends pos- 
teriorly well past the posterior extensions of the 
lateral regions; its posterior margin is broadly 
rounded but possesses a small, median protru- 
sion that forms the junction of the free fourth 
pedigerous segment in the now discernible male 
but which is absent in the female. The lateral 
margins of the protruding median thoracic area 
are continuous with the inner margins of the 
posterior sinuses. The major cephalothoracic 
grooves are similar to those of the preceding 
chalimus but appear more definite and not quite 
as irregular. Two heavily sclerotized regions, 
the lateral strengthening regions, are visible as 
posterior laterally curved, rodlike structures in 
the cephalothorax, originating just anterior and 
slightly medial to the junction of the longitudi- 
nal legs of the cephalothoracic grooves and ex- 
tending to the region of the lateral margins. 
The free fourth pedigerous segment is short; 
its greatest length is approximately one-half its 
greatest width. The posterior portion of the 
median thoracic area of the cephalothorax over- 
laps the anterior end of the segment slightly in 
the female but not in the male. The lateral mar- 
gins are convex, the middle of the segment 
being the broadest region and slightly overlap- 
ping the proximal end of the fourth thoracic 
legs. The posterior margin is distinct although 
slightly irregular. 
Because of the presence of the sixth thoracic 
legs on the male of this stage, the sexes can 
now be differentiated, as the female does not 
possess this appendage pair. As will be discussed 
later, the second antennae can not be used in 
differentiating the sexes of any of the chalimus 
stages of L. dissimulatus. The presence or ab- 
sence of the sixth legs is the best method so far 
found by which the sexes can be told apart in 
both the fifth and sixth chalimus stages. 
The genital segment of the female (Fig. 12 g) 
is as wide as long; the lateral margins are flatly 
or broadly convex. The anterior end possesses 
two lateral, rounded protuberances that are dis- 
tinct in a dorsal view but indistinct in a ventral 
view. The posterior lateral margins are indented, 
the lobate fifth legs arising from the ventral 
surface and projecting posteriorly. The fifth leg 
does not project past the posterior end of the 
genital segment as in the preceding stage. Four 
plumose setae, an additional two setae over the 
