1976] 
Parsons — Morphology of Corixidae 
161 
The thoracico-abdominal boundary follows the same course 
as in Micronecta. The transverse third phragma (Fig. 24, 3PH) 
extends as far as the level of the dorsal opening into the epimeral 
lobe (heavy dotted line). The boundary then bends abruptly pos- 
teriorly, forming a right angle, rather than a gradual curve as in 
Micronecta, and runs along the dorsal edge of the outer wall of 
the lobe in the form of a low ridge (IR). The thoracico-abdominal 
membrane (TAM) attaches along the narrow external wall of the 
ridge and contains the first abdominal spiracle (Figs. 17 and 20, S 1). 
The intersegmental ridge extends as far as the base of the muscle 
process (Fig. 24, MP). The process is larger and more rectangular 
than its homologue in Micronecta but resembles it otherwise and 
bears a similar muscle attaching dorsomedially on the first abdomi- 
nal tergite. As in Micronecta it is incompletely separated from the 
second abdominal segment and its crest is laterally continuous with 
the second abdominal antecosta (AC2). The antecosta, unlike that 
of Micronecta and Diaprepocoris, forms a large, raised apodeme 
just lateral to the muscle process; the anterior face of the apodeme 
provides an extensive surface for the attachment of the very large 
ventral longitudinal muscle of the first abdominal segment. 
Although the relationship of the epimeral lobe to the abdomen 
is very similar in Micronecta and Hesperocorixa, the two insects 
differ in one major respect. In Micronecta, as in Diaprepocoris, 
the posterior part of the lobe does not contact the abdominal pro- 
cess; the two structures are separated by a narrow space in Micro- 
necta and by a much larger gap in Diaprepocoris (Figs. 22 and 23, 
black arrows). In Hesperocorixa, however, the vertical posterior 
edge of the lobe fits into a groove on the ventral surface of the ab- 
dominal process (Fig. 24). The two structures fit so closely to- 
gether that there is little, if any, space between them. This close 
fit creates a barrier between the first abdominal spiracle of Hes- 
perocorixa and the air layer which surrounds the metacoxa (see 
p. 172). 
F. Metathoracic and First Abdominal Spiracles 
I. Metathoracic spiracle 
In all three corixids the orifice of the metathoracic spiracular 
trachea is covered by a sieve-plate and an integumental region 
