350 The Pedigree of Insects 



(Heteroptera and Anoplura) wherein the wings have 

 undergone the greatest modification or degradation, 

 retain a simple, direct life-history, while in that 

 whose fore and hind-wings remain less differentiated 

 (Homoptera) a larval stage markedly differing from the 

 imago is sometimes present, and rarely (male Coccidae) 

 a resting pupal stage also. 



Plectoptera and Odonata. The Plectoptera and 

 Odonata agree in passing through an aquatic larval 

 stage, but differ greatly in the form of their larvge as 

 well as in their structure as imagos. In their per- 

 manently paired genital ducts the Mayflies exhibit an 

 ancient character which has been retained by no other 

 winged insects, while their larvs suggest Bristletails 

 adapted for life in the water. But the specialisation 

 of their wings and sense-organs and the complete 

 division of labour between the larval and perfect 

 stages forbid us to consider them as, on the whole, 

 a primitive group. Mayflies as well as Dragonflies, 

 whose specialisation both in the larval and perfect 

 condition is most marked, must be regarded as isolated 

 off-shoots from the primitive mandibulate stock, giving 

 but few hints as to the course of their evolution. 



Origin of Wings. — It has been suggested how- 

 ever that the aquatic larva of a Mayfly represents 

 closely the ancestral stock whence all insects have 

 sprung, since life in the waters preceded life on the 

 land. And a brilliant speculation (202) has indicated 

 pairs of tracheal gills on the meso- and metathorax as 

 the possible origin of insect wings. The primeval 

 insects forsook, so it is thought, the water for the 

 land ; and the plates, becoming useless for breathing, 

 were enlarged and finally changed into organs of 

 flight. In the preceding chapter (pp. 285-94) reasons 

 have been given for considering insects, as primarily 

 creatures of the land and the air, those which live 



