Johnstonianidae — NEWELL 
401 
Corneas 
The eyes of the Johnstonianidae show little 
variation. There are typically two well-devel- 
oped corneae borne on a small ocular plate 
on either side of the scutum. While these 
frequently protrude prominently, they are not 
so well developed that they could be de- 
scribed as stalked, except in Jobnstoniana , in 
which the ocular plates are definitely cylin- 
drical and raised well above the surface of the 
propodosoma (Fig. 86). About the only vari- 
ant in the form of the corneae is found in 
Centrotrombidium. In this genus only the an- 
terior cornea is developed in the adults of the 
species known to the writer; the posterior 
cornea is represented by a prominent but 
nonhyaline protuberance (Fig. 53). In none 
of the forms seen by the writer are there any 
setae on the ocular plates, either in the larva 
or in the adult. According to Womersley, the 
eyes are absent in Crossothrombium. 
Dorsal Setae of Hysterosoma 
There are two outstanding features of the 
dorsal chaetotaxy of the Johnstonianidae. The 
first of these is the insertion of the dorsal 
setae of the larvae in individual sclerites which 
show no sign of fusion in any of the known 
forms. The second feature is the rather poor 
development of setal ornamentation, espe- 
cially in the adults. As in the larva the setae of 
the adults are also borne on individual scler- 
ites, although these are relatively smaller than 
in the larva. In all species studied, the dorsal 
setae in the adult are completely smooth, and 
are frequently stiff, short, and rodlike (Fig. 
183) . In the larva the setae are always arranged 
in four or five well defined rows (Fig. 131). 
The dorsal setigerous sclerites in the larva are 
always flat or weakly arched, but in the adult 
they are characteristically elevated, forming 
either low hemispheres or short cylindrical 
protuberances above the general surface of the 
cuticle (Fig. 64). 
Dorsal Propodosomal Setae 
In none of the known larvae are there any 
setae outside the scutum. In the adult how- 
ever, there is a tendency toward the develop- 
ment of setae lateral to the scutum and even 
lateral to the ocular plate. In Johnstoniana 
latiscuta (Fig. 86) there are no setae whatever 
lateral to the scutum. In Lassenia lasseni there 
is a single pair of setae, or occasionally more, 
between the scutum and the ocular plate, but 
the great majority of the propodosoma out- 
side of the scutum is bare (Fig. 183). The 
situation in Lassema spinifera is somewhat 
different, for in this species there are quite a 
number of setae surrounding the ocular plate 
although these are by no means as abundant 
as on the dorsum of the hysterosoma; and 
moreover there is an appreciable difference in 
form between the dorsal setae of the propo- 
dosoma and hysterosoma (Fig. 232). In Centro- 
trombidium and Diplothrombium , there are 
generally several setae between the scutum 
and ocular plates, although again the density 
of setae here is in no way comparable with 
that on the dorsum of the hysterosoma. 
Coxal Chaetotaxy of Larva 
The situation with respect to this character 
is not entirely clear owing to the small number 
of species known in each of the genera. In 
each of the four species of Centrotrombidium , 
Diplothrombium , and Johnstoniana which have 
been adequately described there are two setae 
on coxa I and one seta on each of coxae III 
and IV (2-1-1). Only two species are known 
in the genus Lassenia, the coxae of L, lasseni 
having the setal formula 2-2-3, and L. scute l- 
lata having the setal formula 2-1-2. Thus it 
is possible that in the genus Lassenia the 
number of coxal setae in the larvae is only a 
specific character, while in the other thrree 
genera it may prove to be genetically con- 
stant. The supracoxal setae of the legs are 
discussed below along with the other spe- 
cialized setae of the appendages. 
Pars Medialis Coxae 
This is the name applied to the small ex- 
tension of the medial angle of coxa I of most 
