746 Charles Paul Alexander 



dolabina), or are ventral only (Rhamphidaria, Ularia, Epiphragmaria , 

 and most Pediciini). In numljer they range from four (on abdominal 

 segments 4 to 7) in most Pediciini, to five or six (on abdominal segments 2 

 to 7) in Ularia, Epipliragmaria, and other forms. 



Organs of respiration 



The spiracular disk. — The posterior end of the body is usually truncated, 

 bearing the two spiracles and surroimded by a number of fleshy lobes. 

 These lobes vary much in their shape and armature, and range in number 

 from two in the Pedicaria and the Antocharia to six or eight in the Tipuhnae. 

 The inner faces of the lobes are often fined with brown or black horny 

 plates, which serve as points of attachment for the longitudinal muscles. 

 Their arrangement and distribution are of great service in specific 

 classification. 



The Limnobaria usually have the lobes surrounding the spiracular 

 disk indistinct; the Antocharia have two long ventral lobes; the Rham- 

 pliidaria five lobes; the EUipteraria and the Dicranoptycharia four lobes. 



In the Eriopterini, the Elephantomyaria and some Eriopteraria have 

 four lobes; the majority of species of the latter division have five. In 

 these last-named species the disk is almost squarely truncated, and the 

 five lobes are subequal and stellate in appearance. 



The Hexatomini usually have four lobes, with the ventral pair longer 

 than the lateral pair. In a few cases only (Ularia, Dicranophragma, 

 Poecilostola) , a reduced mid-dorsal lol)e is present. In the more specialized 

 forms the lateral lobes also tend to reduce, simultaneously with the 

 elongation of the ventral lobes. In this tribe the disk is fringed with long 

 hairs, which are sometimes excessively elongate, especially on the ventral 

 lobes where they appear as long fringes of delicate hairs that spread out 

 on the surface film of the water as broad fans. Such fringes are found 

 in a large nimiber of Hexatomini and also in the Adelphomyaria (sup- 

 position). Near the tips of the ventral lol^es, but inside this fringe of 

 hairs, are often inserted one or more stiff sensory bristles. 



In the Pediciini, the Pedicaria have two lobes and the Adelphomyaria 

 (supposition) have four. In the former division the lobes are ventral 

 in position and are almost naked. 



The Cylindrotominae have six rather indistinct lobes in Cylindrotoma, 

 and four in the other genera. 



I 



