1920] Walker: Orthopteroid Orders 137 



Thysanura and CoUembola exhibit certain specializations of 

 structure and function, but these are of minor importance — 

 Apterygota being essentially the most generalized group of 

 insects. They exemplify a life cycle without metamorphosis 

 and are, so to speak, larviform, but with the power of 

 reproduction. 



2. THE LIFE CYCLE OF THE ORTHOPTEROID ORDERS. 



E. M. Walker, 

 University of Toronto, Toronto, Ont. 



The Orthopteroid insects (sens, lat.) include all such forms 

 as have mouth-parts of the mandibulate type and undergo a 

 gradual or "incomplete" metamorphosis. 



As in other orders of insects, the extent to which the imma- 

 ture stages ("nymphs") diverge from the adult in form and 

 structure is more or less proportional to the differences in 

 environment and habits. Accordingly they may be divided into 

 two groups, (1) those which are terrestrial throughout life, 

 and (2) those in which the early stages are aquatic. 



Group 1 includes the Blattoidea, Mantoidea, Isoptera, 

 Zoraptera, Grylloblattoidea, Phasmoidea, Orthoptera, Der- 

 maptera, Embiidina, Corrodentia and Mallophaga. Group 2 

 includes the Plecoptera, Ephemerida and Odonata. 



. In Group 1 the habitat and feeding habits are not materially 

 altered during the life cycle, so that the same structural adap- 

 tations are present throughout life, and the metamorphosis is 

 entirely gradual, except at the last moult, when the wings, if 

 present, and the genitalia undergo more or less marked changes. 

 There is, however, little or no histolysis of larval structures. 

 The number of moults is comparatively small, so far as known, 

 being usually four to six, but sometimes reduced to two or 

 increased to seven or eight. A pronymphal stage is sometimes 

 present. The nymphs resemble the adults except in size, details 

 of proportion, chaetotaxy, scuplturing and sometimes colour- 

 pattern, and in the thinner cuticle. The number of antennal 

 and occasionally tarsal joints sometimes increases with growth, 

 and the ocelli, when present, may not appear until the last 

 moult. The wings, when present, appear at an early stage 



