252 H. A. HAGEN. 



2. The palpus has a sharp terminal hook and is dentated ; the middle 

 third of the front margin of the mask is cut straight (some exceptions 

 to be mentioned ) ; eighteen specimens. 



The first group belongs to Onychogomphus, Herpetogomphus and 

 Ophiogomphus. 



Onychogomphus known by but one Asiatic species differs from the 

 others by the prismatic shape of its body, rather similar to Progomphus 

 and by the exceptionally large number of lateral spines on segments 2 to 

 9, characters perhaps to be modified by other species. In this neighbor- 

 hood is to be placed the nympha from the Himalaya with a long trian- 

 gular lobe of the mask ; perhaps it belongs to Heterogomphus. 



The nympha of Herpetogomphus (three specimens) and Ophiogomphus 

 (five specimens) so similar one to the other that I could not find any 

 differences, are directly separated from Onychogomphus by the flat lan- 

 cet-shaped abdomen. 



The nymphae of the second group (eighteen specimens) are somewhat 

 polymorphic. The species belonging to the group of G. plaginfus repre- 

 sent a type difierent from the others by the very long spindle-shaped ab- 

 domen, which is not broader than the head, by the length of 9th seg- 

 ment, by the small hook on the anterior tibia and by only two or three 

 teeth near the terminal hook of palpus. The burrowing for a narrower 

 spindle-shaped abdomen is obviously easier, and explains the smallness of 

 the hook of the tibia. To this group belongs G. plagiatus, G. notatns, 

 G. spiniceps and perhaps G. olivaceus. Of the latter species the male 

 is not yet known, and the rounded lobe of the mask, which is wanting in 

 the other species, makes its place in this group doubtful. 



The nympha of G. pallidus and of the species supposed to belong to 

 G. pilipes by Mr. L. Cabot are exceptional by the shape of the abdomen, 

 which is broad, flat and slopes strongly at tip, the 9th segment long ; the 

 mask has a rounded lobe with a cylindrical tooth in the middle. The 

 length of the 9th segment corresponds with the length of this segment 

 in the imago as well as for the group of G. plagiatus. 



Another group is represented by G. vulgatissimus and three not raised 

 species. The flat lancet-shaped abdomen has the dorsal segments 3d to 

 8th or less, divided by a sharply impressed middle line in two halves ; 

 the mask straight, the palpus strongly dentated. 



The last group, all North American species, only two of them raised, 

 have the dorsal segments not divided, but with strong dorsal hooks, the 

 abdomen is less flat and mostly narrower, the mask straight, the palpus 

 dentated, mostly from base to tip. To this group belongs G. vastus, 

 G. exib's, and related species. 



