414 
PACIFIC SCIENCE, Vol. XI, October, 1957 
DISTRIBUTION OF VARIANTS 
1 
2 
3 
4 
5 
6 
7 
8 
9 
10 
11 
12 
13 
14 
15 
JOHNSTONIANINAE 
a 
a 
b 
b 
b 
a 
a 
b 
b 
a 
a 
b 
c 
a 
7 
Johnstoniana 
a 
a 
b 
a 
b 
a 
b 
b 
b 
a 
a 
a 
a 
a 
a 
Diplothrombium 
b 
a 
b 
a 
b 
a 
b 
b 
b 
a 
a 
a 
a 
a 
a 
Centrotrombidium 
7 
7 
7 
7 
7 
? 
7 
> 
7 
7 
? 
7 
7 
? 
7 
Hirstithrombium 
LASSENIINAE 
a 
b 
a 
b 
a 
c 
a 
a 
a 
b 
b 
c 
b 
b 
b 
Lassenia 
a 
b 
p 
7 
a 
? 
7 
? 
7 
b 
7 
7 
a 
b 
b 
Polydiscia 
7 
7 
7 
7 
7 
7 
7 
? 
7 
7 
7 
7 
7 
7 
7 
Crossothrombium 
coxal area between coxae III. Deutorostral 
setae absent; terminal seta of palpal tarsus not 
eupathidiform. Tarsi each with two claws. 
Centrotrombidium Kramer 1896 
ADULT: Length of idiosoma 500 to 1200 /jl 
in known species. Scutum of both adult and 
larva bearing a single pair of clavate sensilla 
and anterior to these, a single pair of elongate 
normal setae; anterior sensilla absent. An- 
terior end of scutum drawn out into a long 
spine. Ocular plates bearing two refractile 
corneae each, or one refractile cornea and a 
non-refractile lobe. Dorsal hysterosomal setae 
borne on individual sclerites, no large plates 
present. Anal sclerites well developed. Supra- 
coxal setae of legs and gnathosoma and ves- 
tigial setae of legs completely absent in both 
larva and adult. Trochanter of palp with 
anterior wall fenestrated in both larva and 
adult. Palpal tibia with a single subterminal 
spiniform seta; tarsus with one solenidion and 
usually only two eupathidia. Solenidia of legs 
of four recognizably different types; tarsi I 
and II with one or more clavate solenidia 3 on 
posterior aspect. Famulus of tarsi I and II 
located in distal one-fifth of segment, on 
posterior membrane of claw fossa. Eupathidia 
of legs confined to tarsi. 
larva: In addition to those which are com- 
mon to both larva and adult, the larva has 
the following characteristics. Ocular plates 
distinctly bicorneate. Anal sclerites absent. 
Velum small, simple; deutorostral setae ab- 
sent, or represented by vestigial structures. 
Base of gnathosoma without posterorostral 
setae. Femur of all legs divided. A single 
clavate solenidion 2 on tarsus II, Si of tarsus 
I curved, rodlike. Tarsus of all legs with two 
simple, smooth claws. 
remarks: At present only five or six spe- 
cies are known, including the three new ones 
described here. C. schneideri Kramer 1896 is 
rather widely distributed in Europe, and C. 
australasiae is known from Australia. C. misel- 
lum (Berlese), from Mexico, is provisionally 
placed in this genus. The genus is probably 
cosmopolitan. The adults are small, brown 
mites, rather inconspicuous, found crawling 
over wet ground along the margins of streams 
and ponds. The writer has also collected them 
in estuarine habitats. The larvae are probably 
parasitic upon various small arthropods but 
nothing is known about host relationships. 
The known species can be separated on the 
basis of the diagnostic formulae given below. 
FORMULA KEY TO ADULTS OF 
Centrotrombidium 
la. Sensilla of scutum with distal portion 
nearly spherical, length of stalk less than 
twice the diameter of the sphere, and less 
than the distance between the sensillar 
alveoli. 
lb. Distal portion of sensillum clavate or 
pyriform, stalk very long, at least twice 
as long as the expanded tip, and con- 
siderably longer than the distance be- 
tween the alveoli. 
lc. Sensilla not swollen, but slender through, 
out. 
2a. Dorsal hysterosomal setae relatively long- 
slender, tapering uniformly to a very fine 
point (Figs. 19, 22). 
