Studies on the Johnstonianidae (Acari, Parasitengona) 
Irwin M. Newell 1 
The subfamily Johnstonianinae Thor 1935 
(Trombidiidae) was established to include 
Johnstoniana George 1909 and related genera. 
The only comprehensive treatment of the 
group was published by Thor and Willmann 
(1947) who included eight species in the 
genera Centrotrombidium Kramer 1896, John- 
stoniana, Diplothromhium Berlese 1910, Myr- 
micotrombium Womersley 1934, and Hirsti- 
thrombium Oudemans 1928. 
The present study was undertaken with a 
view to placing the systematics of the John- 
stonianinae on a sound morphological basis, 
and to determine the relationship of the group 
to other terrestrial Parasitengona. It soon be- 
came apparent that the mites included in the 
Johnstonianinae differed so significantly from 
other terrestrial Parasitengona that they could 
not logically be retained within the family 
Trombidiidae Murray 1877. 2 The differences 
are much greater than those which have been 
cited, for example, to establish Trombiculidae 
Ewing 1944 as a family separate from the 
Trombidiidae. The group is therefore raised 
to family rank with Johnstoniana George 1909 
as the type genus. 
Interest in the group was stimulated by 
numerous indications that this is perhaps the 
most primitive existing family, terrestrial or 
otherwise, within the Parasitengona. The in- 
dications of this are of ecological, behavioral, 
and morphological nature. 
1 University of California, Riverside, California. 
Manuscript received April 17, 1956. 
2 While many derivatives of the name Trombidium 
Fabricius 1775 were published prior to 1876 (see 
Oudemans 1937: 1349-1361), the first unequivocal use 
of the name Trombidiidae appears to be that of Murray 
(1876) in his Economic Entomology. Accordingly the 
writer ascribes the family name to Murray 1876 rather 
than to Leach 1815, who employed the name Trom- 
bidides, and not Trombidiidae. 
Ecologically the Johnstonianidae are ter- 
restrial in larval, nymphal and adult stages, 
but are rarely found where there is not an 
ample supply of water nearby; in fact many 
of them could be aptly termed subaquatic. 
Thus, ecologically they are in a position from 
which they (or their antecedents) could evolve 
in two directions — either toward strictly ter- 
restrial forms such as the Trombidiidae, or 
toward the subterrestrial aquatic mites such as 
the Limnocharidae, Thyasidae, etc., and 
thence to the more strictly aquatic Parasiten- 
gona. From an evolutionary standpoint, it 
might be hypothesized that the Parasitengona 
parasitic on insects evolved before those on 
terrestrial vertebrates, because insects ap- 
peared first in the fossil record. This assumes 
that there were appropriate predatory Acari 
present when the hosts appeared, but since 
the earliest fossil record of the Trombidi- 
formes ( Protacarus Hirst, Devonian) is at least 
contemporaneous with that of the earliest 
known Insecta, this is not an impossible 
assumption. The next major group of hosts 
to become available to the Parasitengona 
were the Amphibia, and the seemingly close 
structural similarities between Hannemania 
species (which largely parasitize Amphibia) 
and the Johnstonianidae, which parasitize in- 
sects living in moist habitats, is suggestive 
that these are phylogenetically very close. 
This close relationship could have arisen in 
more than one way— either by the direct 
descent of the Amphibia-parasites from the 
early Johnstonianidae, or by simultaneous 
radiation of the Amphibia-parasites and the 
Johnstonianidae from a common ancestral 
mite or group of mites which appeared at the 
same time as or somewhat later than the 
Amphibia. The extent to which the Parasi- 
tengona have become adapted to the Insecta 
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