408 
same time they also emphasize the essential 
unity of the various types. This will be more 
fully treated below. 
The Solenidia of the Legs 
The terrestrial Parasitengona are remarkable 
for the variety of solenidia developed on the 
legs. These must have considerable physio- 
logical, evolutionary and systematic signifi- 
cance, although it will require many years of 
work before these are understood. At the 
present time there are few unequivocal state- 
ments which can be made regarding the form 
and distribution of the various types, because 
of the complexity of study, and the lack of 
comparative studies in the several families. 
However, the most significant findings that 
have emerged from the present study are out- 
lined here to provide at least a tentative be- 
ginning toward the eventual comprehensive 
understanding of the morphology of these 
important organs. In all cases the statements 
should be interpreted as applying most im- 
mediately to the Johnstonianidae, although 
exploratory studies in other families show 
clearly that their applicability extends far be- 
yond the family momentarily under consid- 
eration. 
1. The several types of solenidia are con- 
stant within a given genus. Interspecific varia- 
tions are found in the numbers of a given 
:ype on specific segments of the legs, or 
differences in the position of certain highly 
characteristic types, or in minor but constant 
differences in form of a single type. 
2. All species studied have at least three 
types of solenidia, while most have four. A 
decrease in number of types comes about 
through convergence of form rather than 
through deletion of one or another group of 
solenidia. 
3. The different types of solenidia are recog- 
nizable in larval, nymphal and adult stages, 
although some apparent cases of divergence 
or convergence in form of two similar types 
have been observed in the transition from 
larva to adult. 
PACIFIC SCIENCE, Vol. XI, October, 1957 
4. As a general rule the solenidia of tarsi 
I and II of the larvae are different from each 
other and from the solenidia of the more 
proximal segments of the legs. Tarsus III does 
not possess solenidia in the larvae of the 
Johnstonianidae. Tarsi I and II in the larva 
always bear a single dorsal solenidion, usually 
without, rarely with a companion seta. The 
tarsal solenidia are thicker and more strongly 
marked with spiral or annular structure than 
those of the more proximal segments. 
5. For purposes of convenience in differen- 
tiating the types and brevity in designating 
the types, the solenidia of tarsi'! and II of the 
larvae and their ontogenic descendants in 
nymphs and adults, where these can be dis- 
tinguished, will be designated solenidiai and 
solenidia 2 , or Si and s 2 respectively. Where s x 
and s 2 cannot be differentiated this will be 
interpreted as a convergence in form so that 
Si and s 2 are indistinguishable. The reader 
should recognize that this is not the only 
interpretation that is available, but it should 
be equally apparent that if a different sequence 
of symbols is used each time a decrease in 
solenidiai types is observed, the description 
of this change becomes unduly difficult. It is 
far simpler to utilize a single sequence of 
symbols which is applicable to the majority 
of cases and to fit observed exceptions to this 
rather than to evolve a new system to fit each 
particular variation. Further justification for 
this is found in the fact that the larvae of all 
species of Johnstonianidae studied so far, as 
well as all species of Trombidiidae and Trom- 
biculidae which have been checked, have four 
types of solenidia on the legs which can be 
differentiated by either form or position, or 
usually both. The apparent reductions in sole- 
nidiai types which have been observed to date 
consist entirely of convergences in setal types 
in the development from larva to adult. 
6. The designation solenidion 3 or s 3 will be 
reserved for the type found on the proximal 
segments of the legs, with the exception of 
s 4 (see below). Even in cases in which Si and 
s 2 might be indistinguishable morphologic- 
