494 Mr. H. Eltringham on the 
have the appearance of white spots on the larva. Fig. 1 
is a diagrammatic view of a section of the larval skin 
including one of these plumules. From this it is seen 
that the spicules S, are borne in tufts on small chitinous 
papillae, each of the latter being in direct communication 
with a double layer of special cells H. The spicules them- 
selves are finely pointed barbed structures, the thicker outer 
end being triradiate. They are very easily detached; in 
fact, it is almost impossible to touch the larva without 
displacing them in considerable numbers. The plume-lke 
structure * arises from a chitinous socket, differing little, 
if at all, from the sockets of the larger branched hairs, 
and having at its base several cells G apparently of a 
glandular nature. The plume is quite as easily detached 
as the spicules. 
As in so many similar cases, it is much easier to determine 
the morphology than the physiology of these structures. 
The two layers of hypodermal cells # doubtless secrete 
the spicules themselves, but whether the gland G does 
more than merely secrete the plume, I am at present unable 
to decide. Neither the plumes nor the spicules have any 
appreciable action on litmus paper. During life the plume 
may serve to hold spicules, or even by itself becoming 
detached, to carry them to a distance. At least it seems 
improbable that so complicated a structure should have 
been evolved, merely as a factor in the rather conspicuous 
pattern of the larva. 
The large branched hairs which occur all over the larva 
certainly serve to hold loose spicules, although this may be 
only a part of their function, and they are probably also 
mechanically protective. 
The question whether the urticating properties of these 
and similar larvae are due to chemical or mechanical action 
or to both combined, still remains unsolved. Whilst I 
favour the purely mechanical theory I admit the difficulty 
of accounting for the marked difference of individual 
susceptibility to the urticating spicules of different species 
of larvae. In my own case the spicules of Cnethocampa 
pityocampa applied in small doses produce less irritation 
than those of P. similis, and yet the former species is 
generally regarded as the most ‘“‘ venomous” of all the 
* This structure is proportionately a good deal longer than 
shown in the diagram. 
