394 



INSECT A. 



mentioned above, leading from Peripatus through the Myriopoda and 

 Thysanura to the Orthoptera, contains throughout only forms living 

 on land and adapted for terrestrial life. "We have no reason for 

 assuming that an aquatic ancestral form has been introduced into 

 the series of ancestors of the winged Insecta (Pterygogenea). The 

 manner of life of the aquatic larval forms of the Hemimetabola, 

 as well as their respiratory organs, which are suited to life in water, 

 must be regarded as secondarily acquired. For the same reasons 

 we cannot adopt the view of Dohrn, who, going still further back 

 in the phyletic series, is inclined to refer the tracheal gills of the 



Ephemerid larvae as well as the wing- 

 rudiments to the elytra of the Anne- 

 lidan ancestors of the Insecta (Dohrn, 

 the Pantopoda). It must be pointed 

 out that, in Peripatus, as well as in the 

 Myriopoda, corresponding integumental 

 folds are altogether wanting. We 

 therefore consider that Grassi (Xo. 

 150) is justified in regarding these 

 organs as new acquisitions by the 

 Insectan phylum, and in tracing them 

 back to integumental outgrowths of the 

 lateral margins of the tergal plates that 

 have been constricted off and have 

 become independent, the wing-muscu- 

 lature being derived from the system 

 of dorso-ventral muscles, which is also 

 represented in the other segments of 

 the body. We may perhaps assume 

 that the transition from the creeping method of locomotion to flight 

 was made through the acquisition of a climbing habit, in which 

 distances would occasionally be overcome by springing, a circum- 

 stance which gave rise to the development of parachute-like 

 widenings of the thoracic segments. The transition from such 

 integumental folds, used as a parachute but still immovable, to 

 independent active locomotory organs seems to us fairly plausible. 

 It is perhaps not without significance that the capacity for rising 

 above the surface on which they rest is common among the 

 Thysanura, the Collembola, and the Orthoptera, and that in the 

 latter (e.g., in Psophus stridulus) the wings are scarcely used other- 

 wise than as parachutes. The limitation of wings to the meso- and 



Fig. 194. — Larvae of Caloterm.es 

 rugosus (after F. JIuller). /', 

 wing-like appendages of the pro- 

 tliorax ; /", rudiment of the fore- 

 wing ; / "', rudiment of the hind- 

 wing. 



