176 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I37 



the animal associations of standing-water algae, and which feeds on 

 organisms fixed upon the substratum. This structure will later be 

 presented in three modifications realized by species of the genus 

 Diamcsa frequenting torrents, by Prodiamesa olivacea Mg., detritivo- 

 rous, and by Protanypus morio Zett., predaceous, all belonging to the 

 Diamesa group. 



BUCCAL STRUCTURE OF THE LARVA OF 

 CRICOTOPUS OR. SILVESTRIS FABR.^ 



At first sight, this structure, considered as a typical one, appears as 

 a simplification of the chironomian structure pertaining to the labral 

 and epipharyngeal area and to the hypochilum. The setae and dorsal 

 labral chaetae (sensillae I to IV, chaetae, spinulae of Zavfel), placed 

 similarly as in the Chironomus larva, are shorter and are moreover 

 very variable from one genus to another. The epipharyngeal area 

 does not have the double pectinate chaeta mentioned by the present 

 writer (1957, p. 11 1). The other labral-epipharyngeal elements do not 

 present any particular characteristics. The mandible has no dorsal 

 brush, and the inner ventral brush is rather reduced. There is no mes- 

 sorial brush. Generally, the chaeta and the setae are not divided 

 (except the sensilla III) ; they consequently are hooks rather than 

 combs. 



The most remarkable modifications pertain to the anterior ventral 

 region; in its median part, termed "hypochilum," it is composed, as 

 in Chironomus, of the bandlike sclerite doubling the external wall, 

 termed "rabat" by the present writer; it also shows a row of teeth 

 opposed to the mandible. The labium also is constructed as in 

 Chironomus and bears setiform and rodlike sense organs and cuticular 

 processes that are extremely abundant and varying. But there are 

 notable differences of structure in the adjacent parts. The submaxil- 

 lary wall is not double ; the striated paralabial plates of Chironomus 

 have no equivalent in the Orthocladiinae, and consequently the 

 maxillae are directly inserted on the strengthened subgenal cranial 

 wall (Sg) by interposition of the setae-bearing maxillary sclerite 

 (SMx 3 + 4) ', in front this sclerite is directly continued into the 

 stipes (figs. I, 3, 5). On the other hand, the hypochilum is strongly 



3 For a more detailed description of the mouth parts we refer to Kettisch's 

 (1936-1937) paper on Cricotopus trijasciatus P., the larva of which undermines 

 the leaves of Potamogeton. This author, too, analyzes the functioning of the 

 mouth parts and emphasizes the prehensile function of the mandible. But in 

 Cr. cf. silvestris L. we have not found the tenuous muscle fibers joining the 

 maxillary lobe to the hypopharynx, mentioned by Kettisch (p. 259). 



