REVISION OF SIMOCEPHALUS DAPHNIIDAE 

Fig. 4 S$. vetulus head and mouth parts. 
VARIABILITY 
Age variability is similar in all species (Fig. 15). New-born fe- 
males do not differ much from males: The brood pouch is small 
and the dorsal valve margin almost straight. The prominence on 
the dorso-posterior valve angle, if it present, is not distinct. Cara- 
pace denticles are small and cover less of the valves than in 
adults. Older females have a more distinct and sharp dorso-poste- 
rior valve prominence. The shape of the brood pouch in the adult 
depends on the number of eggs. The head grows slower than the 
carapace. Valve shape in new-born males differs from that of 
adults only in the absence of an embayment in the proximal part 
of the ventral margin. The number of anal teeth correlates with 
size in females. The ocellus in juveniles is shorter than in adults. 
The postabdomen of neonates of both sexes lacks an anal bay, 
supra-anal angle (Figs 12C; 15C), abdominal processes. The 
fourth endite prominence of the first limb has a large hook bear- 
ing a denticle at its end in the adult male (Fig. 8B) and small hook 
lacking a denticle in the juvenile (Fig. 8D). The curved setae of 
the second, third and fourth endite prominences of the second 
limb are short in juvenile males and longer than the base of the 
plumose seta of the first prominence in adults (Fig. 13B—D). The 
morphology of third, fourth and fifth trunk limbs in males and all 
trunk limbs in females does not depend on age. 
Eye and ocellus size are subject to seasonal variation. This was 
discovered in the following way: two series of S. vetulus were 
collected in the same water-body in the Moscow region on 12. 5. 
1990 and 5. 11. 1990. All specimens from the first sample had a 
small eye and ocellus (Fig. 16A) and all those from the second 
(parthenogenetic and ephippial females and males) a large one (Fig. 
16B). Individuals from the sample of 5. 11. 1990 were kept at room 
temperature. By the 17th day the size of the eye and ocellus in all 
cases had become small (Fig. 16C).A similar result was obtained for 
S. serrulatus. 
Ocellus size is also affected by illumination intensity. It decreases 
in darkness (Jermakov, 1924) and if the ventral part of the head is 
covered by epibionts (personal observation) (Fig. 16D). Ocellus 
shape varies within populations. In females of Simocephalus s. str. it 
is straight or curved, widened in the middle or bifurcated at the end. 
In males of these species and in both sexes in species of other 
subgenera it is round or rhomb-like. The frons in S. (Coronocephalus) 
bears a variable number of denticles. Individuals with and without a 
prominence at the ventral head margin occur in all species except S. 
gibbosus, S. elizabethae and S. obtusatus. A dorsal embayment 
between carapace and head is more or less developed in all species. 
Sometimes, there is a small prominence on the head near this 
embayment (Fig. 16F). 
There are pigmented spots in the valve tissue. Their shape and 
colour differ within populations. The colour is green, brown or 
orange and as a rule correlates with the colour of the gut contents. 
According to Green (1966) carotenoid pigmentation depends on the 
food composition. 
The number of denticles at the ventro-posterior angle of the 
valves varies from two to six. No correlation between number of 
denticles and size was observed. There is some variability in shape 
of the postabdomen and abdominal processes (Fig. 17). 
