57 



III. DiASTATOMMA Bumi, 



Oj)h /()(/()nq)/( i(s Selys. 

 IV. Herpetogomphus Selys. 



Diastatomma and Herpetogomphus are very closely related, 

 and no good character has yet been found to sejmrate them 

 when in the nymphal stage. They are then similar in form to 

 the common gomphid larvae of the genera which follow, but 

 may be easily distinguished from them all by the widely diverg- 

 ing wing-pads, the inner margins of which separate at an angle 

 of 60° or more instead of being approximately parallel as in 

 the genera which follow. In exuviae of the latter forms, how- 

 ever, the wing-pads are often more or less separated. Her- 

 petogoitqjhus has not been found east of the Mississippi. Dias- 

 tatomma is widely distributed in North America, but both the 

 nymphs and imagos are quite rare in Illinois, owing prob- 

 ably to the small extent of rocky stream-beds and rapid cur- 

 rents. The rivers near Rock Island and Golconda, in which 

 examples have been found, afford favorable situations of this 

 kind. Three species of the nymphs are in our collections, — one 

 found in Illinois, — readily separable by the number of lateral 

 spines, the length of the lateral appendages, etc., as stated in 

 the key. 



Nymph stout, little flattened. Head abruptly sloping for- 

 ward from the ocelli. Labruni pilot-shaped. Antenna? with 

 the two basal segments globular, third segment twice as long 

 as both basal, much flattened and laid close beside the labrum. 

 Fourth joint a minute rudiment. Median lobe of mentum 

 rounded, with border of short blunt teeth and a double series 

 of fringing scales. Lateral lobes nearly straight, not terminat- 

 ing in an end hook and minutely denticulated within ; movable 

 hook short, arcuate. 



Legs rather short. Fore and middle tibiae with external 

 hooks, wing-cases divaricate, strongly sloping downward toward 

 the sides. Dorsal hooks on abdominal segments on 2 or 3 to 9. 

 Tenth segment not inclosed by the 9th but triquetral, exceeding 



