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Psyche 
[June 
well, the exposed supra-alar air store also communicates, at the 
gap, with the spiracular chamber. 
The way in which the epimeral lobe is modified to form the spi- 
racular chamber differs in the three corixids. In Diaprepocoris 
(Fig. 22) the dorsal part of the lobe does not extend as far pos- 
teriorly as in the other two insects. The spiracular chamber is thus 
formed by the large space between the abdominal projection and 
the posterodorsal wall of the lobe. This space can receive oxygen 
not only through the gap beneath the forewing (white arrow) but 
from the air store surrounding the metacoxa (black arrow). In 
Micronecta (Fig. 23) and Hesperocorixa (Fig. 24) the dorsal part 
of the lobe extends posterior to the level of the spiracle (position 
of spiracle indicated by heads of arrows in Figs. 23 and 24). If the 
outer wall of the lobe were dilated, as in Diaprepocoris, the spi- 
racular chamber would be reduced in size. In both Micronecta 
and Hesperocorixa, however, the outer wall of the lobe is recessed 
(Figs. 23 and 24, C) immediately ventral to the spiracle, forming 
an enlargement of the subalar space. The spiracular chamber of 
Micronecta communicates, albeit narrowly, with the air which 
surrounds the metacoxa (Fig. 23, black arrow). In Hesperocorixa, 
however, the posterior edge of the epimeral lobe lies closely against 
the ventral surface of the abdominal projection, creating a barrier 
between the spiracular chamber and the metacoxa (Parsons 1970). 
In this respect Hesperocorixa differs from the other two corixids. 
II. Phylogenetic Implications 
According to Popov (1971) the ancestral stem of the Corixidae 
gave rise to six subfamilies, three of which survive at the present 
time. The Diaprepocorinae arose somewhere near the boundary 
between the Triassic and Jurassic, the Micronectinae during the 
Jurassic, and the Corixinae during the second half of the Creta- 
ceous. Popov (1971) placed considerable emphasis on three plesio- 
morphic features of the Diaprepocorinae, the presence of ocelli, 
the short pronotum, and the relatively undeveloped embolium of 
the forewing. 
The condition of the pronotum and embolium appear to have 
respiratory as well as phylogenetic significance. In Hesperocorixa 
and, to a lesser extent, in Micronecta, the lengthened pronotum 
increases the size of the I-II air store and the buoyancy of the an- 
terodorsal part of the body. In Diaprepocoris these functions are 
