EVOLUTION 329 



subject, that the open type of wing- development preceded 

 the hidden type in the evolution of the class ; the Exoptery- 

 gota as a group are more primitive than the Endopterygota. 

 In all those orders whose members show a marked contrast 

 between the adult and the larva, we recognised evidence for 

 an increasing divergence between the early and the final 

 stage of the life-history (see pp. 183-6). To estimate 

 approximately the relationship between the various orders 

 of winged insects it is necessary therefore to take into account 

 the form of the creature both in the immature and adult 

 conditions. 



The orders of winged insects — whether exopterygote or 

 endopterygote — are distinguished mainly, as regards the 

 adults, by the characters of their jaws and their wings, and 

 these are alike structures of great biological im.portance, as 

 on them respectively depend the feeding and the movement 

 of the creatures. Comparison of the various types of jaws 

 among insects assures us that the mandibulate condition, 

 in which the mandibles are adapted for biting solid food- 

 stuffs, is more primitive than the various modifications of 

 the mouth-parts for piercing and sucking, because biting 

 mandibles of essentially the same form are found in many 

 orders of both the sub-classes of winged insects, in the 

 Orthoptera, Isoptera, Corrodentia, Odonata, Coleoptera, 

 Neuroptera, Mecoptera, and Hymenoptera, for example. 

 The members of these orders agree not only in the build 

 and mode of working of the mandibles but also in the 

 general structure of the maxillae. The parts of a typical 

 maxilla, such as that of an earwig (see p. 18) or cockroach 

 can be recognised in all mandibulate insects whether their 

 mode of wing growth be open or hidden. It is evident, 

 therefore, that such mandibles and maxillae were characteristic 

 of most primitive winged insects and that they have been 

 inherited with relatively minor modifications by a large 

 proportion of the existing orders. The characteristic parts 

 of the hinder pair of maxillae that form the labium can also 

 be recognised in the members of these and other orders ; 

 only in the more highly specialised groups — Coleoptera, 



