Lepeophtheirus dissimulaius- — Lewis 
205 
area in the frontal organ of the preceding stage. 
Yolk material is still visible in the alimentary 
tract and is concentrated in an enlarged area 
just posterior to the ocular region. 
Eight specimens of the third chalimus lasted 
for approximately 33 hr at room temperature 
(approximately 23 C) at which time they 
moulted into the more elongate fourth chalimus. 
The elongate condition of the fourth chalimus 
indicates a cyclic condition in the growth of 
the first four chalimus stages ( Fig. 5 ) . The av- 
erage greatest length of the first chalimus (11 
specimens), excluding setae, is 1.54 times the 
greatest width, of the second chalimus (45 
specimens) is 2.04 times the width, of the third 
chalimus (25 specimens) is 1.54 times the 
width, and of the fourth chalimus (74 speci- 
mens) is 1.95 times the width. This cyclic 
tendency, from stubby to elongate to stubby to 
elongate, in the relationship between the great- 
est length and greatest width in the first four 
chalimus stages can not be explained by the 
growth of any single structure or body part. 
There appears to be a cyclic growth tendency 
in several body parts. For example, the cephalo- 
thorax length as a per cent of the total length is 
79.1, 74.3, 79.5, and 73.6 for the first four chali- 
mus stages respectively; the maximum width of 
the genital-abdominal segment as a per cent 
of the maximum length of the segment is 186.0, 
153.6, 180.0, and 153.8 (genital segment only 
for this last measurement) for the first four 
stages respectively. Certainly, the inclusion of 
the second pedigerous segment into the cepha- 
lothorax affected the length of the cephalothorax 
as a per cent of the total length measurements 
in the second chalimus but the fact that the 
third pedigerous segment was included in the 
cephalothorax in the third chalimus did not ap- 
pear to affect the trend in that stage. 
The cephalothorax of the fourth chalimus 
(Fig. 9 d) is ovoid in outline as in the preceding 
chalimus stages. The greatest length of the ceph- 
alothorax, including the frontal region but not 
the frontal filament, is slightly more than 1.3 
times the greatest width. The frontal region 
and frontal plates are now distinct and the divi- 
sion of the region from the cephalothorax is 
complete. The division between the frontal re- 
gion and the cephalothorax forms an irregular 
groove curving anteriorly from its origin on the 
anterior lateral margin. The anterior margin of 
the frontal region is broadly curved, the an- 
terior lateral surfaces forming laterally project- 
ing lobes, the posterior lateral margin connect- 
ing these lobes to the lateral margins of the 
cephalothorax. The lateral cephalothoracic mar- 
gins are generally convex, although wavy; the 
posterior lateral corners are rounded. The lateral 
posterior margins, outside the posterior sinuses* 
are bilobed, consisting of the broadly rounded 
posterior lateral extensions of the lateral regions 
and the narrow, lobate lateral surface of the 
median thoracic area. The posterior sinuses are 
distinct and slender, generally V-shaped al- 
though sometimes U-shaped. The median tho- 
racic area extends posteriorly well past the pos- 
terior extensions of the lateral regions and is 
irregularly tapered to the slightly rounded pos- 
terior margin. The lateral margins of the median 
thoracic area are continuous with the inner 
margins of the posterior sinuses. The major 
cephalothoracic grooves, as in the preceding 
stage, form an H. The anterior legs of the H, 
however, curve laterally from the junction with 
the cross groove anteriorly to their termination 
lateral and just posterior to the eyes. The lateral 
strengthening regions of the adult are not yet 
visible. 
The free fourth pedigerous segment is -short, 
its greatest length approximately two-thirds its 
width. The anterior end of the segment is over- 
lapped by the posterior extension of the median 
thoracic area of the cephalothorax. The lateral 
margins taper both anteriorly and posteriorly 
from the widest point, in the middle of the 
segment, at the junction of the fourth thoracic 
legs. The posterior margin is distinct, forming 
a flatly convex groove. 
The genital segment (Fig. 12^) is slightly 
shorter than the fourth pedigerous segment; its 
greatest length, excluding the slightly projecting 
fifth legs, is approximately two-thirds its width. 
The widest point is in the posterior medial re- 
gion of the segment. The anterior lateral mar- 
gins curve convexly, the posterior lateral mar- 
gins concavely. The fifth legs project as dactyli- 
form processes from the concave posterior lat- 
eral margins and extend slightly past the poste- 
rior end of the segment. The legs are tipped by 
two small plumose setae. The posterior margia 
of the segment is distinct although irregular. 
