G. H. F. Nuttall, W. F. Cooper and L. E. Robinson 349 
side by a prominent raised lip (PI. XXIII, figs. 5—7), which over¬ 
hangs the opening and effects its closure when the lip is depressed 
by muscular action. The ostium does not appear in the male or 
nymph 1 . 
It is only when cleared and mounted specimens of the entire 
spiracle are examined that its complex structure becomes apparent. 
That which on examination in situ appears to be a simple sieve-like 
plate, resolves itself into a series of three superposed layers, each 
differing from the other two in structure. The superficial layer exhibits 
a regular reticulate pattern which, as will be seen later, is due to a 
thickening of the chitin on its under surface; the meshes of this 
reticulum are more or less circular, and each is perforated in its central 
portion by one of the minute pores ( e.p .) already alluded to. It is 
almost impossible to define these pores in cleared entire preparations 
of the spiracle on account of their extremely thin margins being 
obscured by the underlying parts. Immediately beneath the super¬ 
ficial layer is a large air space, traversed by an arrangement of 
innumerable delicate chitinous rods or pedicels (p.), the slightly ex¬ 
panded bases of which arise from a thick basal layer of chitin, the 
latter forming the deepest layer and the most substantial portion of 
the spiracular plate. The basal layer, like the superficial layer, is 
porose, but its pores (ip.) are not pervious to air, being occupied 
by protoplasmic extensions from the hypodermal cells underlying the 
spiracle. The internal pores coincide in position with the superficial 
pores and are those seen in ordinary cleared and mounted prepara¬ 
tions. The intermediate or pedicellar layer is formed by the system 
of pedicels which support the superficial layer. The arrangement of 
the pedicels shows a regular system ; this is readily seen in PI. XXII, 
figs. 2 and 4. The pedicels are triangular in cross section and are 
slightly curved in such a manner that their upper extremities are 
brought together and fuse with one another on the under surface of 
the superficial layer; it is this fusion of the pedicels which forms the 
reticulate thickening of the superficial layer. 
By the study of a series of vertical transverse and longitudinal 
sections of the spiracle, it becomes possible to interpret the relationship 
of the parts described in the preceding paragraph (PI. XXII, fig. 3 and 
PI. XXIII, figs. 1—10). The continuity of the thin superficial layer is 
1 The terms macula and ostium are both new : the term pore has been very loosely 
used in general descriptions and appears to refer to the macula. We see, however, that 
the macula may contain no pore or aperture communicating with the tracheal system. 
