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PACIFIC SCIENCE, VoL XI, October, 1957 
families, but at present this appears to be the 
situation in the Johnstonianidae. In none of 
the larvae of the Johnstonianidae studied so 
far are there any setae behind the tritorostrals 
corresponding to the posterorostrals of some 
genera. 
In the adults, of course, the base of the 
gnathosoma is generally covered with a con- 
siderably greater number of setae. An inter- 
esting exception is found in Lassenia spinifera 
(Fig. 248), in which only one pair of setae is 
added behind the tritorostrals; in L. lasseni 
(Fig. 180) many more setae are found here. 
In adults of Centrotrombidium (Fig. 17) the 
number of setae is apparently never great, but 
these are small species. The largest number of 
setae found behind the tritorostrals is in 
Diplothrombium and Johnstoniana (Figs. 125, 
66 ). 
It is difficult to say whether or not the setae 
found behind the tritorostrals are to be con- 
sidered the ontogenic descendants of the 
tritorostral setae, but this does not appear to 
be the case, for the tritorostrals often retain a 
fairly characteristic appearance, differing rather 
markedly from the setae behind them (Figs. 
23, 106). We would therefore have to consider 
these setae as having arisen de novo in the 
postlarval instars. 
Velum 
This structure appears to show few variants 
of any significance in the family. It is a simple 
structure of moderate size in all species studied 
by the writer. 
Posterolateral Arms of Gnathosoma 
Only moderate trends are noticeable within 
the family, with Johnstoniana and Diplothrom- 
bium having essentially no lateral arms on the 
posterior margin of the gnathosoma and with 
Centrotrombidium having only moderately de- 
veloped arms here. The maximum develop- 
ment of these structures is found in Lassenia. 
Within this genus there is quite a bit of differ- 
ence between the two known species, with L. 
spinifera having relatively longer posterolateral 
arms than L. lasseni. 
Chelicerae 
The form of the chelicerae in the Jonhston- 
ianidae is fairly uniform, and is best seen in 
Figures 74 and 85. The tarsus or digitus mo- 
bilis is scythe-shaped with the dorsal margin 
ranging from nearly smooth to serrate (Fig. 
7) to dentate (Fig. 247). The digitus fixus is 
membranous and shows no particular variants 
of generic importance. Perhaps the most sig- 
nificant variant in the form of the chelicerae 
is found in the tendency toward a downward 
flexure in the chelicerae of adults of the genus 
Lassenia. In Lassenia lasseni this flexure is only 
slightly developed, but in L. spinifera it is very 
pronounced (Figs. 200, 245). One very in- 
teresting bilateral anomaly was found in a 
single female of Centrotrombidium distans (Fig. 
24) in which a typical seta and alveolus were 
found dorsally at the base of the digitus fixus. 
No other individuals of this species were 
found to have such a seta here, and certainly 
none of the other Johnstonianidae studied 
had a seta in this position, or anywhere on the 
chelicera. Such a seta is found at this point in 
many genera of Eleutherengona, however, and 
also in many of the Parasitiformes. The ap- 
pearance of this seta in this anomalous indi- 
vidual not only suggests that the Johnston- 
ianidae were derived from a group which did 
have a seta on the chelicera, but also raises 
the question why such a structure, apparently 
so irrevocably lost in the course of evolution 
of the group, should suddenly reappear as a 
bilateral variant in a single female of Centro- 
trombidium distans . Further details of this will 
be found under the description of the species. 
Segmentation and General Form of the Palp 
With a single exception the palpi of the 
Johnstonianidae are provided with five free 
segments. In larvae of Lassenia lasseni (Figs. 
210, 213) there is a tendency toward the fusion 
of the femur and patella on the dorsal side; 
