400 SYNOPTIC COLLECTION. 



hairy appendages is drawn completely into the ninth 

 segment of the abdomen. 



Specialization by reduction is illustrated in the adult 

 stage by Caenis (PI. 962, C. macrura) in which the second 

 pair of wings have disappeared and the insect resembles 

 the two-winged Diptera. 



Order 3. ODONATA. 



These insects, like the Ephemeroptera, have become 

 adapted to aquatic life. There can be little doubt, how- 

 ever, that their ancestors were terrestrial, air-breathing 

 animals with an open tracheal system like that of Campo- 

 dea. 



A dragon-fly, Palaeophlebia superstes (PI. 963, fig. i, 

 with the wings on one side and with two legs removed) 

 has been discovered in Japan, which possesses, perhaps, 

 more primitive characters than any other member of its 

 order. Unfortunately the larval form is unknown, but 

 judging from the adult, the larva must be more primitive 

 than most young dragon-flies. The broad head, the 

 separated eyes (fig. 2), the nearly equal wings, with their 

 simple venation (fig. i), are all generalized characters. 

 Although it is true that the larvae of the Odonata do not 

 resemble Campodea so closely as the larvae of the. 

 Ephemeridae, nevertheless, the body in the most general- 

 ized family, represented by Calopteryx (spelled also 

 Calepteryx) virgo (PI. 964, fig. i ; No. 965, an allied 

 form, Hetaerina) is long with little concentration of regions 

 and but slight modification of the thoracic segments. 

 Like the young mayfly, the young Calopteryx has three 

 caudal appendages (fig. i) which serve both as locomo- 

 tor and as respiratory organs. 



The second pair of maxillae or labium have become 

 modified into a mask (fig. 2), so called because in the 

 more specialized genera it entirely conceals the mandibles 



