102 Annals Entomological Society oj America [Vol. VI, 



itants of twenty stones measuring about seven by eleven inches 

 revealed the following inhabitants listed in the order of abun- 

 dance, Simulium, Blepharocera, May-flies (Epeorus, Iron. 

 Baetis), Stone-flies and Parnidae. In competition with such 

 structures as the sucking disks of Blepharocera and the limpet 

 like form of the water-penny (Parnidae), Iron has developed 

 a successful hold fast of its own. The first pair of gill lamellae 

 are very large and scoop shaped with their hinder edges over- 

 lapping the succeeding lamellae, and their front edges meeting 

 beneath the posterior portion of the thorax. The lamellae 

 diminish in width posteriorly and the last pair are incurved 

 beneath the abdomen. The edges of the lamellae have a thick- 

 ened border and when closely pressed to the supporting surface 

 a successful holdfast is formed. This is one of three closely 

 allied genera, Epeorus, Iron, and Rhithrogena, which represent 

 remarkable modifications for life in rushing water. All three 

 possess closely overlapping gill lamellae and but two caudal 

 setae. The main differences are in the shape of the ventral 

 abdominal disk which in Epeorus is incomplete, the first and 

 last pairs of lamellae being distant ; in Iron nearly complete, the 

 first and last pairs of lamellae nearly meeting; in Rhithrogena 

 the disk is completed by the perfect apposition of these lamellae. 

 The mouth-parts (PI. X, fig. 1) are completely hidden from 

 above by the flaring margin of the head with its bordering fringe 

 of soft hairs. Viewed from beneath the small labrum (PL 

 X, fig. lr) may be seen curving downward and backward over 

 the tips of the mandibles and maxillae (md. a. and mx. a) to 

 meet the median flaps of the broad labium (1. 1. e.) The labium 

 is flat and its outer surface (1) fits close down to the surface upon 

 which the insect is foraging. Along the anterior margin of the 

 outer surface of the labial palp is a series of incurving hairs 

 (1. 1. a.), behind these a set of overlapping plates, and still 

 farther backward a single strongly chitinized scraper, (1. c.) 

 On the inner surface of the palp is a semi-circular patch of 

 inwardly directed hairs (1. b.) Closely apposed to the inner 

 surface of the labium is the outer surface of the hypopharynx 

 (fig. 1 hy), made up of two lateral, and one median portion, 

 distinct, except at their bases. The separation of the two 

 lateral portions from the median portion leaves a gutter-like 

 trough between them on the inner side. The labial palpi are 

 freely movable horizontally. They are moved outward, then 



