Johnstonianidae — Newell 
399 
solenidia taper very little throughout their 
length, and the distal end is usually rounded. 
Ornamentation is almost invariably wanting, 
and the cuticle is optically inert in polarized 
light. The eupathidia are similar to the soleni- 
dia in certain respects, but there is usually no 
internal structure (an exception is illustrated 
in Fig. 238). External ornamentation is more 
likely to be found than in the solenidia, in 
the form of short barbs (Figs. 215, 216, 217) 
and the cuticle is nearly always partially ani- 
sotropic. Birefringence is most noticeable in 
die basal portion. The walls of the eupathidia 
are typically of uniform thickness throughout 
—slightly thicker than the solenidia and thin- 
ner, relatively, than the normal setae. Notable 
transformations in structure of the eupathidia 
are found in certain cases, as in those on the 
tarsus of the palp. For example, in the genera 
Centrotrombidium , Johnstoniana , and Diplo- 
thrombium , typical eupathidia are found on the 
palpal tarsus of the adult (Figs. 59, 83, 159), 
while in the larva the apparent ontogenic 
forerunners of these are heavy, pectinate, 
sometimes elaborately formed setae, with lit- 
tle or no trace of a central canal (Figs. 29, 
92, 132). 
The other types of setae provide no par- 
ticular difficulties in interpretation and will be 
discussed below with reference to their char- 
acteristics and distribution in the Johnstonia- 
nidae. 
Scutum 
A number of interesting and illuminating 
trends can be seen in the structure and chaeto- 
taxy of the scutum of the Johnstonianidae. 
The first is a reduction or a modification of 
the anterior pair of sensilla, leading to their 
eventual disappearance, or their transforma- 
tion into setae which are not at all sensillar 
in form. While it might be argued that the 
trend is the reverse, that is, toward the de- 
velopment of the anterior pair of sensilla, this 
does not appear to be the most likely alter- 
native. In Johnstoniana there is a well defined 
anterior pair of sensilla in both larva and adult 
(Figs. 70, 90). Also, in the larva of Lassenia 
scutellata there is a well defined anterior pair 
of sensilla, borne on a small sclerite separated 
from the remainder of the scutum by a narrow 
band of striated membranous cuticle (Fig. 
249). In the adults of Lassenia spinifera , these 
anterior setae (Fig. 232) are scarcely recog- 
nizable as sensilla because the alveoli are not 
to any sensible degree different from those of 
the other setae of the scutum. But there is 
little doubt that the two short simple setae 
on the spine of the adult scutum actually are 
the anterior sensilla. In the larva of Lassenia 
lasseni the anterior sensilla are present, al- 
though the alveoli of these are not as well 
developed as the alveoli of the posterior pair 
of sensilla (Fig. 214). Nevertheless these are 
m. questionably the anterior sensilla. They are 
short and distinctly barbed, in marked con- 
trast to the posterior pair which are long, 
slender, and smooth. In fact the anterior sen- 
silla more closely resemble the other setae on 
the scutum than they do the posterior sen- 
silla. In the adult of the species (Fig. 183) 
there is only one well defined pair of sensilla. 
At the same time there is usually a pair of 
stiff, rodlike setae near the anterior margin of 
the scutum, somewhat set apart from the rest 
of the setae. If one were to examine only the 
nymphs and adults of Lassenia lasseni there 
would be considerable justification in stating 
that this species was characterized by the 
possession of a single pair of sensilla. Yet 
there is no doubt that the anterior two rodlike 
setae on the adult scutum are derived from 
the anterior sensilla of the larva, and are there- 
fore homologous with them, regardless of 
their remarkable transformation in form. Thus 
within the one genus Lassenia we see a partial 
obscuring of the anterior pair of sensilla be- 
tween the larval and adult stages. 
Genera in other families of Parasitengona 
should be re-examined to determine those in 
which the anteromedian setae of the scutum 
are homologous with the anterior sensilla for 
it is evident from the Johnstonianidae that 
this can be expected. Such is almost certainly 
