84 ODONATA. 



useful in the secondary capacity of a motor by being 

 forcibly expelled in a jet from the rectum. The 

 reaction of this stream against the still water behind 

 the abdomen gives a sudden forward impulse to the 

 body, and the walking pupa becomes transformed into 

 a darter driven by hydraulic pressure. In Agrion there 

 are three broad, leaf-Hke gills (PL III., Fig. 41, p. 73) 

 at the extremity of the abdomen, which are used as 

 respiratory organs, and which serve also as rudders. 



Young dragon- flies can be collected in March or 

 April, and kept in the schoolroom, where their habits 

 and transformations can be observed. The pupae of 

 this species (Libellula trimaciilata) stay at this season 

 on and in the mud at the bottom of the aquarium. 

 From twelve to twenty- four hours before the last pupa 

 skin is shed, however, they remain at or near the sur- 

 face. They are usually quiet, resting on a twig or 

 water-plant up to a short time before the final transfor- 

 mation begins. One which was watched by J. M. 

 Arms on the evening of May i6th had not appar- 

 ently changed its position on the following morning. 

 Another observed by her at the surface on the morn- 

 ing of May 23d was quiet till 10.27 a.m., when its 

 motions became quick and restless. When first 

 observed, a distinct longitudinal suture was seen along 

 the middle of the thoracic rings, and on either side, 

 extending obliquely downward and backward from the 

 median suture in front of the wing-pads, were two 

 other sutures. On the head were two sutures extend- 

 ing obliquely forward ; these are seen in Fig. 42, -J, 

 which is a dorsal view of the empty pupa skin drawn 

 after it had been in alcohol a short time. At 10.2 7 the 



