450 



ONTOGENY 



The various parts of the body will now be considered with ref- 

 erence to their form during the different periods of development. 



Head. — The compound eyes during the life of the embryo are 

 small and dichoptic and situated on the lateral aspect of the head. 

 After eclosion they become larger, are sometimes expanded dorsad, 

 but never become holoptic until the adult stage. The embryonic an- 

 tennae are composed of three segments, the second segment being 

 longer than all the others together, and the third segment nothing 

 more than a spur at the tip of the second. The increase in number 

 of segments takes place by division of the second, which continues to 

 divide until the antenna has seven segments in all. There is little, if 

 any. variation in the diameter of the different segments of most 

 nymphal antennae, but the proximal segments of a few are sometimes 

 greatly developed and much larger than anv of the distal ones. In 

 the adult antennae, the apical segments are setiform and the number 

 of segments varies from four to six. The labial palpi and the median 

 lobe are without setae or fixed hooks. The cleft is usually obliterated 

 after eclosion, but remains practically unchanged in the nymphs of 

 some species. The labial palpi of the young nymph are soon after 

 eclosion provided with fixed hooks, and 'the median lobe is furnished 

 with rows of setae. The adults have no rows of labial setae, but 

 these are scattered promiscuously over the surface. The condition of 

 the mandibles and maxillae is not known for the embryonic stages, 

 but the nymphal condition is much simpler than that of the adult. In 

 this stage the mandible is not biramous except in a few cases. The 

 adult mandible, however, is divided into two parts, one composed of 

 a number of teeth and the other of several cutting edges forming a 

 Z when viewed from the edge. The galea-lacinia of the nymphal 

 maxilla is not as specialized as that of the adult, which bears a greater 

 number of fixed hooks and setae. 



Thorax. — The thorax of the embryo consists of three equal seg- 

 ments, each with a pair of appendages. Very little can be said of the 

 sclerites in the embryonic stages, but the segments of the nymph are 

 all about equal in size. The legs are widely separated and the in- 

 vaginations of all furcae are usually prominent. The suture sepa- 

 rating the proepimeron from the proepisternum is indistinct in the 

 earlier nymphal stages, but becomes more distinct with age. In the 

 mesothorax and metathorax, the interpleural suture is distinct in all 

 zygopterous nymphs and in the adults of the family Agrionidae. In 

 the Anisoptera it is indistinct in all stages. The infraepisterna and 

 supraepisterna are separated by furrows in the nymph, but there are 

 no definite sclerites formed until the adult stage. The mesonotum is 



