1920] Walker: Orthopteroid Orders 141 



Ephemerida. Naiads phytophagous, diverging from the 

 adults more widely than in the Plecoptera and showing a wider 

 range of adaptation in form and structure. Life cycle varying 

 from a few weeks to three years, aquatic except in the last two 

 stages (subimago and imago), both of which have functional 

 wings but live only a few hours to a few days and take no food. 

 Number of moults large, apparently sometimes 30 or more. 

 The most marked changes acquired on reaching the subimago 

 stage are the enlargement of the compound eyes, the develop- 

 ment of ocelli, reduction of antennae and mouth-parts, develop- 

 ment of wings and genitalia, loss of the tracheal gills, elongation 

 of the cerci, with loss of their seise, besides more or less marked 

 changes in general form. Adult phase generally appearing in 

 early summer, usually crepuscular or nocturnal and serving 

 only for the mating function. Eggs deposited in large numbers, 

 without capsule, sometimes attached to other objects. 



Odonata. The habitats and corresponding modifications of 

 the naiads of this group are similar to those of the preceding, 

 but their habits are wholly predaceous. Developmental period 

 varying from a few months to three years or more, the number 

 of moults variable, even in the same species, 11 to 15 in known 

 cases. A pronymphal stage is present. 



The principal adaptations of the naiad are : Modification of 

 the labium as a prehensile arm; (2) special respiratory organs, 

 viz., three leaf -like terminal tracheal gills in the suborder 

 Zygoptera, which also serve as fins, or numerous gills in the 

 rectal chamber in the suborder Anisoptera. Wing pads of 

 naiads reversed in position as in Orthoptera. Adults compar- 

 atively long-lived, active, predaceous, diurnal, showing the 

 following changes of structure at the final moult in relation to 

 the change of habits: Enormous enlargement of compound 

 eyes and development of ocelli ; shortening and change of form 

 of labium, the larval structure being absorbed by histolysis; 

 modification of thoracic segments and legs in adaptation to 

 habits of flight and perching; elongation of abdomen and 

 development of genitalia. There is a short quiescent stage, 

 equivalent to a pupa, in which no food is taken. Oviposition 

 endophytic or exophytic, the eggs in the latter case being 

 sometimes scattered, sometimes enclosed in a gelatinous 

 envelope, or attached to objects in water; generally several 

 hundred deposited. 



