MOUTH PARTS OF THE NYMPH. 



are broad at the posterior portion, but fray out anteriorly. This is the prevailing 

 type among those forms inhabiting streams with gravel or sand bottom. 



The molar surface reaches its maximum of development in Lachlania 

 (fig. 56) and Homeoneuria (fig. 62). Eaton's nameless Chilean nymph (fig. 63) 

 has no molar surface. He suggests that it is predatory. 



Lacinia. The lacinia of the May-fly mandible has been variously named. 

 Eaton, 1883, writes: "The endopodite is often represented by a slender jointless 

 movable appendage attached to the inner base of the inferior lobe, or sometimes 

 by a tuft of hairs." Folsom, 1900 (p. 107) states: "'It is an interesting fact 

 that Heymons '96, distinctly represents a mandibular palp for the larva of 

 Ephemera, — a rare condition ; indeed Packard, '98, terms this appendage of 

 nymphal Ephemerids a 'lacinia-like' process, although Heymons states that it is 

 lateral in position, and so figures it." Lestage, 1917, states that a "prostheca" 

 is present. In fig. 1, p. 219, he erroneously labels it a canine and states that the 

 prostheca is absent in that form. 



Many varieties of mandibular lacinia exist among the different genera. 

 There are well developed forms. Many of these are articulated at the base 

 (Baetis, fig. 60; Polymitarcys, fig. 50; Siphlurus, fig. 51; Euthyplocia, fig. 59). 

 Others are anchylosed ( Choroterpes, fig. 55; Leptophyes, fig. 52; Heptagenia, 

 fig. 53). Some laciniae bear teeth at the apex (Polymitarcys, fig. 50; Leptophyes, 

 fig. 55; Euthyplocia, fig. 59). Long brush-like bristles are present on the lacinia 

 of Leptophyes (fig. 52), Polymitarcys (fig. 50), and Choroterpes (fig. 55). In 

 Siphlurus (fig. 51) and Euthyplocia (fig. 59) the lacinia is covered with short 

 hairs. Sometimes the lacinia is reduced to a slender lobe (Ameletus, fig. 61, Lc). 

 or is absent entirely (Rhithrogena, fig. 48). Among the Heptageninae the lacinia 

 is diminished in size. In Heptagenia it is present on the left mandible (fig. 53, 

 Lc) and absent on the right (fig. 54). A row of hairs is often found on those 

 forms where the lacinia is weak or absent. In Rhithrogena (fig. 48) the hairs are 

 pectinate. These hairs are cuticular appendages, and entirely different in origin 

 from the lacinia. 



Canines. A generalized May-fly mandible possesses two teeth at the apex 

 (PI. I, fig. 12, oc, ic) known as outer and inner canine. When in position, they 

 are more or less ventrally directed. The two canines of the right mandible of 

 Hexagenia (PI. II, fig. 28) dovetail between the outer and inner canine of the 

 left mandible. The raised surface of the epipharynx fits into the anterior cavity 

 formed by the two outer canines. This explains its position a little to the right 

 of the center in many forms (PI. II'I, fig. 34; IV, 43, 44, Ep). In the Baetinae 

 and Ephemerinae the canines are toothed and used for tearing or holding. In the 

 Heptageninae they are more or less scoop-shaped. 



A survey of the mandibles of our North American forms confirms the 

 systematic grouping of the three sub-families and reveals generic characters as 



13 



