LARVAL HEAD OF SOME CHIRONOMIDAE — GOUIN I79 



processes, which in all the other forms, chironomian as well as ortho- 

 cladian, are rather strong, rigid, and short, are in the genus Diamesa 

 represented by tufts of long, tenuous setae, appearing to obstruct the 

 atrium before the hypochilum. On its sides, the labium presents a row 

 of short scales. Moreover, the lacinia is provided with setae instead 

 of the knife-bladelike chaetae which the maxilla of Cricotopus bears ; 

 the ventral mandibular brush is made of tenuous setae. The whole 

 gives the impression that the "mouth" is obstructed. On the other 

 hand, the labral processes seem to be stronger ; in effect, the sensillae 

 I to IV have the form of minute points, and the chaetae and spinulae 

 are represented by short spinulae situated in front of the ventral 

 tormal arch ("margo labralis" of Zavfel). The hooklike form and 

 the nondivision of the centroepipharyngeal chaetae are also points 

 to be noted. 



Prodiamesa olivacea Mg., a Pelobiontic Species 



This detritivorous species of the Diamesa group, inhabiting mud of 

 slow-flowing streams, presents some characteristics convergent with 

 those of the CJiironomus larva, especially in the labral and epipharyn- 

 geal processes. In effect, here, we find again the lateral and median 

 labral chaetae ("chaetae" and "chaeta media" of Zavrel) finely pec- 

 tinated, occupying the whole width of the ventral labral surface, a 

 position very reminiscent of that of the Chironomus larva. In no 

 lasiophagous species have these chaeta either this form or this dis- 

 position. The structures of the other organs have not such a clear 

 convergence character. In the U-shaped piece, the hooks ("chaetulae 

 laterales" of Zavfel) are rather long and very simple, but they do 

 join ; the premandible is strong, its distal tip is bifurcated and wide 

 without a messorial brush. The labial processes are very different 

 from that of the Diamesa larva : they include sensorial rods and setae, 

 denticulate-edged scales and scales with minute points, disposed in 

 several rows and strata ; therefore the labium is not brushlike as in 

 Diamesa. The hypochilum is characteristic of the species : its rather 

 irregular outline, its wide basal winglike expansions with whiskers 

 made of long setae, the deep pigmentation of the whole ventral region 

 forming a design like an X, all give this larva an extremely charac- 

 teristic aspect. Anatomically this region is not different from that 

 of orthocladian larvae. The winglike setigerous expansions are only 

 lateral processes of the hypochilan base as in Cricotopus and are thus 

 not homologous with the striated plates of Chironomus. 



