Johnstonianidae — NEWELL 
397 
would suggest that the only thing that might 
possibly have prevented the parallel emerg- 
ence of the latter two groups would have been 
the unavailability of a suitable prototype mite 
at the time the insects were first becoming 
established. The groups with larvae parasitic 
on higher vertebrates appear to be more modi- 
fied than the Johnstonianidae and Hannemania 
(although not more so than the Trombidiidae 
or Erythraeidae which parasitize insects) . 
So far as their habits are concerned, the 
larvae of the Johnstonianidae are all parasitic, 
but the relationship between larva and host is 
very loose. Among the Parasitengona there 
are two extremes in the relationship of larva 
to host — in some species the larva is little 
more than a predator, preying upon a number 
of host individuals during its development, 
and detaching readily when disturbed. At the 
other extreme we find larvae which, once 
firmly attached, remain with the host until 
they complete their larval development almost 
regardless of what stimuli may intervene. A 
simple test of the degree of host fixity is to 
drop the host with the attached larvae into a 
vial of 60 per cent alcohol. Forms in which 
host fixity is highly developed will usually 
remain attached, unless they had not become 
firmly attached to the host or unless they were 
in the process of detaching themselves at the 
time of capture. Forms in the other group 
detach readily regardless of how long they 
have been on the host, and for this reason 
they are called ’'self-detaching larvae.” All 
Johnstonianidae which the author has studied 
in the living state have larvae of the self- 
detaching type. 
Morphologically they appear to be more 
generalized than other Parasitengona. For 
example, there are two pairs of sensilla on the 
scutum in most species, such as are found in 
the Erythraeidae and most Smarididae, but 
with obvious trends toward modification or 
loss of the anterior pair as in the Trombidiidae 
and Trombiculidae. The simplicity of the 
body setae and those of most leg segments 
would appear to be a primitive characteristic 
contrasting with the more ornate setae found 
in other families. The possession of a large 
complement of supracoxal, vestigial, and ros- 
tral setae would also seem to be primary in the 
Parasitengona, secondary reductions in these 
being most notable in the Trombidiidae and 
Trombiculidae. It should be pointed out, 
however, that notable differences in these 
occur within the limits of the family. The 
possession of well- developed paragenital scler- 
ites also seems to be a primitive character- 
istic. These are probably found in all genera 
of terrestrial Parasitengona, although in many 
they lack setae and are nearly invisible. The 
generalized structure of the palpal tarsus of 
the larva, and the presence of a single sub- 
terminal spiniform seta on the adult palpal 
tibia, appear in sharp contrast to the greatly 
reduced tarsus of the larvae of the Trombi- 
diidae, and to the elaborately developed 
ctenidium of the adults of many genera out- 
side of the Johnstonianidae. These are some 
of the principal morphological indications of 
the primitive nature of the group. The detailed 
morphology of these and other structures will 
be discussed below, along with others not 
mentioned here. 
ACKNOWLEDGMENTS 
The present study was initiated under a 
contract between the Office of Naval Research 
(Nonr-782(00), and the University of Hawaii 
while the writer was on the staff of the Hawaii 
Agricultural Experiment Station. It was con- 
tinued under a research grant (NSF-G1833) 
from the National Science Foundation to the 
University of California. Much of the work, 
including the field study program for 1953, 
has been supported by research grants from 
the University of California. Without the 
assistance of these foundations and institu- 
tions, the present work would have been most 
improbable. 
Mari Riess of the University of California 
at Riverside made about 200 of the camera 
lucida drawings; while the remainder were 
