Johnstonianidae — Newell 
409 
ally, so that they effectively comprise a single 
type, the designation s 3 will still be applied 
as indicated here, for the reason explained 
immediately above. Solenidia 3 are generally 
small, very slender, usually but not always 
lacking internal structure, and are probably 
invariably the most numerous type in both 
larval and postlarval stages. 
7. In the larvae of all genera studied to date, 
a fourth type of solenidion is found dorsally 
on tibia I and will be designated solenidion 4 
or s 4 . This is a little larger than s 3 and typically 
shows some degree of internal structure. It 
may be intermediate in size to s 3 and s 4 , and 
usually the distinction between these is con- 
siderably more difficult to appreciate in the 
adult than in the larva. The identification of 
s 4 is also confused by those cases in which a 
solenidion very similar to Si is found on tibia 
I in the position normally occupied by s 4 . 
This is true of Lasse ni a, new genus, in which 
one distidorsal seta of tibia I has a companion 
seta and is of the same form as Si. This might 
be interpreted either as a case in which Si is 
found on the tibia, or, as the writer has done, 
a case in which there is a strong convergence 
between s 4 and s 4 . The situation is in no way 
simplified by the fact that throughout the 
protonymphal and deutonymphal instars of 
Lassenia there is a progressive diminution of 
this seta so that in the adult apparently only 
one type of solenidion can be found on tibia 
I with any degree of certainty— namely s 3 . 
8. As can be seen from the foregoing, the 
differences between solenidial types may be 
very marked, or there may be convergences 
which make interpretation of types exceed- 
ingly complex at times. However, we can only 
go so far in simplifying a complex situation, 
and perhaps the more surprising thing would 
be to find that all patterns of solenidial varia- 
tion in the Johnstonianidae or the terrestrial 
Parasitengona in general could be smoothly 
fitted into a single scheme. If we fail to 
achieve this, we may yet succeed, if in failing 
we discover the reason for doing so. 
The Johnstonianidae are a very fortunate 
group for studying the morphology of the 
solenidia, because in most genera the four 
types can be recognized not only in the larva 
but in the adult as well. This is especially so 
since s 2 in Centrotrombidium , Diplothrombium , 
and Johnstoniana , are of very unusual form 
and readily distinguishable from s 4 and s 4 . In 
Lassenia larvae, s 4 and s 2 are of similar form 
and size, differing chiefly in the presence 
of a companion seta at the base of s 4 . How- 
ever, the companion seta is never retained in 
the postlarval instars, so this difference cannot 
be utilized in differentiating the two types of 
solenidia in the nymphs and adults. Conse- 
quently in this genus it is all but impossible 
to differentiate between s 4 and s 2 in the adult; 
moreover this situation extends to s 4 as well, 
as pointed out above. 
One interesting feature of s 2 is that this 
type, while it appears first on tarsus II of the 
larva, is actually more abundant on tarsus I 
in the adult than it is on tarsus II. In other 
words they arise de novo on tarsus I in the 
postlarval stages, and in greater numbers there 
than in the site at which s 2 originally appeared 
in the larva. This is true in all genera (except 
possibly Lassenia in which the true state of 
affairs has not been ascertained because of the 
convergence in solenidial types in the post- 
larval stages). 
The Eupathidia of the Legs of the Larva 
Each of the four genera studied by the 
writer has a characteristic arrangement of the 
larval eupathidia. In all genera, the eupathidia 
are confined to the tarsi, and in all cases tarsus 
I has two eupathidia. Variations are found in 
the number of eupathidia on II and III, and 
in the presence or absence of companion 
setae. Both Centrotrombidium and Johnstoniana 
have eupathidial formulae of (2-1-0), but 
only in Johnstoniana is there a companion seta 
at the base of the dorsal eupathid. Diplothrom- 
bium has a formula of (2-1-0) in both species 
seen by the writer, but there is no companion 
seta with any of the eupathidia. In the two 
