l86 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I37 



typical structures. They include the following dispositions and ele- 

 ments ; their morphological interpretation is given by Gouin (1957). 



a. The cranium is ventrally closed by a median sclerite (the "hypochilum"), 

 double-walled, forwardly toothed, forming a rather convex vault. Its post- 

 occipital margin is dorsally incomplete. There is a rudimentary tentorium. The 

 frontal suture limits a triangular clypeofrontal area. 



b. The very different labral and epipharyngeal ornamentations include : ( i ) 

 the tornial sclerite which continues on the ventral face as a thin transversal bar ; 



(2) the four pairs of sensory setae (sensillae labri I to IV) ; (3) the pair of 

 premandibles articulated in a syndesis to the central tormal bar; (4) the diverse 

 ventral chaetae (chaetae and spinulae of Zavfel) of which the median chaeta 

 is often distinct from the others; (5) articulated to the tormal bar, the centro- 

 epipharyngeal piece ("piece en U," "ungula" auct.) ; (6) in the centro-epi- 

 pharyngeal area, the epipharyngeal comb, backed against the tormal bar, and 

 chaetae ("chaetulae," Zavfel); (7) four pairs of muscles: the tormal muscle, 

 two messorial muscles, the labro-epipharyngeal muscles. 



c. The cibarial and pharyngeal musculature is reduced to a few fiber groups. 



d. The mandible has obtuse molar teeth and an internal ventral brush ; the 

 condylic points of articulation are disposed on the "phragmata" and on an 

 oblique axis, apposing the mandible to the hypochilan arch. There are two 

 antagonistic muscles ; the adductor, very powerful, is divided into four fiber 

 groups. 



e. The maxilla is regressive, its elements not very distinct, the sclerites re- 

 duced ; its mobility is slight. There are a stipital muscle and a lacinial muscle 

 which are rather feeble. The maxilla and mandible are closely connected. 



/. The labium is membranous and regressive, hidden in great part by the 

 hypochilum; a pair of premental muscles gives it a weak longitudinal mobility. 

 Numerous sensorial elements are on its anterior end. The reduced hypopharynx 

 is at the back of the labium from which it is separated by the salivary duct. 

 The suspensorium constitutes the frame of the salivary syringe. The retractor 

 angulorum oris (rao) is absent. 



g. The muscular origins are grouped: (i) on the clypeofrontal triangular 

 area; between the antennal socles (cibarial muscles); toward the middle (an- 

 terior messorial muscle) ; distally, toward the summit (posterior messorial 

 muscle, pharyngeal muscles, tormal muscle) ; (2) in the dorsal lateral and 

 ventral region of the posterior half of the cranium (mandibular muscles) ; 



(3) in the middle of the lateral ventral region (maxillary muscles) ; (4) 

 between the posterior tentorial pits (labial muscles). 



Differential characters of these three structural types: 



a. The orthocladian structure is very close to the generalized organization 

 plan. The labral and epipharyngeal processes tend to be spinelike or hook- 

 shaped; the diamesan structure is an important modification of it, chiefly char- 

 acterized by the setiform aspect of the labral and labial processes. Two diamesan 

 forms are excepted : Prodiamesa olivacca Mg, detritivorus, with characters con- 

 vergent with the chironomian structure; and Protanypus morio Zett., car- 

 nivorous. 



b. The podonomian structure is distinct from the generalized organization 

 plan and from the orthocladian structure by the construction, the dimensions, 



