1976] 
Parsons — Morphology of Corixidae 
171 
appears to be the case in Hesperocorixa, Diaprepocoris, and Mi- 
cronecta. The capacity of a spiracle to inhale oxygen depends 
upon its porosity and size and its degree of communication with a 
well-oxygenated air store. In all three corixids the first abdominal 
spiracle is larger than the other spiracles, faces onto an enlarged 
part of the subalar space, and is widely open, lacking any sort of 
modification ( Diaprepocoris and Micronecta) or bearing only a 
highly porous collar ( Hesperocorixa , Fig. 30). 
The more posterior abdominal spiracles, although they face 
directly onto the well-oxygenated ventral abdominal air layer, 
are much smaller than the first abdominal one and lie in a region 
of less metabolic activity. The mesothoracic spiracle communi- 
cates broadly with both the I-II and ventral thoracic air stores, and 
Parsons (1970) believed it to be a major inhalant spiracle in Hes- 
perocorixa. This now appears to be unlikely. In Hesperocorixa 
it possesses a sieve-plate with an inner membrane, and in all three 
corixids it is considerably smaller than the First abdominal spiracle. 
The fine structure and porosity of the mesothoracic spiracles of 
the three corixids, however, has yet to be investigated by means 
of the scanning electron microscope. The metathoracic spiracle is 
smaller and much less porous than the first abdominal one in all 
three insects. It is covered by a sieve-plate which, at least in Hes- 
perocorixa, limits its porosity (Fig. 28). 
The inhalant function of the first abdominal spiracle is reflected 
not only in its fine structure but in the gross morphology of the 
region surrounding it. It lies in an enlarged part (“first abdominal 
spiracular chamber”, Parsons 1970) of the subalar air space. When 
the spiracle lies within the metathoracic epimeron, as in most Hy- 
drocorisae (Fig. 21), the relatively small portion of the epimeron 
which surrounds it is recessed on the body, forming the spiracular 
chamber. In the Corixidae, however, the spiracle lies in the tho- 
racico-abdominal membrane and its spiracular chamber is formed 
by the modification of the greatly enlarged metathoracic epimeral 
lobe which lies ventral to it. 
All three corixids possess a gap, between the forewing and the 
body, which leads directly into the spiracular chamber (Figs. 11-13 
and 22-24, white arrows). Through this gap the spiracle can receive 
oxygen from the ventral air layer on the exposed parts of the epi- 
meral lobe (Figs. 22-24, EML) and abdominal projection (AP); 
in Hesperocorixa, and quite probably in the other two insects as 
