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PACIFIC SCIENCE, Vol. XI, October, 1957 
the case in Hannemania , Leeuwenhoekia , and 
related genera in which the resemblance to 
some of the Johnstonianidae is augmented 
by the possession of an anteromedian spine 
on the scutum. If these are homologous with 
the anterior sensilla, then it is further likely 
that the single anteromedian seta found in 
many of the Trombiculidae is also homolog- 
ous with the anterior sensilla, which have 
somehow or other become fused. Only in 
such genera as Gateria , Walcloia , Gahrliepia 
and Schongastiella do the anterior sensilla (or 
their paired or unpaired homologues) appear 
to be totally lacking. The recognition of the 
essential unity of these setae at the anterior 
end of the scutum will help greatly in the 
analysis of evolutionary trends within the 
Parasitengona. The modifications of the an- 
terior sensilla in the Johnstonianidae provide 
the key. 
In Diplothrombium monoense , we find that it 
is the adult rather than the larva in which the 
anterior sensilla are best developed (Figs. 110, 
129). And, finally, in Centrotrombidium the 
anterior sensilla are completely absent in both 
larva and the adult. Some authors have re- 
ferred to the anterior pair of setae on the 
scutum of Centrotrombidium as a second pair 
of sensilla, but a comparison of Figures 4 and 
110 will show that this is not the case; rather 
the anterior pair of setae in Centrotrombidium 
are undoubtedly the homologues of the setae 
which, in Diplothrombium , lie between the two 
pairs of sensilla; the anterior sensilla are to- 
tally lacking. 
Another interesting variant is found in the 
degree of duplication of the setae of the 
scutum other than the sensilla. In Centrotrom- 
bidium and Diplothrombium , the setae anterior 
to the posterior sensilla undergo little or no 
reproduction in the transformation from the 
larva to adult, except perhaps in D. micidium . 
The larva of this species is as yet unknown 
(unless it should prove to be the larva de- 
scribed here as D. cascadense) so it is not 
possible to state positively whether or not 
there has been a duplication of these setae in 
this particular species. If we assume that the 
larva does have a single pair of setae between 
the anterior and posterior pairs of sensilla, 
which is probably the case, then there is a 
moderate degree of duplication of these setae 
(Fig. 160). 
In Diplothrombium monoense there is a mod- 
erate degree of duplication of these setae as 
a rule, for there is usually a second pair of 
setae near the margin of the plate between the 
anterior and posterior pairs of sensilla (Fig. 
110). Considering the situation in D. mici- 
dium, it is probable that these can be regarded 
as a duplication of the setae in question. The 
situation in Johnstoniana and Lassenia is some- 
what more complex, for there are usually five 
to nine or more setae which we could regard 
as the ontogenic descendants of the inter- 
sensillar setae of the larva. 
In addition to the chaetotactic features de- 
scribed above, there is a further variant in the 
structure of the scutum. In both Centrotrom- 
bidium and Diplothrombium , there is a notice- 
able tendency toward the formation of a pos- 
terior stalk with a feebly sclerotized lateral 
expansion in the adult (Figs. 4, 63, 110, 160). 
No corresponding development is found in 
the scutum of either Lassenia or Johnstoniana. 
Associated with the tendency toward the dif- 
ferentiation of the posterior lobe of the scu- 
tum, is the degree of development of the 
lateral portion of the scutum outside the 
crista. The broadest scutum is found in John- 
stoniana, a somewhat narrower plate in Las- 
senia , and the narrowest scuta are found in 
Diplothrombium and Centrotrombidium. 
There is also a noticeable difference in the 
crista metopica, for this structure is well de- 
veloped in the larva of Johnstoniana and Diplo- 
thrombium, very feebly developed in the pos- 
terior part of the scutum of Centrotrombidium , 
and essentially absent from the larval scutum 
of Lassenia. A unique variant in the form of 
the posterior sensilla is found in Centrotrom- 
bidium, in which the distal portion of the 
shaft is greatly expanded and pyriform to 
spherical in form. 
