112 
The Respiratory System of Mallophaga 
continuous at the margin of the external aperture. Sometimes, as in 
Tetrophthalmus, the chamber projects above the surface as a hemi¬ 
spherical protuberance, on the summit of which is the spiracle. In 
Gyropus it has somewhat the shape of a kettledrum, the rim of which 
is flush with the surface, and the centre of the flat surface pierced by 
the spiracle. In Trichodectes the arrangement at the surface is the same, 
but the rim is thickened and sculptured, and the chamber the shape 
of an elongate cone. The prothoracic vestibules are often remarkably 
different from those of the abdomen. In Trimenopon a very long cylin- 
drical vestibule with reticular pattern on its thick chitinous wall connects 
the external opening with the bulla. In Trichodectes setosus (Fig. 13) 
the vestibule is enormously enlarged, with scaly sculpture. Contrary 
to what is the case with the abdominal stigmata, the prothoracic 
vestibule is proportionately larger in the Ischnocera than in the Ambly- 
cera, as the following diametric measurements show. 
Measurements in Millimetres of Prothoracic Vestibules. 
Trichodectes setosus 
0-075 
Goniodes curvicornis 
0-019 
Heteroproctus hilli 
0039 
Gliricola gracilis 
0-019 
Lipeurvs asymrnetricus 
0-033 
Heterodoxus longitarsus 
0-019 
Ornithohius fuscus 
0-030 
Lipeurus fasciatus 
0-017 
Aptericola gadowi 
0-028 
Pseudomenopon tridens 
0-017 
Docophoroides brevis 
0-028 
Trabeculus heteracanthus 
0-014 
Ooniocotes gigas 
0-028 
Latumcephalum macropus 
0-014 
Oxylipeurus sinuatus 
0-025 
Rallicola hisetosa 
0-013 
Philopterus occidentalis 
0-019 
Giebelia hexakon 
0-013 
The sculpture of the vestibule differs considerably in different forms. 
A common type in the Ischnocera is that in which the whole inner 
chitinous surface is raised into parallel ridges. In other genera the 
sculpture is reticulate, or scaly, the scales in some cases being so small 
and elongate as to look like hairs. This sculpture probably serves to 
hinder dust particles from entering the tracheae. 
The term btdla I employ for what is usually a distinct chamber 
internal to the vestibule, which gives support to the occluding apparatus, 
and with which the stigmatic branch of the trachea actually connects. 
In some forms, e.g. Helerodoxus, there is little distinction between 
vestibule and bulla; in others, as Trichodectes, the bulla is reduced to 
a narrow chitinous tube. The precise relations of these parts vdll be 
seen in Figs. 16, 18 and 19. 
The occluding apparatus has received attention only from Mjoberg. 
He has published (1910, p. 221) a diagrammatic figure of this apparatus 
