34^ The Pedigree of Insects 



the jaws into the head as characterises the Collem- 

 bola has taken place ; while one of these genera, 

 Campodea, is the only insect known to retain in adult 

 life a pair of tubercles representing the tritocerebral 

 appendages of the embryo (103). The ectotro- 

 phous Thysanura then furnish us with a provisional 

 starting-point for our higher insect orders, while the 

 entotrophous division leads on — though only for a 

 very short way — towards the lowly divergent shoot 

 of the Collembola (206). 



Dermaptera, Orthoptera, and Platyptera. — The 

 Dermaptera, Orthoptera and Platyptera, have many 

 structural features in common and may be considered 

 together. All have normal biting jaws, the second 

 maxilise being as a rule incompletely fused. Most 

 have ten evident abdominal segments with appendages 

 on the tenth. All attain the perfect state without any 

 marked change of form. Comparing the three orders, 

 the Dermaptera are seen to retain a markedly prim- 

 itive character in their mesodermal genital ducts, 

 though their wings are highly specialised. In the 

 close likeness between the fore and hind-wings the 

 Termitidje, a family of Platyptera, show the most 

 primitive condition of those organs among the three 

 orders. In the Orthoptera and the Perlaria the hind- 

 wings are specialised by the presence of a folding anal 

 area, but most of the insects of these groups retain 

 the primitive long jointed cercopods so characteristic 

 of the Thysanura. The aquatic nymphal life of the 

 Perlaria, however, marks them off distinctly from the 

 Orthoptera and foreshadows the orders (Plectoptera and 

 Odonata) with aquatic larvae. In the Orthoptera are 

 to be found the most vigorous development of species, 

 and the most marked specialisation of structure within 

 the section we are considering, and they on the whole 

 must be considered higher than the other two, though 



