1976] 
Parsons — Morphology of Corixidae 
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the exoskeleton. These ducts (indicated by dotted lines, Fig. 6) 
open onto the intersegmental groove (Fig. 19, EC), between the 
metathoracic episternum and the mesothoracic epimeron, which 
leads to the lateral ostiole. 
In Hesperocorixa the subalar space external to the metathoracic 
pleural sulcus (Fig. 17, PL III) is enlarged by a pronounced, hori- 
zontal, trough-like indentation (AT) which extends from the in- 
tersegmental boundary to a level posterior to the metathoracic 
coxal process. This “air trough” is formed by both the dorsal por- 
tion of the metathoracic episternum and the ventral portion of the 
metathoracic epimeron. It is bounded ventrally by a pronounced 
ridge (W) which appears to represent the anterior part of the epi- 
sternal lateral lobe. The costal margin of the forewing fits closely 
against this ridge, closing off the air trough ventrally. The raised 
episternal ridge performs the same wing-holding function as does 
the horizontal episternal groove of Diaprepocoris (Fig. 15, W) but, 
unlike the latter, extends anteriorly from the coxal process (Fig. 20, 
PR) rather than lying far ventral to it (Fig. 18). In Hesperocorixa 
a possible homologue of the episternal groove of Diaprepocoris 
is a short groove (Fig. 17, G) which lies posteroventral to the wing- 
anchoring ridge, at the posteroventral edge of an indented region 
(ID) of the episternum. On this indentation the ventral thoracic 
air layer is greatly thickened. Unlike the rest of the exposed epi- 
sternum, the indentation lacks long hydrofuge hairs and is covered 
only with short ones. 
In Micronecta the metathoracic air trough is less well-developed. 
Anteriorly it is formed only by the recessed dorsal portion of the 
metathoracic episternum (Fig. 16, ES III). The anterior part of 
the epimeron (EM III) is entirely convex, like that of Diaprepocoris 
and typical Hydrocorisae, rather than partially concave as in 
Hesperocorixa. Only the portion of the epimeron which lies at 
the level of the coxal process is concave, forming the posterior 
part of the air trough (AT). 
The episternal wing-holding mechanism of Micronecta consists 
of a low horizontal ridge (Fig. 16, W) which, like its more pro- 
nounced homologue in Hesperocorixa, forms the ventral boundary 
of the air trough. Ventral to this ridge is a deep groove (G) which 
runs obliquely anteromedially and ends at the ostiole of the meta- 
thoracic gland (O). This groove may be homologous with the 
wing-holding groove of Diaprepocoris. Its function is difficult 
