1903] NEEDHAM, COCKERELL: — NYMPHS OF ODONATA 137 



of Argia that live in quiet waters and come into direct competition with nymphs of 

 other genera, have the gills quite thin and flat. 



(B. ) The Arroyo Pecos ^ near Las Vegas. 



The Arroyo Pecos (by no means to be confused with the Pecos River) is a 

 gully to the east of Las Vegas, where a small stream cuts through the fossiliferous 

 Pleistocene beds, and even into the underlying Cretaceous shales. The stream is 

 small and the water is so alkaline that drivers of vehicles crossing it will not usually 

 allow their horses to drink. Here and there are pools and marshy places. On 

 June 7 search was made for Odonata in a marshy place in the arroyo, and along 

 the borders of an artificial pond not far off. Agrionines only were observed breed- 

 ing, but these were exceedingly numerous. The males were holding the females 

 by the neck with their abdominal appendages, while the latter oviposited in the 

 ■water. At the same time nymphs were leaving the water, and several placed in 

 paper bags gave forth adult flies. It would seem that either the hatching season 

 is quite prolonged, or else the ovipositing occurs veiy shortly after emergi nee. 

 Three species were represented, as set forth below. Enallagma civile was very 

 abundant, but IscJinnra perparva was scarcely less numerous, while /. damula 

 occurred in fair numbers around the pond. 



Enallagma civile Hagen. 



This widely distributed North American Enallagma was bred from nymphs 

 collected at Arroyo Pecos, Las Vegas, N. Mexico on the 7th of June, 1902. The 

 nymphs of this genus are so much alike that it is hardly worth while to repeat the 

 detailed description given in the Bulletin of the New York State Museum. It will 

 be sufficient here to state the more distinctive characters of this species. The 

 immature stages were unknown at the time the above mentioned Bulletin was 

 written. 



Length 19-20 mm., gills 6 mm. additional. Form as in others of the genus. Gills with- 

 out color pattern save for scanty pigmentation along the larger tracheae. Mental setae of 

 the labium four each side, the innermost small, but half as long as the others. Lateral setae 

 5. End of the lateral labial lobe with end hook as usual, but above it on the margin are 

 three larger followed by three smaller, obsolescent teeth, the latter covering the outer angle. 



Ischnura perparva Selys. 



This little western species was also bred from nymphs collected at Arroyo 

 Pecos, Las Vegas, N. Mexico, on June 7, 1902. Its nymph agrees closely with 



